DRAFT
TECHNICAL SUPPORT UOCUMENT FOR. BETERMNATION OF
Soon ENGINEERING* PRACTICE STACK.HEIGHT
JuorlL I97E
11* i. ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF AIR* NraisE> AND RADIATION;.
QFFICE OF AIR DUALITY PLANNING AND: STANDARDS-
RESEARCH TRIANGLE FARK* NORTH CAROLINA
DRA.FT
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Table of Contents
Page
1.0 Overview and Recommendations 1
2.0 Technical Basis for GE? Stack Height ' 5
2.1 Description of Aerodynamic Effects 5
2.2 Building Effects 7
2.3 Quantitative Rationale for GE? Equation and Excessive 12
Concentration
2.4 Terrain Influences 18
2.5 Minimum Stack Height 21
3.0. Determination of GEP Stack Height 23.
3.1 Initial Assumptions 23
3.2 Simple-Structures . 24
3.3 Complex Structures 29
3.4 Terrain Obstacles 35
3.5 Framework for Demonstrating GEP Stack Height 35
4.0 Air Quality Estimates 38
References . 43
Annotated Bibliography A-l
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1.0 Overview and Recommendations
Section 123 of the Clean Air Act Amendments of 1977 requires
the Administrator to promulgate regulations to assure that the control
of any air pollutant under an applicable implementation plan shall not
be affected by (1) stack heights that exceed good engineering practice
or (2) any other dispersion technique. Good engineering practice (GEP)
is defined with respect to stack heights, as "the height necessary to
insure that emissions from the stack do not result in excessive concen-
trations of any air pollutant in the immediate vicinity of the source as
a result of atmospheric downwash, eddies and wakes which may be created
by the source itself, nearby structures or nearby terrain obstacles."
The GEP definition is based on the observed phenomenon of
disturbed atmospheric flow in the immediate vicinity of a structure or
terrain obstacle. Some finite stack height is therefore necessary to
insure that emissions do not result in excessive ground-level concen-
trations. The GEP definition identifies the minimum stack height at
which significant adverse aerodynamic effects are avoided.
This document provides technical support for the definition
and specification of GEP stack height and reasonable minimum stack
height. The remainder of this section proposes applicable definitions
of GEP stack height. Section 2 summarizes available technical informa-
tion and provides a technical basis for the GEP definitions. Section 3
provides examples of factors and methods to be applied in determining
GEP heights. Section 4 identifies the procedures and the assumptions
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that should be used in determining emission limitations based on GEP
heights. An annotated bibliography is included that provides a repre-
sentative selection of statements found in the scientific literature
concerning the stack height for which adverse aerodynamic effects may be
a problem.
The scientific literature in general indicates that a case
specific review is integral to assuring the prevention of adverse
aerodynamic effects in the immediate vicinity of a given source.
However, the literature also identifies generalized formulations which
are designed to establish the minimum stack height to prevent this
phenomenon. The following recommendations are based on these gen-
eralized findings:
0} GEP stack height to minimize the adverse impact of struc-
tural obstacles should be based on the formulation
HG = H + 1.5L (1)
where: HG = Good engineering practice stack height
H s= Height of the structure or nearby structure
L = Lesser dimension (height or width) of the structure or
nearby structure
Both the height and width of the structure are determined from the
frontal area of the structure, projected onto a plane perpendicular to
the direction of the wind. If the structure is asymmetrical, the GEP
stack height should be based on the plane projection lying upwind from
the source [stack! which results in the greatest justifiable height.
As new studies and research are reported, additional guidance will be
provided.
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The plane projection may have a multitude of heights or widths, par-
ticularly for a multilayered structure. Each combination of the height,
H, and lesser dimension (height or width), L, must be evaluated for each
segment of the structure to determine which one results in the greatest
GEP stack height as defined fay Equation (1). The area in which a nearby
structure, can have a significant influence on a source should be limited
to five times the height or width of the structure, whichever is less,
downwind. The area of influence becomes diminishingly small as the
height to width ratio of a structure increases. Thus structures such as
stacks and radio or TV transmission towers should not be considered in
GEP stack height determinations. Assumptions associated with the deter-
mination of GEP stack height and appropriate examples are presented in
Section 3. Complex structures with a multitude of heights and widths
are covered in Section 3.2.
C2] The GEP stack height required to minimize the adverse
impact of terrain obstacles should be determined on a case-by-case basis
until further studies allow a general formulation to be set forth.
Field studies designed to evaluate the specific situations under the
variety of adverse meteorological conditions are the best source of
information. Where field studies are not possible, comparable fluid
model studies are acceptable. In order for the general definition of
"nearby" to follow the expressions of Congressional intent, stack
height credit for facilities should not be given to overcome the aero-
dynamic effect of terrain features which are beyond 1/2 mile (800
meters}, away from the source.
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(3) There are circumstances where pollutants from a major source
may be released near ground level or from a stack with no supporting
structure on which to base GEP calculations. To avoid natural atmos-
pheric effects which may cause excessive concentrations around very low
sources, it is recommended that a stack height of 30 meters be defined
as good engineering practice, without demonstration of necessity.for any
major source.
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2.0 Technical Basis for GEP Stack Height
2.1 Description of Aerodynamic Effects
Atmospheric flow is disrupted by aerodynamic forces in the
immediate vicinity of structures or terrain obstacles. The aerodynamic
forces evolve from interacting frictional forces and pressure gradients
induced by the local obstruction. The surface friction and pressure
gradients combine to retard the atmospheric surface layer flow enough to
produce regions where the flow is locally distorted, causing an area of
stagnation (cavity) to develop. The flow within the stagnant region is
highly turbulent and conceptually perceived as circulating eddies. The
outer boundary of the eddy or cavity region extends from the point of
separation to reattachment downwind, as shown in Figure 1. The wake is
defined as the entire region of the flow field that is. disturbed by the
obstacle. The upper boundary of the wake is called the "envelope", as
shown in Figure 1. The reattachment point is taken as the ground-level
position where the flow is no longer drawn back towards the backside of
the building. Downwind, beyond the reattachment, the flow readjusts
itself to a boundary layer appropriate to local surface roughness. For
sharp-edged obstacles the flow distinctly separates at the leading
edges. For rounded obstacles the point of separation can vary greatly.
The disrupted flow in the immediate vicinity of either building struc-
tures or terrain obstacles can both enhance the vertical dispersion of
emissions from the source and reduce the effective height of emissions
from the source. For elevated sources these aerodynamic effects tend to
cause an increase in the maximum ground-level concentrations.
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Additional discussions of the aerodynamically induced disrup-
tion around obstacles can be found, for example, in Cermak (1976),
Halitsky (1968), Scorer (1968) and Batchelor (1967). A review of the
literature clearly indicates that the aerodynamic influences and the
extent of the wake are highly dependent on the particular shape and
design of the obstruction. The extent of the wake also depends on the
characteristics of the approaching atmospheric flow. Presently, theo-
retical and quantitative understanding of the extent of obstacle in-
fluences are limited. Further examinations of the extent of influence
for a wide range of structures and terrain obstacles are needed.
UNDISTURBED REGION
REATTAGHMENT
Figure 1. Diagrammatic sketch of envelope and cavity regions behind
a building (SIDE VIEW).
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2.2 Building Effects
The scientific literature in general indicates that a case
specific review f$ integral to assuring the prevention of adverse
aerodynamic effects in the immediate vicinity of a given source. How-
ever, the literature also identifies generalized formulations which are
designed to establish minimum stack heights to prevent this phenomenon.
One such formulation is the "2.5 times rule," which specifies that
stacks designed to discharge their effluent at least 2.5 times the
height of the highest nearby structures would escape building influ-
ences. This rule apparently arose during the early part of this century
as a practical formula. Hawkins and Nonhebel (1955) report that the
rule had been successfully used by the British electricity generating
industry during the previous 20 years. A British government report
CBeaver, 1956} which summarizes the informed opinion at that time,
presents the 2.5 times rule as successfully used in practice. According
to Sutton 0960) the rule was probably originally deduced by Sir David
Brunt from W. R. Morgan's study of the height of disturbances over a
ridge in connection with an investigation into the disaster of an
airship.
No matter what the origins of the rule may be, it can be
called a reasonable working rule that is extensively referenced and
generally supported by scientific literature. In some instances where
application of the 2.5 times rule was considered impracticable, in-
dividual evaluations of the specific case have been made.. Most of these
studies were conducted as scale model studies in a wind tunnel where the
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design parameters could be easily adjusted to determine the necessary
stack placement and height. Unfortunately, field studies have been
limited to a few case-specific problems. The following are among the
most significant findings from studies of building wake effects.
Evans (1957) estimated the smoke visualized shape and size of
the cavity region for nearly two hundred variations of basic building
shapes in a wind-tunnel study. He found that regardless of the height
of the building the pattern of the air going over the top of the build-
ings appeared the same. Examination of the published sketches shows the
cavity to extend from the ground vertically to about 1.5 times the
height of the building. In the case of pitched roofs the height scale
should be taken as the height of its apex. When the width of the
building was increased from 1 to 8 times its height, the depth of the
downwind cavity increased from 2 to 5 times the building height. As the
width of the building was further increased to 28 times its height, the
downwind extent was found to increase at a somewhat smaller rate to 9
times the building height.
Wind-tunnel tests defining the influence of block-type struc-
tures on smoke emissions from roof-mounted chimneys were conducted by
Lord et al. (1964). An examination of their results shows that the
freight of the cavity is nearly equal to the building height plus
one-half times the building height or width, whichever is less. How-
ever, the maximum vertical extent of the disturbed flow above the cavity
was found to be equal to the building height plus up to 3 times the
building height or width, whichever is less.
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Halitsky (1968) reviewed several wind-tunnel studies of flow
near structures. One of the studies (Halitsky, e£ aj_._, 1963) demon-
strated that the wake tn the lee of a rounded building is not as great
as that found in a study of sharp-edged buildings (Halitsky, 1963).
Meroney and Yang (1971) found that for a stack less than 1.5 times the
building height the plume was downwashed into the lee side of the
building. When the stack height was increased to 2.0 times the height
of the burlding the influences were greatly diminished.
A formulation that prescribes the stack height sufficient to
avoid significant building influences has been presented by Lucas (1972)
and Briggs 0973). They state that a stack should equal the height of
the building plus 1.5 times the height or width, whichever is less.
Snyder and Lawson [1976) in a series of wind-tunnel tests showed that
this formulation is adequate for a stack close to a building whose
height is three times its width, and for a building whose width is twice
its height.
Robins and Castro (1977) examined the wind-tunnel flow field
in the vicinity of a model cube. They found the cavity region to extend
up to 2 times the building height downwind. The downwind zone, where
the flow was significantly affected, extended, however, to 5 building
heights. The effluent from a stack 2.5 times the building height having
a stack exit velocity 3 times the wind speed, was found to be insigni-
ficantly affected by the building.
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Huber and Snyder (1976) evaluated a series of wind-tunnel
studies designed to examine building wake effects near a building whose
wtdtfi was twice its height. The size of the cavity was found to be
approximately 1.5 building heights above ground level in the vertical
and 2.5 building heights downwind. In evaluating the building influence
on dispersion, aerodynamically generated turbulent flow was found to
rapidly decay in the region 3 to 10 building heights downwind. The most
significant disturbed flow occurred within 5 building heights downwind.
A significant building influence on ground-level concentrations was
found for cases with the stack less than 2 times the building height.
The building influences were found to be significantly reduced for a
stack 2.5 times the height of the building.
In the vicinity of building structures where mechanically
generated turbulence dominates the undisturbed atmospheric flow, wind-
tunnel modeling has been found to be very reliable. However, near the
outer boundaries of the wake, differences can be significant. In the
above early studies of Evans (1957) and Lord et al. (1964) no attempt
was made to simulate an atmospheric-like boundary layer. However, more
recent studies, such as those described in Huber and Snyder (1976) and
Robins and Castro 0977), use methods similar to those suggested by
Counihan (1969) for producing a simulated atmospheric boundary layer.
Thus, preference should be given to the results in the most recent
studies.
A review and evaluation of the current literature as reflected
tn the annotated bibliography reveals a consensus that the height of the
cavity downwind of structures extends to the height of the structure plus
10
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0.5 times the height or width, whichever dimension is less. However,
significant influences on plume behavior are found to extend farther.
The well established 2.5 times rule is found to be the consensus opinion
as the stack height necessary to avoid significant effects for buildings
whose projected width is greater than its height, although individual
studies show some deviation. For tall buildings, where the width is
less than the height, the stack height need only be equal to the height
of the building plus 1.5 times its width. Thus, the good engineering
practice stack height has been determined to be equal to the height of
the structure plus 1.5 times the height or width, whichever is less.
This determination is most applicable to sharp-edged structures. The
effects of rounded structures are likely not as great as those for
sharp-edged structures, although there is very little information
available.
The downwind extent of the highly turbulent region where there
are significant effects is unfortunately not as well defined. Based on
the current literature, it is recommended that, for the purposes of
determining GEP stack height, the downwind extent of the wake be taken
as 5 times the height or width of the structure, whichever is less,
i.e., 5L from Equation (!)• This choice is most applicable to struc-
tures whose width is less than ten times its height. In situations
where the structure is wider than ten times its height there may be
significant adverse effects extending farther downwind. Evaluation of
the wind-tunnel results of Evans (1957) indicates that for extremely
wide buildings the maximum extent of adverse effects likely do not
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extend beyond ten times their height. In the wind-tunnel studies and
literature review reported by Huber, et al. (1976) on flow over two-
dimensional obstacles, a maximum extent of ten times the obstacle height
was found, except in the case of very thin obstacles where the extent
can be much greater. Again, very little information was found for
rounded structures which are unlikely to have as great a downwind
influence as do sharp-edged structures. It is not anticipated that
structure influences will exist beyond 800 m.
2.3 Quantitative Rationale for GEP Equation and Excessive
Concentration
Little of the literature on building effects presented above
and in the annotated bibliography contains specific data that can be
used in evaluating building influences. Design stack height near
buildings has been based mostly on theory and experience with minimal
supporting data. Also, some of the data available cannot be used
because no measurements of concentrations in the absence of buildings
were taken for comparison. Specific data available from the literature
concerning cavity and wake height are summarized in Figures 2 and 3.
In Figure 2 the cavity height (hc) is found to be well repre-
sented by
h = H + 0.5L (2)
c
The scatter of data appears evenly distributed about the line with slope
equal to 1.0. The three sets of data included in Figure 2, were taken
from wind-tunnel studies where smoke was used to visualize the region
where flow was, circulating.
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2.6
2.4
2.2
2.0
I 1.8
cc
Lk
a
LU
H-
I 1.6
tn
1.4
O LORD, ETAL (1964)
QHUBER&SNYDER(1976)
EVANS (1957)
CAVITY HEIGHT ESTIMATED FOR
90 SLOG. TYPES. AVG = 1 .5 -
STO DEV = 0.36
hc = H-c0.5L(EQ2) Q
WHERE: hc = CAVITY HEIGHT
H =• BLDG.HEIGHT _
L = LESSER DIMENSION
(BLDG.HEIGHT OR WIDTH)
1.0
0.3
: <9
0.8 1.0 1.2 1.4 1.5 1.6
he/H (EQUATION 2)
Figure 2. Cavity height estimate.
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Figure 3 presents data from studies defining the necessary
stack height to avoid significant wake effects. The wake height (hw)
estimate has been used above to define GEP stack height as formulated by
h. = H + 1.5L (3)
w
The data from Meroney and Yang 0971) and Lord e_t al. (1964) came from
observations of the plume centerline visualized through smoke. The wake
height estimate was defined as the minimum plume centerline height found
to be unaffected by the building. The other data are from an exam-
ination of vertical concentration profiles. For these data, the wake
heights were defined at the plume centerline height where profiles both
with and without the building were judged to be essentially the same.
One must be very careful in interpreting the data in Figure 3. The
visualized studies can be strongly biased by the observer's eye and are
extremely sensitive to the density of the smoke. The information from
concentration profiles is influenced strongly by where the traverse
through the plume is made and the judgment in determining what con-
stitutes a significant concentration difference. In all these studies a
hiaher stack would have been required if the objective was to determine
the height at which there was no building wake effect on emissions.
The data presented in Figure 3 show Equation 3 to estimate the
lower bound of measurements. Although the consensus opinion in the
scientific literature strongly supports using Equation 3 to determine
GEP stack height, actual studies can show the need for a much taller
stack depending on one's interpretation of what is a significant in-
fluence. To more precisely define that height for a specific stack,
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4.6
4.4
4.2
4.0
3.8
3.6
3.4
3.2
3.0
i 2.8
as
ik
2 2.6
H~
2-2
2.0
1.8
1-6
1.4
1.2
1.0
0
a UKEGUCHI.ETAL. (1967)
A HUBER&SNYDERM976)
O SNYOER & LAWSQN (1976)
• MERONEY& YANG (1971)
• JENSEN & FRANK (1965)
A SHERLOCK & STALKER (1940)
O LORD, ETAL. (1964)
hw = Hf 1.5L(EQ3)
WHERE: HW = WAKE HEIGHT
H =BLDG. HEIGHT
L = LESSER DIMENSION
(BLOG.HE1GHT OR WIDTH)
8-
8"
•o-
1.1 1.3 1.5 1.7 1.9 2.1
hw/H (EQUATION 3)
2.5
Figure 3. Wake height estimate.
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ground-level measurement both in the wake of and in absence of the
building are needed to assess the increase in maximum concentrations.
The ground-level measurements must be sufficient to determine the
location of the maximum concentration which may occur at a different
position in the wake of the building than found in absence of the
building. The increase in maximum concentration is simply the diff-
erence between the maximum concentration found in the wake of the
butldfng and that found in absence of the building. This concentration
increase can be assessed to determine whether the increase constitutes
an excessive concentration. In practice successive runs varying the
physical stack height would be conducted until the concentration in-
crease due to the building influence is less than the excessive con-
centration criteria.
Only three data sets having ground-level measurements that
included increased maximum concentrations were found which can be used
to determine excessive concentration. They are presented in Figure 4.
A theoretical estimate (Britter, et al., 1966) of increased maximum is
also presented. The theoretical estimate assumes the building is very
much wider than it is high, and should be considered as providing an
upper estimate. For all three data sets the plume rise was very small
and thus plume centerline height is nearly equal to stack height. The
two sets of data where the building width is twice the building height
and the data for the square building placed at a 45° diagonal to the
ijrind are very similar. One should expect some differences among build-
rng types. Ground-level maximum concentrations associated with a stack
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OBRITTER, ETAL. (1976)
THEORY: W»H
DHUBER&SNYDER (1976):
WIND TUNNEL RECT. SLOG
AUKEGUCHI ETAL. (1967)
WIND TUNNEL RECT. SLOG
• ROBINS 3. CASTRO (1977)
WIND TUNNEL SQUARE BLDG.
SQUARE BLDG.
STACK HEIGHT
8LDG. HEIGHT
BLDG. WIDTH
Figure 4. Increased maximum (excessive) ground-level concentrations
in building wake .
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2.5 times the building height, which is GEP for these cases, is found to
be increased by roughly 20-40% by the building wake. It can thus be
concluded for similar situations that an increase in maximum concentra-
tions less than 20% is less than expected for a GEP stack height while
an increase in maximum concentrations greater than 40% is excessive.
2.4 Terrain Influences
Terrain obstacles are generally much larger than most struc-
tures. Atmospheric phenomena on these scales can have a great influence
on the development of aerodynamic forces, beyond those found in the wake
of low-lying structures. Very few definitive evaluations of the
extent of significant adverse effects in the wake of terrain obstacles
are found in current literature.
The review of published field studies presented by
Hufaer et al. (.1976) strongly supports the assertion that, on the leeward
side of a mountain ridge, a circulating eddy with strong downwash and
dispersion characteristics can exist. Many of these studies are con-
tained in the annotated bibliography. However, information that could
define the point where the flow separates and the size and extent of the
cavity was not found. The point of separation appears to be a function
of mean flow speed and direction, atmospheric stability, downslope and
upslope angle of the ridge sides, and the location of the ridge with
respect to surrounding terrain.
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For a particular situation, the greatest cavity occurs when
flow separation occurs at the ridge apex. Both field studies and fluid
modeling results confirm a natural expectation that the more obtrusive
the ridge, the larger the cavity region. Obstructions with salient
features should exhibit definite separation at their edges under all
atmospheric conditions. The size of the cavity region is greatest for
isolated ridges with steep sloping sides. Stable atmospheric conditions
act to restrict the size and extent of the cavity region. Under highly
stable flows other phenomena, such as lee waves and rotors, may be found.
Terrain features that most adversely affect flow are two-dimensional in
nature. Lateral air motion around a hill under neutral stability results
in a smaller eddy size than would be observed for a two-dimensional
ridge.
Sporn and Frankenberg (1966) and Frankenberg (1968) recognized
the potential for adverse terrain influences in the late 1960's when
their pioneering experience with tall stacks began. A wind-tunnel study
was conducted for the Clifty Creek plant since preliminary evaluations
indicated that there would be unusual difficulties from an aerodynamic
standpoint. An abrupt rise of the .terrain to a plateau approximately
100 m above plant level was found in the prevailing downwind direction.
The authors indicate that the results of the wind-tunnel study showed
that stacks with a gas exit velocity of 36 m/s and a height twice the
plateau height (200 m) would be adequate to insure that the plume would
not intercept the boundary layer flow along the hillside and be immedi-
ately brought to the ground. The Kyger Creek plant presented no special
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terrain problems so the stack height was determined from diffusion
calculations only. The results of the analyses at Clifty Creek and
Kyger Creek were used as a guide in determining the necessary stack
design for newer facilities. For example, the stacks at the Cardinal
Power Plant were constructed 251.8 m high; this makes them about 1.5
times the height of the surrounding terrain, Frankenberg et al. (1970).
Williams and Dowd (1969) report that wind-tunnel studies of
gaseous diffusion have been used in many cases to help determine stack
heights. It has been observed, however, that for scaling ratios larger
than 600:1, consistent and repeatable results become difficult to
obtain.
A recent study, "Plume Dispersion in Complex Terrain," by Johnson
and Mage (1978) was found to provide some specific cases applicable to
assessment of potential terrain effects for two AEP power plants.
The stack of the Mitchell Power Plant is more than 2.5 times higher than
the maximum terrain features in the vicinity of the plant, while the
stacks at the Kammer power plant are nearly equal to the elevation of
the surrounding terrain. The horizontal spread of the plume from the
stacks of the Kammer power plant were found on the average to be twice
as large as the spread found for the Mitchell power plant.
Because of the complex air flow over terrain and the general
uniqueness of each situation, no simple definition of GEP stack height
is possible as has been recommended for building and other structures.
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Until further studies better define the extent of the region where
significant terrain influences can affect nearby (within 800 meters)
sources, determination of 6EP stack height in the vicinity of terrain
obstacles should be made on a case-by-case basis.
2.5 Minimum Stack Height
In the case of very low structures or where there is essen-
tially no structure to which a stack is attached, application of the 2.5
times rule may yield answers which have little or no meaning. Isolated
release points may require some physical height for security, safety or
other public health reasons. Excessive ground-level concentrations may
also result from such low level releases, due to adverse meteorological
phenomena in the lowest few tens of meters of the ground. The height
of this layer often called the surface boundary layer, varies not only
with certain meteorological factors but also among the definitions used
by micro-meteorologists such as Sutton (ca. 50 m.}(1953), Busch (30 m or
so] (.1973), and others. In this layer the vertical atmospheric struc-
ture is largely a function of thermal and mechanical turbulence, i.e.,
surface heating by the sun or cooling by terrestrial radiation, and the
surface roughness caused by obstacles to air flow.
To mfnimize the influences of these natural atmospheric
effects, one alternative is to consider that good engineering practice
should not preclude the construction of stacks up to a reasonable height
of 30 meters. This will certainly minimize the deleterious effects of
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stable conditions, allow reasonable dilution to take place in the short
travel time to nearby locations and permit a larger spectrum of atmos-
pheric eddy sizes to act in the dispersion process. It should be noted,
however, that no reasonable stack height will eliminate instantaneously
high concentration peaks associated with looping plumes.
Thus it is recommended that the equation (1), HQ = H + 1.5L,
be applied unless the resulting height is less than 30 meters. If this
is the case, a stack height credit up to 30 meters height could be
allowed.
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3.0 Determination of GEP Stack Height
3.1 Initial Assumptions
GEP stack height is designed to insure that emissions from a
stack do not result in excessive concentrations as a result of aero-
dynamic effects from nearby structures or terrain features. Excessive
ground-level concentrations will not result when: (1) the emission
point is well above the disturbed flow, (2) the effluent rise is suf-
ficiently great to keep a significant part of the effluent plume above
the disturbed flow or (3) the wind direction places the stack outside
the area of disturbed flow. It is assumed that the wind speed and
direction that may result in wakes, eddies or downwash are always
possible for stacks less than GEP height. Plume rise is not considered
in the determination of GEP stack height. Under high wind speeds plume
rise near the source is likely to be negligible. For most sources, even
those with a relatively high exit velocity, a wind speed of 15-20 m/s
will result in significantly reduced plume rise and thus increase the
potential for adverse effects. The critical':conditions for determining
GEP stack height are high winds associated with neutral atmospheric
stability. Such conditions must be considered when demonstrations are
required.
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3.2 Simple Structures
GEP stack height has been defined to be equal to the height of
adjacent or nearby structures plus 1.5 times the structure height or
width, whichever is less. Both the height and width of the structure
are determined from the frontal area of the structure, projected onto a
plane perpendicular to the direction of the wind. If the structure is
asymmetrical, the GEP stack height should be based on the plane pro-
jection lying upwind from the source Cstack) which results in the
greatest justifiable height.
In some situations the projected area may be very irregular,
thus resulting in a multiplicity of scales. Note that structural pro-
tuberances are seldom a significant factor in determining GEP stack
height. The most significant scales are, of course, those that result
in the source being considered within the immediate vicinity of the
structure, and those that then require the highest GEP stack height.
For the purpose of determining GEP stack height, the immediate vicinity,
R, is limited to 5 structure heights or widths, whichever is less,
downwind from the trailing edge of the structure.
Figure 5 illustrates an application to three types of build-
ings. A GEP stack should have an emission point above the shaded
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R3-5H3
TOP VIEW
R2=5W2=0.5H2
H3-
SIDE VIEW
Figure 5. Determination of the immediate vicinity, R, for three types
of structures.
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regions of the vertical cross-section or the stack should be placed
outside the shaded region of the horizontal cross-section to avoid
adverse aerodynamic effects. Note for both the tall, thin structure and
the short, long structure the expected sphere of adverse influence is
less than that found for the moderately tall cubical structure.
A. Low Structures
The immediate vicinity downwind from a uniform low structure
(one whose width all around is greater than its height) is very easy to
determine. It is 5 times the structure height, downwind in all direc-
tions from the trailing edge of the building. The vertical extent of
disturbed flow is 2.5 times the. structure height throughout the entire
vicinity of the structure. Thus GEP stack height is defined as 2.5
times the structure height. This determination for a low structure is
presented in Figure 6 where the sphere of influence is outlined.
Figure 6 also depicts the maximum projected structural width affecting
each of the four given sources. Mote that these projected widths are
only valid for a wind which is perpendicular to the actual or the cross
sectional surfaces. Since the projected width for all directions is
greater than the height, the width scale is not a factor in determining
GEP stack height.
B. Tall Structures
The width scale becomes the significant factor in determining
GEP stack height whenever the structure is taller than it is wide. In
Figure 7, the structure is assumed to be tall and thin (one whose width
all around is less than its height). The determination of the struc-
tural width and resulting downwind extent of aerodynamic effects for
26
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TOP VIEW
— I I
i im i i i i i i i 11
W///////////M
' 2.5H - GEP
STACK HEIGHT
._ H
SIDE VIEW
Figure 6. Determination of the immediate vicinity, R, and the maximum
structural width for four stacks placed nearby a low structure.
27
-------
O.SH-
1.75H-GEP STACK HEIGHT
Figure 7. Determination of the structural width and downwind extent of
the immediate vicinity for four stacks placed nearby a tall,
th.in structure.
28
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each of four sources is given in Figure 7 for the specified directions
of the wind. The immediate vicinity, R, is 5 times the projected width,
downwind from the trailing edge of the structure. Note that the extent
is highly dependent on the wind direction. The GEP stack height for a
tall structure is determined to be equal to the structure height plus
1.5 times the projected structure width. GEP for the situation of
source 3 in Figure 7 is equal to 1.75 times the height of the structure.
Since the projected width of the structure is dependent on the wind
direction, all directions of wind projecting downwind towards the source
need to be assessed. The maximum allowable GEP for sources nearby a
tall structure is then equal to the structure height plus 1.5 times the
maximum projected structure width.
3.3 Complex Structures
A. Tiered Structures
Figure 8 presents a more complex, tiered structure. Tier 1 by
itself has an immediate vicinity, R, extending downwind for five heights.
The addition of tier 2 which is equal in height to tier 1 causes both
the vertical and downwind extent of the region of significant influence
to double since the height scale is the overall height which still is
less than the width. The projected area downwind of tier 3 which is
placed above tier 2 has a height 4 times greater than its width, as can
be seen from examination of Figure 8. However, the downwind region of
influence extends downwind less than the influence of tier 2. Should a
29
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GEP3=1.4H3._
GEP2=Z.5H2- -
-H3
/ t / i i/i r/
-HI
SIDE VIEW
Figure 8. Variation in the determination of the immediate vicinity
for additions to a tiered structure.
30
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source be located directly downwind of tier 3, although out of its
influence, GEP is then based on the influence of tier 2. Note that the
influence of tier 2 totally engulfs the influence of tier 1.
For the situation presented in Figure 8, GEP stack height is
equal to GE?3 0-4H3) for all sources downwind of tier 3 and placed
within R-. GEP for sources farther downwind but not beyond R2 is equal
to GE?2 (2.51^). For sources outside of the projected width of tier 2,
however within the projected width of tier 1 and downwind distance R-,.
the GEP stack height is equal to GEP-| (Z.SH^. Other orientations of
the building to the wind can result In different determinations of GEP
stack height where the projected width is less than its height. For the
building design in Figure 8 only the influences of tier 3 change the GEP
determination since only its projected width is less than its height.
The influence of additional tiers has been assumed to be
complementary. Very little information relative to such situations is
found in the present scientific literature. The influence of tiers may
not be exactly complementary since additional tall tiers similar to tier
3 in the above example, may result in some streamlining of the flow
around the lower tiers and thus some reduction in their effects. Since
such effects are likely minimal, it is recommended that, until further
evaluations are reported, the effects of tiers should be considered
totally additive as presented here.
B. Tapered Structures
A tapered structure is presented in Figure 9. This situation
differs from the tiered structure only in that there is now a continuous
31
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array of widths and heights to consider. The four selected heights and
widths are representative. Above the H£ level of the structure the
width is less than its height. Since the width decreases further with
increasing height both the determination of GEP stack height and the
immediate vicinity which are now functions of the width have little
effect on the determination of GEP stack height. Figure 9 shows that
the overall determination of the immediate vicinity is set by the H?
level and the overall GEP stack height set by the H3 level for sources
within R3 are only slightly greater than that set by the H2 level. The
overall GEP stack height is the outermost boundary of the outlined
regions presented in Figure 9. The region near the top is not a sig-
nificant factor in determining GEP stack height in this situation, since
the width is negligible.
Figure 10 presents a cooling tower structure. In this appli-
cation use of the width at the top of the cooling tower was found to
result in the outermost boundary of influence and thus the overall
determination of GEP stack height. The immediate vicinity is therefore
given in Figure 10 as 5 times the width at the top of the tower structure
and the GEP stack height is given as the height plus 1.5 times the width
at the top.
As is the case for tiered structures, very little information
relative to such situations are found in the present scientific litera-
ture. In addition, as pointed out in the literature review presented in
Section 2.2, the influences of rounded obstacles may not be as great as
that found for sharp-edged buildings as a result of variation of the
32
-------
CO
co
W/2 = 2H2
W3 = 0.5H3
Wa = O.I4M4
SIDE VIEW
Figure 9. Determination of the immediate vicinity for a tapered structure.
-------
co
3H
WIND DIRECTION
l>
TOP VIEW
1.9HGEP STACK HEIGHT
•iiui/inniiiiiiiiuniii run ///////////////////////////////
I
SIDE VIEW
Figure 10. Determination of the immediate vicinity near a cooling tower structure.
-------
flow separation line. The affect of rounded edges and widths also
results in some streamlining as discussed for the tiered structures.
The above determination is recommended until further evaluations are
reported in the scientific literature.
3.4 Terrain Obstacles
GEP stack height for new major sources and proposed stack
height increases for existing sources to minimize the adverse impact of
terrain obstacles should be determined on a case-by-case basis. Field
studies designed to evaluate the specific situation under the variety of
adverse meteorological conditions are most preferred. Where field
studies are not practicable, comparable fluid model (wind tunnel, water
channel, etc.) studies or mathematical analyses that are developed are
acceptable.
3.5 Framework for Demonstrating GEP Stack Height
As presented above, case-by-case demonstrations should be used
for GEP determinations for terrain obstacles. Also a demonstration may
be used to justify a stack height taller than the GEP definition
[Equation 1) when air quality violations are a result of excessive
concentrations due to the influence of nearby structures. Where field
studies are not practicable, comparable fluid model (wind tunnel, water
channel, etc.) studies or mathematical analyses that are developed are
acceptable. In field studies and fluid model or mathematical simu-
lation, some quantitative evaluation of the obstacle's influence is a
necessary part of demonstrating GEP stack height. Quantitative anlayses
are necessary since GEP stack height is to be based on
35
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"the height necessary to insure that emissions from the stack do not
result in excessive concentrations of any air pollutant in the immediate
vicinity of the source."
Comparable fluid model studies require certain similarity
criteria to be considered. Discussion of similarity criteria can be
found, for example, in Snyder (1972), Sundaram, e_t al., (1971), Cermak
(1970), and Halitsky (1968). Comparable mathematical analyses that are
developed must also satisfy physical laws and be well supported by field
or fluid model data. Nondimensional parameters that characterize the
flow in the atmosphere must be matched in the model medium. This con-
sideration is necessary to' insure that the flow in the medium accurately
simulates that in the atmosphere.
Organizations that desire to provide GEP demonstrations should
present a description of the elements of their studies. This should
include a complete description of the facility and how necessary simi-
larity criteria are met. A well-developed simulated atmospheric bound-
ary layer is required. In most cases only consideration of a neutrally
stable atmospheric condition is necessary since GEP determination is
based on the physical stack height assuming high wind speeds which
result in negligible plume rise. Full development of aerodynamically
induced disruptions is also required. A reliable system for quantita-
tively measuring both ground-level concentrations and vertical and
horizontal concentration profiles across the stack plume is necessary.
36
-------
Concentrations must be measured both in the wake of the obstacle and
with the obstacle removed in order to demonstrate resulting excessive
concentrations.
Modeling simulations rely on the continuing development and
refinement of state-of-the-art techniques. The specific criteria and
procedures for an adequate modeling 6EP determination are being con-
solidated into a separate guidance document which will be published by
EPA..
37
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4.0 Air Quality Estimates
When any dispersion model is used for determining an emission
limitation, it is the intent of the stack height regulation that the
stack height specified for use with dispersion models be no greater than
the GEP height. The GEP stack height based on the physical configura-
tion of the source and any nearby structure should be determined by the
procedures in the preceding section.
For many sources, the GEP stack height may be lower than the
existing stack height, resulting in higher estimates of air quality
impact. Therefore, for certain sources, this guidance will dictate more
stringent emission limitations than currently required by SIPs. Even
though a somewhat less conservative approach might have been suggested,
the guidance presented here seems most consistent with the proposed
stack heights regulation and with the intent of Section 123 of the 1977
Clean Air Act Amendments.
In some cases, a greater GEP height may be justified, based on
nearby terrain effects. Specifically, a GEP stack height based on
terrain features within one-half mile (800 meters) of the source may be
used for determining an emission limitation if adequate justification is
demonstrated through a field study or fluid model study. When the above
38
-------
procedures are inapplicable or yield a 6EP height less than 30 meters,
the GEP height should be specified as 30 meters for model input.
In the event that the actual stack height is less than the GEP
height, the stack height specified for use with the dispersion model
should be limited to the actual height. In that case, it is possible
that excessive pollutant concentrations may occur in the immediate
vicinity of the source due to atmospheric downwash, eddies and wakes
created by the source itself or nearby structures or terrain features.
Such adverse effects should be accounted for when estimating the air
quality impact of a source. Guidance concerning such effects has been
provided in several reports (Huber and Snyder, 1976; Huber, 1977; and
Budney, 1977).
For GEP stack heights, on the other hand, such adverse at-
mospheric effects are avoided and specific modeling techniques can be
recommended for estimating the air quality impact of a source. A simple
screening analysis may first be conducted to eliminate from further
consideration those new sources that clearly will not cause an air
quality problem. Screening procedures (Budney, 1977). provide a con-
servative estimate of maximum concentrations; i.e., a substantial margin
of safety is incorporated to insure that maximum concentrations will not
be underestimated. If a more refined analysis is necessary for a new
source or if a SIP revision is being considered for an existing source,
the analysis should be consistent with techniques recommended in the
Guideline on Air Quality Models CEPA, 1978). The guideline makes
specific recommendations concerning air quality models, data bases and
general requirements for concentration estimates.
39
-------
Case Examples
There are three basic situations in which modeling analyses will be
applied to emissions from GEP stacks.
Case 1: Terrain less than GEP height.
Recommendation: Apply a "flat-terrain" dispersion modeling
technique, as recommended in Section 4.3 of the Guideline on
Air Quality Models. Use the GEP stack height as the specified
physical stack height input to the model.
Example:
ACTUAL.
HEKSHT
WIND
PLUME CENTERLINE
FOR MODELING
ASSESSMENT
GEP HEIGHT
40
-------
Case 2: Terrain greater than GEP height.
Recommendation: As discussed earlier, a GEP stack is theo-
retically high enough to avoid downwash, eddies and wakes
caused by nearby elevated terrain. However, even though the
stack is tall enough, or the source is located so as to avoid
adverse aerodynamic effects, there is still the possibility of
plume interaction with elevated terrain features further
downwind. High concentrations may occur on the downwind
elevated terrain due to the effluent plume coming close to or
impacting on it. If the GEP stack height were to theoret-
ically result in plume impingement, then allowable emissions
should be calculated as if impingement occurred. Thus, flat-
terrain modeling techniques will not suffice. Some techniques
for estimating ambient concentrations on elevated terrain have
been identified (.Burt, 1977 and Egan, 1975) and should be
considered as discussed in Section 4.3 of the Guideline on Air
Quality Models.
Example:
ACTUAL.
HEIGHT
GEP HEIGHT
WIND
PLUME CENTERLJfVE
FOR MODELING
ASSESSMENT
-------
Case 3: Multiple Source Impacts
Recommendation: Many situations are anticipated in which
there will be significant contributions to ambient concen-
trations due to sources other than the one in question. In
such cases, first estimate the air quality impact of the
source in question, as discussed for Cases 1 and 2. Then
superimpose the air quality impact of the other sources to
estimate the total air quality impact. For the calculation of
contributions from other sources, GEP-based emission rates
should be used in conjunction with GEP stack heights as input
to the modeling assessment. The emission limitation for the
source in question generally should be determined such that
the National Ambient Air Quality Standard and allowable
concentration increments will be met even after natural
background and the additive impact of other sources are con-
sidered. Guidance is available for estimating contributions
from other sources (Budney, 1977 and EPA, 1978).
-------
REFERENCES
Satchel or, G. K., 1967: An Introduction to Fluid Dynamics. Cambridge
University Press, Cambridge, (Great Britain), 325-331.
Beaver, S. H. (Chairman), 1954: Report of Government Committee on Air
Pollution. Her Majesty's Stationary Office, Cmd. 9322, November.
Briggs, G. A., 1973: Diffusion Estimation for Small Emissions. Atmos-
pheric Turbulence and Diffusion Laboratory, NOAA, Oak Ridge, TN,
(Draft) ATDL No. 75/15.
Budney, L. J., 1977: Procedures for Evaluating Air Quality Impact of
New Stationary Sources. Guidelines for Air Quality Maintenance
Planning and Analysis: Volume 10 (EPA-450/4-77-001, QAQPS Guide-
line Number 1.2-029R), Environmental Protection Agency, Research
Triangle Park, NC, October.
Burt, E., 1977: Valley Model User's Guide. (EPA-450/2-77-018), Environ-
mental Protection Agency, Research Triangle Park, NC, September.
Busch, N. E., 1973: On the Mechanics of Atmospheric Turbulence. In:
Workshop on Micrometeorology, by American Meteorological Society,
Science Press, Ephrata, PA, Chapter 1.
Cermak, J. E., 1970: Laboratory Simulation of the Atmospheric Boundary
Layer. American Institute of Aeronautics and Astronautics, 3 rd
Fluid and Plasma Dynamics Conference, Los Angeles, CA, June 29-
July 1, No. 70-751.
Cermak, J. E., 1976: Aerodynamics of Buildings. In: Annual Review of
Fluid Mechanics. Vol. 8, Van Dyke, M and W. G. Vincenti (Co. Ed.),
Annual Reviews Inc., Palo Alto, CA, 75-106.
Counihan, J., 1969: An Improved Method of Simulating an Atmospheric
Boundary Layer in a Wind Tunnel. Atmospheric Environment, 3_, 197-214.
Egan, B. A., 1975: Turbulent Dissfusion in Complex Terrain. Lectures on
Air Pollution and Environmental Impact Analysis, American Meteo-
rological Society, Boston, MA.
Environmental Protection Agency, 1978: Guideline on Air Quality
Models, CEPA-450/2-78-027) Research Triangle Park, NC, April.
Evans, B, H., 1957: Natural Air flow Around Buildings, Research Report
NoT 59, Texas Engineering Experiment Station, Texas A&M College System.
43
-------
Frankenberg, T. T., 1968: High Stacks for the Diffusion of Sulfur Dioxide
and Other Gases Emitted by Electric Power Plants, Am. Ind. Hyd. Assoc.
J_., 29_, 181-185.
Frankenberg, T. T., I. Singer, and M. E. Smith, 1970: Sulfur Dioxide in
the Vicinity of the Cardinal Plant of the American Electric Power
System. Proc. 2nd Int. Clean Air Cong. Washington, DC.
Halitsky, T., 1968: Gas Diffusion Near Buildings. Meteorology and
Atomic Energy - 1968. D. H. Slade (Ed.), Chapter 5-5.
Hawkins, J. E. and G. Nonhebel, 1955: Chimneys and the Dispersal of
Smoke. J. of the Institute of Fuel, 28_, 530-545.
Huber, A. H. and W. H. Snyder, 1976: Building Wake Effects on Short
Stack Effluents. Third Symposium on Atmospheric Turbulence Diffusion
and Air Quality, Raleigh, NC, Oct. 19-22, pp 235-241.
Huber, A. H., W. H. Snyder, R. S. Thompson, and R. E. Lawson, Jr., 1976:
Stack Placement in the Lee of a Mountain Ridge. U.S. Environmental
Protection Agency, EPA-600/4-76-047, Research Triangle Park, NC,
Sept.
Huber, A. H., 1977: Incorporating Building/Terrain Wake Effects on Stack
Effluents. AMS-APCA Joint Conference on Applications on Air Pollu-
tion Meteorology, Salt Lake City, Utah, Nov. 29 - Dec. 2, pp. 353-356.
Johnson, F. G., and D. T. Mage, 1978: Plume Dispersion in Complex
Terrain. Presented at the Annual Meeting of the APCA, Houston,
Texas, paper no. 78-73.10.
Lord, G. R. W. D. Baines, and H. J. Leutheusser, 1964: On the Minimum
Height of Roof-Mounted Chimneys, Results of an Exploratory Wind-Tunnel
Study. Report TP-6409, Technical Publication Series, Dept. of
Mechanical Engineering, University of Toronto.
Lucas, D. H., 1972: Choosing Chimney Heights in the Presence of Buildings.
Proceedings of the International Clean Air Conference, Melbourne,
Australia, May 15-18, pp. 47-52.
Meroney, R. N. and B. T. Yang, 1971: Wind-Tunnel Study on Gaseous Mixing
Due to Various Stack Heights and Injection Rates Above an Isolated
Structure. USAEC Report No. COO-2053-6.
Robins, A. G. and I. P. Castro, 1977: A Wind Tunnel Investigation of
Plume Dispersion in the Vicinity of a Surface Mounted Cube-I. The
Flow Field, II. The Concentration Field. Atmospheric Environment,
V7, 291-311.
Scorer, R. S., 1968: Air Pollution. Pergamon Press, Oxford, England,
pp 107-123.
44
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Snyder, W. H., 1972: Similarity Criteria for the Application of Fluid
Models to the Study of Air Pollution Meteorology. Boundary-Layer
Meteorology. 3_, 113-134.
Snyder, W. H. and R. E. Lawson, Jr., 1976: Determination of a Necessary
Height for a Stack Close to a Building—a Wind Tunnel Study.
Atmospheric Environment, 1J3. 683-691.
Cramer, H. E., and J. F. Bowers, Jr., 1976: West Virginia Power Plant
Evaluation. Prepared for U.S. EPA Region III, Philadelphia, PA, May.
Sundaram, T. R., 6. R. Ludwig, and G. T. Skinner, 1971: Modeling of the
Turbulence Structure of the Atmospheric Surface Layer. American
Institute of Aeronautics and Astronautics, 9th Aerospace Sciences
Meeting, New York, NY, Jan. 25-27, No. 71-136.
Sutton, 0. G., 1953: Micrometeorology, McGraw-Hill, NY.
,Sutton, 0. G., 1960: Discussion before the Institute, in London, 23.d,
May 1960. J. Institute of Fuel. 33_. 495 (comment).
Williams, D. H., Jr., and J. T. Dowd, 1969: Design and Construction
Features of the 1600 MW Mitchell Plant. Combustion, August, 19-23.
45
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ANNOTATED BIBLIOGRAPHY
Sherlock, R. H. and E. A. Stalker, 1940: The Control of Gases in the
Wake of Smokestacks. ASME Journal, June, 455-458.
A wind-tunnel investigation was used to determine whether an addi-
tion to the height of the existing stacks would prevent downflow of
stack gases into the area surrounding the Crawford Station of the
Commonwealth Edison Company, Chicago. An additional study of the
nature and cause of the behavior .of the gas in the wake of smokestacks
is reported. The turbulent region immediately adjacent to the down-
stream surface of the stack was found to cause plume downwash. If the
gases thus brought down come within the influence of the turbulence flow
over the roof of the building, they were then quickly brought to the
ground behind the building.
Zero downwash into the wake of the smokestack was observed when the
stack gas exit velocity was greater than twice the wind velocity.
Downwash was approximately one stack diameter below the top of the stack
when the stack gas exit velocity was only twice the wind velocity. The
model study of Crawford Station demonstrated the need for a stack increase
of 50 feet to prevent downwash from any direction, provided that the gas
velocity is high enough to prevent the first step of downwash. This
additional increase results in the stack being approximately 2.5 times
the highest part of the building structure.
A-l
-------
Davidson, W. F., 1951: Studies of Stack Discharge Under Varying Con-
ditions. Combustion, September, 49-51.
The problem encountered in designing stacks for the new Astoria
Station in New York City is reviewed. Design of the stack to have a
height greater than 2.5 times the height of the power station is stated
as a long time recognized "rule of thumb". However, the author believes
that despite the importance of this factor, except for stacks of limited
height and the number of investigations made, it is still impossible to
give any rules or criteria that can be used with reasonable assurance to
predict the stack performance of a. new station. Thus, carefully planned
wind-tunnel tests seem to be required. In the case of Astoria Station,
increase in stack height was originally limited by nearby airport runways.
A wind-tunnel model was tested to determine the necessary exit gas
velocity to provide a sufficient plume height to minimize adverse
building effects.
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Strom, G. H., 1952: Wind-Tunnel Techniques used to Study Influence of
Building Configuration on Stack Gas Dispersal. Industrial Hygiene
Quarterly. 13, 76-80.
Wind-tunnel experimentation is presented as a research tool that
has yielded answers difficult, if not impossible, to obtain by other
means. Stack gas dispersal in the presence of buildings and other
nearby structures is given as the most frequently investigated problem
in the wind tunnel. Wind-tunnel modeling is suggested when use of
empirical rules for stack height such as requiring a stack to be 2.5
times the building may lead to unnecessarily high and costly structures,
Discussion of wind-tunnel modeling methods and criteria then follow.
-------
Beaver, S. H. (Chairman), 1954: Report of Government Committee on Air
Pollution. Her Majesty's Stationery Office, Cmd. 9322, November.
A committee was appointed in July, 1953, with the following terms
of reference:
"to examine the nature, courses and effects of air pollution
and the efficacy of present preventive measures; to consider
what further preventive measures are practicable; and to make
recommendati ons."
Discussion of desirable stack height is taken from Appendix VI.
APPENDIX vr
THE INFLUENCE OF CHIMNEY DESIGN AND HEIGHT ON THE
DISPERSION OF FLUE GASES FROM INDUSTRIAL CHIMNEYS
Memorandum by the Industrial Suh-Cotnmince
Introduction
The original function of high chimneys was to create draught for the furnaces.
With the introduction of mechanically created draught early in the century,
many factories were equipped with only short chimneys and as a consequence
smoke dispersal was not good. More recently, however, there has been a trend
towards use of high chimneys in order to improve dispersion by discharge into
the higher levels of the air.
We have found that the information on chimney design and height and the
effect of chimney height on probable conditions on the ground to the lee of
the chimney is widely scattered and in general inaccessible to industrial engineers.
We have therefore felt it necessary to go into the subject in some detail in this
appendix. The following is a summary of the best informed opinion at present.
but further investigation may cause these opinions to be revised.
1. Down-draueht
When a wind blows across a building or a hill a down-draught is created on
the Ice side. (1) It is important that chimneys should discharge their smoke high
enough for it to escape these down-draughts if possible.
A rule used successfully for about 20 years by the Electricity Industry is that
the height of a chimney shall be at least 2J times the height of the highest
adjacent building. When the chimney is sited in hilly country or among buildings
which make it impracticable to apply the "2^ times" rule, wind tunnel tests on
models may be necessary to determine where to site the chimney and how high
to make it to avoid down-draughts. Pending further research on the subject.
a good working rule for low buildings is to make the chimney not less than
120 feet high—(hough discretion must of course be exercised for small
installations.
1 Down-wash
Down-wash is the drawing downward of chimney smoke by the system of
stationary vortices or eddies that form in the lee of a chimney when a wind
is blowing. If the velocity of emission of the smoke is not great enough to
overcome down-wash some of the smoke will be drawn by these eddies down
into the down-draughts of the buildings beneath.
The down-draught will then carry the smoke to the ground. Experiments
have shown that down-wash will not occur if the velocity of emission is sufficiently
high. It is clear to us that further research on the design of chimney mouths is
required.
Reference (2) gives a graph showing for a given wind speed the minimum
velocity of emission for avoiding down-wash.
3. Chimney heicht and dispersal of smoke and gases
At whatever height smoke is discharged, graviu will eventually bring the
larger particles of dust and soot to the ground. Morcuvcr. because of the natural
turbulence and mixing of the atmosphere, a proportion of the liner particles
and gases in the smoke wiil reach the ground, although their motion is unalTected
by gravity. The higher the point of discharge the greater wiil be the dilution of
the gases and dust by the time .they reach the ground.
-------
Corby, G. A., 1954: Airflow Over Mountains: A Review of Current
Literature. Quart. J. Roy. Met. Soc., 80., 491.
The work of J. Forchtgott, who gathered about 35 different sets of
observations involving five different mountain ridges located in Bohemia
is reviewed. Mountain airflow is classified into four main types: (1)
undisburbed streaming, (2) standing eddy streaming, (3) wave streaming,
and (4) rotor streaming. The case of standing eddy streaming corres-
ponded to the situation of boundary layer separation at the ridge apex
with cavity formation in the lee. This type of flow is reported to have
been observed frequently. Forchtgott implied that this situation was
predominant under moderate wind speed and wind shear conditions. Even
for the cases with smooth waves above, some form of turbulent wake was
found in the lee of the ridge. No discussion of the extent of the
region of modified airflow is presented.
-------
Hawkins, J. E. and G. Nonhebel, 1955: Chimneys and the Dispersal of
Smoke. J. of the Institute of Fuel, 28, 530-545.
To avoid parts of a smoke plume being blown rapidly to the ground
by local disturbances of the wind, the authors report that it is neces-
sary to choose minimum heights of chimney and exit velocities of flue
gases which are related to the height of surrounding buildings, diameter
at chimney and local ground contour. Disturbances of the atmosphere set
up by the wind flowing past the chimney and over buildings can, under
certain circumstances draw the smoke rapidly to the ground so that the
efficiency of the chimney as a smoke disperser is much impaired. The
region of so called "down-draughts" is stated to stretch from the top of
the windward face of the building, rise to about twice the building
height and stretch for about six times the height downwind of the building.
These dimensions are stated to be approximate and to increase with the
cross-wind width of the building. Also similar effects occur in the lee
of hills.
It is reported that a committee appointed by the Electricity Com-
missioners (Great Britain) proposed the rule that, to discharge flue gas
clear of down-draughts, chimneys should be 2.5 times the height of the
heighest adjacent building. "This rule has been used successfully by
the electricity generating industry during the last 20 years, although
there is some evidence that at high wind speeds cool gas plumes can be
brought down by down-draught even though the chimney height satisfies
the 2.5 times rule." The usefulness of wind-tunnel tests as an indica-
tion of how high the chimney height should be to avoid down-draught, in
-------
difficult cases is stated. For large plants in complicated locations,
advice is given to obtain confirmatory data by observation of the spread
of smoke from smoke generations and observations of the trajectories of
"zero-buoyancy" balloons. It is noted that when a chimney is discharging
into a region of down-draughts and turbulence behind a building, changes
in the velocity of emission or temperature of the flue gas as it emerges
from the chimney will make little or no difference to conditions on the
ground. The work of Sherlock and Stalker (1940) is referenced in determin-
ing the necessary exit velocity to avoid the drawing-down of the smoke
plume by the chimney wake. Also stated is the likelihood that a more
intense wake-region will occur for a square-shaped chimney in comparison
to the circular chimney.
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Scorer, R. S., 1955: Theory of Airflow Over Mountains: IV-Separation
of Flow form the Mountain Surfaces. Quart. J. Roy. Met. Soc., 81, 340-350,
According to the author, the flow separation point is stationary
when there is a salient edge at the top of a hill or ridge. Numerous,
but limited, field studies relating to the zone of recirculation and
instances of intense mixing and general downdrafting in the leeward
regions of ridges are cited. Details are insufficient to draw firm
conclusions relating to formation of separated flows. No specification
of the size of the region modified is given.
-------
Evans, B. H., 1957: Natural Air Flow Around Buildings. Research Report
No. 59, Texas Engineering Experiment Station, Texas A&M College System.
The shape and size of the downwind eddy caused by the model building
was determined in a wind tunnel study for nearly two-hundred variations
of the basic building shape. The downwind eddy was defined as the area
between the building and the point downwind of the building where some
particles of the air close to the ground are found to flow upwind toward
the building. Smoke patterns were used to determine the observed dimen-
sions of the eddy. The shape of the building, the roof type, the posi-
tion of openings, and the orientation with respect to the wind, were all
found to have an effect on the air flow over the building. Several
significant findings are reported. It was found that regardless of the
height of the building the pattern of the air going over the top of tall
building appeared the same. For pitched roofs the depth of the downwind
eddy increased due to the increase in the height of the building. When
the building was extended in the downwind direction the depth of this
downwind eddy decreased. When the width of the building (perpendicular
to the wind direction) was increased from one times its height to eight
times its height, the downwind depth of the eddy increased from 2 to
5.25 times its height. As the width of the building was further increased
to 28 times its height the downwind depth of the eddy increased at a
somewhat smaller rate to 8.75 times its height.
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Scorer, R. S., 1959: The Behavior of Chimney Plumes. Int. J. of Air
Pollution, 1, 198-220.
The 2.5 times rule concerning chimney heights is presented as being
a well-known commendable rule because it is comprehensible as a prac-
tical working rule: it has no precise theoretical justification, and if
experience proved it to be inadequate it could be changed by Act of
Parliament! It is also argued that architects should accept the chimney
heights necessary for the proper dispersal of pollution as a requirement
and design buildings with the chimney as an integral part instead of as
an undesirable appendage. Also in the lee of a cliff there may be
eddies into which, if a chimney is sited in the downdraught of the eddy,
the plume may be carried down to the ground bodily. This is more serious
than being diffused down by ambient turbulence. A case at Hope Cement
Works near Sheffield is discussed. A problem of downdraught was solved
by installing a 150 meter chimney which reaches above the eddies down-
wind of the nearby hill.
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Nonhebel, 6., 1960: Recommendations on Heights for New Industrial
Chimneys. J. Institute of Fuel, 13, 479-495.
A review of the present state of knowledge and experience, and
recommendations are put forward as the basis of discussion between
industrialists and those responsible for the administration of the Clean
Air Act of 1956. This technical review was felt necessary since no
detailed technical advice had so far been issued by any governmental
department to assist those frequently faced with diffulty in deciding
the height of chimney required under the provisions of this Act.
Appendix VI of the Beaver Report (1954) is referenced as providing
guidance on technical considerations governing the height of chimneys.
Where a chimney rises from or is adjacent to a high, large building the
recommended height is stated to be at least 2.5 times the height of the
building. For small plants (reference to very low buildings appears to
be intended) the Beaver Report (1954) makes the recommendation that
chimney heights be not less than 120 feet high. The author goes on to
point out that where there is a choice in the orientation of a large
building to which is attached a chimney, the longitudinal axis should be
at right angles to the prevailing wind. Additionally suggested is that
when the chimney of a large plant is to be built among a group of high
buildings which makes it costly to apply the "2.5 times rule," the only
satisfactory solution is to make tests with models in a wind tunnel to
determine its minimum height and its position with respect to the
buildings. For small installations where the chimney- plume ts. not
-------
expected to be seriously affected by downdraughts exerted by neighboring
building a sliding scale of minimum stack height from 60 feet to 120
feet for plants with steam output up to 33,000 Ib/hr. This minimum
height is suggested to insure adequate dispersal of flue gases and is
based on specified estimates of maximum desirable ground-level concen-
trations.
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Sutton, 0. G., 1960: Discussion before the Institute, in London, 23.d,
May, 1960. J. Institute of Fuel, 33_, 495 (comment).
It is pointed out that the 2.5 times rule be strictly applied only
to a building which is very long across wind, and only near the central
point. Sutton believes the origin of the rule was deduced by Sir David
Brunt from W. R. Morgan's study of the height of disturbances over a
long ridge, in an investigation into the disaster of the airship R. 101;
if a wind were blowing perpendicular to the longside of a building, the
disturbances should extend upwards to about 2.5 times the height of the
roof. Another significant point raised by Sutton was that since it is
impossible to take every factor into account in the mathematics of
atmospheric turbulence, the only thing to do is look at a situation with
the aid of scaled-down models.
-------
Scorer, R. S. and C. F. Barrett, 1962: Gaseous Pollution from Chimneys.
Int. J. of Air and Water Pollution, 6, 49-63.
The wake region of the building is given by a vertical circular
cylinder centered on the building of height 2.5 times the height of the
building and of horizontal radius equal to 3.5 times the width of the
building. For building whose width is less than its heights, the wake
region is of height 2.5 times the maximum width.
Skinner, A. L., 1962: Model Tests on Flow from a Building Ventilation
Stack. Atomic Energy Establishment, Winfrith, Report AEEW-R 227,
November.
Wind tunnel tests were conducted on a model of a building to assess
the minimum requirements for a stack which would effectively disperse
the ventilation air clear of the building wind eddies and also avoid
recirculation into the inlet grille. A stack 2.25 times the average
roof height was found to be just sufficient.
Davidson, B., 1963: Some Turbulence and Wind Variability Observations
in the Lee of Mountain Ridges. J. Appl. Meteor. ,2 (4), 463-472,
The results of a number of ballon releases made in two valleys in
Vermont are reported. Ballon releases were made at several positions
along the sides of ridges that had approximately 20° slopes. Balloon
paths were determined using theodolites. The limited results could not
be used to confirm a point of separation or the extent of a leeward
cavity region. The extreme turbulence generated in the lee of the
ridges, however, appeared to be dissipated at most elevations at a
distance of 4 to 6 heights downwind.
-------
Thomas, F. W., S. B. Carpenter, and F. E. Gartnell, 1963: Stacks—How
High? J. of the Air Pollution Control Assn.. T_3_ (5), 198-204.
TVA experience has demonstrated that when stacks are less than
twice the height of the main powerhouse structure, the plume may, during
high velocity wind, be caught in the turbulent vortex sheath and brought
to the ground level in relatively high concentrations very near the
plant and sometimes re-enter the building air supply. Also, extensive
wind-tunnel tests are stated to have demonstrated that downwash does not
pose a problem where the stack height is at least 2.5 times the height
of the powerhouse or other nearby structures and appropriate efflux
velocities are provided.
-------
Buettner, K. J. K., 1964: Orographic Deformation of Wind Flow. Uni-
versity of Washington, Seattle, Washington. Prepared for U.S. Army
Electronics Research and Development Laboratory, Fort Monmouth, New
Jersey, under Project No. 1AO-11001-B-021-01, Contract No. DA 36-039-SC-
89118. 70p.
The general features of flow over a ridge are treated theoretically
and experimentally. A ridge station was constructed on the lee side of
the Ipsut Pass area of Mount Rainier National Park in Washington as part
of a study of the effect of terrain obstacles on the fallout of particulate
matter through the atmosphere. Tracer particles of zinc sulfide were
released and collected. Data were collected for 5 days during which the
airflow approach was perpendicular to the ridge. During the period of
experimental set-up, only light-moderate winds were observed. The most
common wind field occurrence is reported as a "vortex sheet flow" with
the atrstream separating from the ridge top and forming a wake zone in
the lee of the ridge. For this flow, the wind field was constant above
and zero below a plane representing the wake zone. Only a small amount
of particulate penetrated down through the horizontal vortex sheet. A
j
contaminant released in the calm zone is reported to meander in an unpre-
dictable manner. Previously, a lee eddy with the main airstream moving
first horizontally away from the ridge, then down, and then up again close
to the valley bottom was visually observed. At this site, such a flow
pattern was believed to exist only for strong winds. Laminar flow com-
plicated by thermal winds is reported to occur when stable settled
conditions prevail and the gradient wind at ridge level was less than
6 knots (3.1 meters per second).
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Eimern, J., R. Karschon, L. A. Razumova, and G. W. Robertson, 1964:
Windbreaks and Shelterbreaks. World Meteorological Organization
Technical Note No. 59.
Part of this report summarizes the literature on the influence of
shelterbelts on air flow. The region leeward of shelterbelts is reported
to have reduced winds, and a degree of turbulence and eddying of the
flow in the lee. According to one reference, the air flow is affected
up to even three or four times the height of the belt.
Most of the literature is concerned only with defining the downwind
extent of the region of reduced winds. The literature offers a wide
range of distances. It is reported that according to West European,
North American as well as Russian experiences, the rule of thumb applies
that the shelter zone extends to 30 times the obstruction height.
However, for a wind reduction of 20 percent and more the effect is noted
to only 20 times the height. Moreover, the extent is very dependent on
the permeability, shape and width of the belt, roughness of the ground
surface, thermal stratification of the air. No discussion defining the
vertical extent that shelterbelts can effect stack effluents is given.
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Lord, G. R., W. D. Baines, and H. J. Leutheusser, 1964: On the
Minimum Height of Roof-Mounted Chimneys, Results of an Exploratory
Wind-Tunnel Study. Report TP-6409, Technical Publication Series,
Dept. of Mechanical Engineering, University of Toronto.
Wind-tunnel tests of smoke emission from roof-mounted chimneys
on both block-type and pyramidal structures are described. The tests
were performed in a constant velocity low turbulence wind field. The
wind velocity was equal to the stack emission speed. Four conditions
defining a minimum stack height are given, each corresponding to a
different degree of plume distortion by the structures. For a given
stack location, building configuration, and wind direction, the height
of the stack necessary to meet each of the four conditions is reported.
A discussion of building wake effects is included. The point is
made that even if the source is above the wake, the effluent may later
enter the region of influence. At several building heights downstream
the turbulent region is stated to be about twice the building cross-
section. For the tests, the stack was placed over the center of the
building. The vertical extent of building influences was found to
scale with the building width for tests where the building height is
greater than its width. The height above the building of the stack
at which the smoke plume just began to be distorted due to the influence
of the building was 3 times the building width. The height above the
building of the stack at which smoke began to be entrained to the stag-
nant wake of the building was 0.5 times the building width. For the
tests when the building width was greater than the building height, the
vertical extent of the building influences were similar to the above
definitions, however, with the height scale replacing the width scale. ,
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Moses, H., G. H. Strom, and J. E. Carson, 1964: Effects of Meteorological
Engineering Factors on Stack Plume Rise. Nuclear Safety. 6_, 1-19.
This paper contains a review and discussion of several reports
concerning desirable stack height near buildings and terrain. Movies
of smoke flow patterns over buildings with small stacks at Argonne
National Laboratory were said to illustrate "cart wheels" forming on
the lee side with a diameter several times the height of the building
and thus providing high concentrations of contaminant. The wind-tunnel
studies of air flow around buildings by Evans (1957) Halitsky (1962) and
Strom (1.962), are discussed. The likely origins of the-2.5 times rule
of thumb, which has been used by the British Electricity Industry since
the 1930's is presented in light of comments of Sutton (1960). It is
reported that the Dutch require that a stack must only
be 1.5 times the height of the highest building in the neighborhood. It
is concluded that no elementary rule, such as a 1.5 or 2.5 times rule,
can be applied to all situations. The air flow in mountainous areas is
stated to be quite complicated with terrain irregularities located many
stack heights upwind and downwind influencing plume motions. It is
suggested that whenever a potential pollution problem results from an
effluent emitted by a stack located in all but perfectly uniform terrain,
wind-tunnel studies should be considered.
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Gloyne, R. W., 1965: Some Characteristics of the Natural Wind and Their
Modification by Natural and Artificial Obstructions. Scientific Horticul-
ture, XVII, 7-19.
Some characteristics of wind field modification by natural obstruc-
tions are reported. An eddy flow 2 barrier heights in vertical extent
and 10 to 15 barrier heights in horizontal extent to the leeward side of
a "near solid" barrier was diagrammed. At ground level, the region of
disturbed flow extended to about 30 barrier heights. Downwind of a
steeply sloped, wooded hill with a wind blowing at right angles to its
lehgth, the disturbed flow is reported to also extend downwind to about
30 times its height. Additional discussions relevant to wind modifica-
tions were also presented, and the point is made that each case must be
assessed separately. Slope angle and thermal stability and wind speed
were influential factors in determining the extent of terrain-induced
disturbances.
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Jensen, M., and N. Frank, 1965: Model-Scale Tests in Turbulent Wind.
Danish Technical Press, Copenhagen.
A large number of systematic wind-tunnel studies of concentration
downwind from an isolated chimney and a chimney on a house are reported.
An evaluation of the data indicates some building influence even for a
stack height three times the house height.
Halitsky, J., G. A. Magony, and P. Halpern, 1966: Turbulence Due to
Topographical Effects. New York University, New York, Geophysical
Laboratory Report No. TR-66-5. 75 p.
Comparisons between the author's wind tunnel model results and
Davidson's (1963) field observations in the lee of Green Peak, Vermont
are reported. Best agreement resulted for the higher model wind speeds
suggesting that tests of this type be run with a minimum ridge height
Reynolds number of 1 x 10 . The field observ;
flow generally fitted the model test results.
Reynolds number of 1 x 10 . The field observations of a cavity and wake
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Ukeguchi, N., H. Sakata, H. Okamoto, and Y. Ide, 1967: Study on Stack
Gas Diffusion. Mitsubishi Technical Bulletin No. 52, August.
The authors reported that downdraughts occur where the structures
and/or buildings stand near the stack, but these can be prevented on the
whole with the increase of stack height to 2.5 times greater than the
structures and/or buildings surrounding the stack. They stressed that
downdraughts produce very high ground level concentrations, depend on the
layout of structures and/or buildings, and must be avoided. A wind-
tunnel study examined the influence of a nearby building complex on
plume diffusion and found only a small effect when the stack was 2.5
times the building height and a negligible effect when the stack was
over 3 times the building height. No general rules are given as being
applicable to the effects of topagraphy; thus wind-tunnel models are
used to assess air quality impact.
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World Meteorological Organization, 1967: The Airflow over Mountains.
WMO, Geneva, Switzerland. Report No. 98. 43 p.
The World Meteorological Organization technical note concludes
that over rugged terrain, whether the flow aloft is smooth or other-
wise, it usually rests on a turbulent wake. Although little descriptive
detail of such regions is presented in the report, many potographs showed
the wave structures above the wakes, as revealed by cloud formations.
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Berlyand, M. E., 1968: Meteorological Factors in the Dispersion of Air
Pollutants in Town Conditions. Symposium of Urban Climates and Building
Climatology, Brussels, October.
The author mentions that the character of air motion changes considerably
near hilly relief and can substantially influence pollutant dispersion.
The increase of concentration was reported to sometimes occur even if the
pollutant sources are located on elevated places, but near leeward slopes
where wind velocity decreases sharply and downward currents arise. He
states that at present numerical solution of the equations of motion and wind
tunnel experiments are carried out for each case. Experiments on models
of separate plants and buildings have permitted determination of zones
in which downward currents and pollutant stagnations are possible. The
"2.5 times rule" is referenced as the recommended stack height in order
to avoid considerable increases of concentration.
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Halitsky, T., 1968: Gas Diffusion Near Buildings. Meteorology and
Atomic Energy - 1968, D. H. Slade (Ed.), Chapter 5-5.
A detailed discussion of flow separation and wake formation near
buildings is presented. The introduction of a building into a back-
ground flow is stated to cause changes in the velocity and pressure
fields. The new fields are called aerodynamical^ distorted, with the
amount of distortion measured by the difference between the distorted
and the background properties. The author presents a literature review
of flow near characteristic structures. It appears that the flow
downwind of sharp-edged buildings is disrupted to a greater extend than
for rounded buildings. No definition of the vertical or horizontal
extent of the building wake which could be used to determine the height
of a stack sufficient to avoid adverse influence is presented.
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Scorer, R. S., 1968: Air Pollution. Pergamon Press, Oxford, England,
pp 107-123.
The author discusses the consequences of a separated flow in the
wake of obstacles. Several examples of adverse influences on chimney
effluents in the wake of buildings and steephills are presented. The
examples are quite descriptive of the problem; however, no specific
definitions to the size and extent of wake effects are given.
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Strom, G. H., 1968: Atmospheric Dispersion of Stack Effluents. In:
Air Pollution. Vol I, Stern, A.C. (Ed.), Academic Press, New York,
Chapter 8.
A brief discussion of the effects on plume dispersion induced by
terrain and buildings is presented. The results of several wind tunnel
experiments are presented. The need for experimental procedures is
stated since there are no accurate analytical procedures. The adverse
effects were seen to be greater when the wind was normal to the long
dimension of the building. The desirability of designing stacks high
enough to have the plume remain clear of the highly turbulent regions is
stated. No specific definitions of the extent of the highly turbulent
retions is presented. Evans (1957) is referenced as providing guidance
when experimental data is not available for specific cases.
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Forsdyke, A. G., 1970: Meteorological Factors in Air Pollution.
Technical Note No. 114, World Meteorological Organization, Geneva,
Switzerland.
The following sentence is the only mention of stack height in
relation to the effect of building eddies which, if the chimney is not
high enough, will bring high concentrations of the pollutant down to
ground level in puffs. "To overcome this effect it is required in some
countries that the chimney height shall be at least two and one half the
height of the building from which it rises."
Pooler, F., Jr. and L. E. Niemeyer, 1970: Dispersion from Tall Stacks:
An Evaluation. Presented at 2nd International Clean Air Congress,
Washington, DC, December 6-11, 1970, Paper No. ME-14D. 31 p.
The authors present as part of a study evaluating dispersion from
tall stacks, several situations in which unexpectedly high ground level
concentrations could be associated with mountain lee effects. On days
with neutral flow, the plume from a stack located 13 ridge heights
downwind from a 450-m ridge was carried down to ground level within a
very short distance. This phenomenon could well be a result of the
strong downwash that occurs near the leeward edge of a standing eddy.
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World Meteorological Organization, 1970: Urban Climates and Building
Climatology. Proceeding of the Symposium on Urban Climates and Building
Climatology, Jointly organized by the World Health Organization and WMO,
Brussels, October 1968, WMO Technical Note No. 108, 109.
Concern for potential adverse building effects upon plume disper-
sion was mentioned in several of the symposium presentations. Only one
of the authors alluded to the "2.5 times rule" as referenced by Hawkins
and Nonhebel (1955). One of the general conclusions as reported by
T. J. Chandler was that "there is an urgent need to define much more
vigorously the physics of the urban surface—particularly its thermal
and aerodynamic properties." He also concluded that wind measurements
within the cubic of the city are clearly dependent upon very local
conditions which "makes it very difficult to use such field observations
to construct any general theory although one simple models of airflow
around single structures may still prove of practical use. Wind tunnel
and similar laboratory techniques have a very real contribution to make
in these enquiries."
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Orgill, M. M., J. E. Cermak, and L. 0. Grant, 1971: Laboratory Simu-
lation and Field Estimates of Atmospheric Transport - Dispersion Over
Mountainous Terrain. Colorado State University, Fort Collins, Colo.
Technical Report No. CER70-71MMO-JEC-LOG40.
An extensive literature review relating to both field and fluid
modeling studies and a discussion as to how mountainous terrain can
alter atmospheric airflow is presented. The authors report that, for
neutral airflow over a mountain, a large semipermanent eddy occurs on
the lee side. An area in the central Rocky Mountains of Colorado was
chosen for a field and laboratory study of transport and dispersion over
irregular terrain. Two different atmospheric conditions were simulated:
the thermal stability used in the wind tunnel model was near-neutral in
the lower levels and stable in the upper levels for one case and totally
neutral throughout for the other case. Field data yielded information on
the mean velocity and dispersion characteristics over the local terrain.
Totally neutral atmospheric stability conditions were observed on only
one day. No specific information as to where and when boundary layer
separation occurs or the size or shape of the cavity region in the lee of
ridges is reported in either the field or laboratory study results. The
purpose of the report is to generalize on flow patterns in complex ter-
rain on a much larger scale.
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Meroney, R. N. and B. T. Yang, 1971: Wind-Tunnel Study on Gaseous
Mixing Due to Various Stack Heights and Injection Rates Above an
Isolated Structure. USAEC Report No. COO-2053-6.
This wind-tunnel study examines the influence of a simple cubical
structure on the dispersion of a tracer gas released from short stacks
at varying heights and exhaust velocities. Both smoke visualization and
quantitative concentration measurements were made. The conclusions of
this study include;
(1) For a stack less than 1.5 times the building height high exhaust
velocities cannot prevent some immediate downwash.
(2) As the stack height increases, the effect of building entrain-
ment decreases. Exhaust velocities, for stack heights greater than
twice the building height, apparently need only be high enough to avoid
downwash behind the stack itself.
(3) Building orientation apparently aggravates entrainment even
for a simple cubical structure, however, the effect is not a major con-
sideration here. (For more complicated building complexes, the influences
may be more significant.)
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Yasuo, I., 1971: Atmospheric Diffusion Theory of Factory Exhaust Smoke
and its Applications. Water Engineering Series, Published by the Japan
Society of Civil Engineers, Hydraulics Committee, July.
The author presents equations for providing air quality estimates
that are intended for flat land. When the stack height is less than 2.5
times the height of buildings (or the mountains near the stack), it is
suggested that the exhaust gas will be swept down into the turbulence
area caused by the buildings. When this phenomena occurs, simulation
methods using wind tunnels and other special techniques are used.
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Lucas, D. H., 1972: Choosing Chimney Heights in the Presence of Buildings.
Proceedings of the International Clean Air Conference, Melbourne, Australia,
May 15-18, pp. 47-52.
A chimney 2.5 times the height of any adjacent building is reported
to follow the widely accepted rule of thumb to avoid effects by building
turbulence. The fact that the building width must also be relevant in
deciding the effect of the building is discussed. The essential dif-
ference for a tall thin building is that flow around the building
reduces the effect of flow over the building. It is generalized for all
buildings that a building wake has a height above the building of 1.5
times the height or width of the building, whichever is less. The extent
of the turbulent wake is reported to be pronounced for a distance down-
wind of approximately five building heights or half-widths, whichever is
less. While there is no abrupt cut-off in fact, it is considered
convenient to take the effect as declining progressively to zero from 5
to 10 building heights or ha If-widths, whichever is less.
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Schultz, J. F., 1972: Self Pollution of Buildings. Proceedings of the
1972 National Incinerator Conference, New York, NY, June 4-7.
It is suggested that good design for a chimney or exhaust system is
to locate them above the eddy area. Otherwise, there will be recycling
of exhaust products into the air intake to contaminate the entire build-
ing. The vertical extent of the eddy over a cubical building is given
according to Evans (1957) as 1.5 times the building width.
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Shingi, K., 1972: Wind Tunnel Experiment on Ascent Height of Exhaust
Gas. Central Research Institute of Electric Power Industry Report,
71053 (Translated from Japanese), 26 pp.
The results of wind tunnel experiments on the ascent height of
exhaust gas from thermal and nuclear power plants are reported, and
studies are made of the ascent height with relation to down-washing,
down-draught, and the stack type. The laws of wind tunnel similarity
are also discussed. It was found that stack down-washing does not occur
if the ratio between the exhaust gas speed and wind speed is more than
two. For the power plants studied, down-draught in the wake of the
building did not occur even when the stacks were much lower than 2.5
times the building height, if the exhaust gas rate was large enough.
The author comments that the 2.5 times law does not have a theoretical
basis making it applicable to all cases.
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American Society of Mechanical Engineers, 1973: Recommended Guide for
the Prediction of Airborne Effluents. Smith, M. (Ed.), New York,
AMSE, 85 p.
One section of the book- discusses the influence of buildings and
irregular terrain. It is reported that few quantitative diffusion
experiments have been made in irregular terrain; however, visual
observations of plume behavior in a variety of situations have been made.
The plume from a stack placed in the cavity leeward of a valley ridge is
said to become thoroughly diffused before passing downwind to the wake
region where the flow was in the direction of the upper wind. The air
flow disturbed locally by buildings is shown to influence that portion
of the plume which penetrates the disturbed flow region. Changes in
building shape and orientation to the wind are reported to affect the
cavity dimensions and flow to a marked degree, but the gross dimensions
of the displacement zone and wake for sharp-edged buildings appear to
be a function primarily of the frontal area of the building presented
to the wind. Also for rounded buildings, both the displacement zone
and wake are smaller than for sharp-edged buildings since separation
usually occurs downwind of the center of the building where the direction
of the surface flow just prior to separation is horizontally downwind
rather than normal to the wind. No quantitative definitions of the
vertical or downwind extent of the region of adverse influences near
buildings or terrain are given.
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Briggs, G. A., 1973: Diffusion Estimation for Small Emissions. Atmos-
pheric Turbulence and Diffusion Laboratory, NOAA, Oak Ridge, TN,
(Draft) ATDL No. 75/15.
A method for estimating air quality concentrations for emissions
influenced by buildings is presented. The plume is considered to be
within the region of building influence only when the estimated source
height is less than the building height plus 1.5 times the building
height or width, whichever is less. The "cavity" region where there is
circulation of the flow within the wake of the building, is defined to
equal the building height plus 0.5 times the building height or width,
whichever is less.
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Peterka, J. A. and J. E. Cermak, 1975: Turbulence in Building Wakes.
Presented at 4th International Conference on Wind Effects on Buildings
and Structures, London, United Kingdom. Colorado State Univ. Report
No. CEP 74-755 AP-JEC 34.
The mean velocity and turbulence characteristics in the wake of
simple rectangular-shaped buildings were measured in a boundary-layer
wind tunnel. The mean velocity deficit, turbulence excess, and longi-
tudinal vorticity relative to the undisturbed turbulent boundary layer
are presented and discussed. The conclusions of this study include;
(1) The turbulence wake effects of single buildings height do not
extend beyond 15 to 20 building heights and can be much less for a tall,
narrow building.
(2) Mean velocity effects in the wake do not extend beyond 15 to
20 building heights except when the angle at flow is such that corner
vortices are formed over the building roof.
(3) Within the primary wake region, the wake can extend 4 to 5
building heights in the vertical direction and 4 to 5 building widths in
the lateral direction for a strongly three-dimensional building.
(4) The data show that the wake characteristics of tall, narrow
buildings and low, long buildings are different. Furthermore, neither
the characteristics for a building of complex shape nor for a group of
buildings has been investigated.
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Smith, D. G., 1975: Influence of Meteorological Factors Upon Effluent
Concentrations On and Near Buildings with Short Stacks. Presented at
the 68th Annual Meeting of the Air Pollution Control Association,
Boston, Mass, June 15-20.
Field data of concentrations from stack emissions near a scaled-
down model of an industrial building is presented. The tests were
conducted for selected conditions of atmospheric stability, aerodynamic
roughness of upwind fetch, and wind orientation angle of the building.
The exit velocity was greater than twice the wind speed for all tests
to eliminate stack downwash as available. The study was designed to
measure the amount of effluent reaching the building and ground surfaces
in the downwind wake cavity of the building under a variety of stack
heights. Concentrations along the lee wall of the building were
measurable, even when the stack was 2 to 2.5 times the building height.
However, much higher concentrations were found when that stack was less
than 1.5 times the building height.
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Britter, R. E., J. C. R. Hunt, J. S. Puttock, 1976: Predicting
Pollution Concentrations Near Buildings and Hills. Presented at
the Conference on Systems and Models in Air and Water Pollution, at
the Institution of Measurement, London, Sept. 22-24.
Several simple mathematical representations of different parts of
the flow field near buildings and hills are presented. These models are
based on theoretical arguments applicable to two-dimensional flow.
Reliable calculation methods for the mean turbulent flow around obstacles
(three-dimensional is implied) are stated to not exist. The effects of
the distorted flow, in the wake behind two-dimensional bluff surface
obstacles in a turbulent boundary layer, upon emissions of various
height and downwind locations is evaluated. A source elevated to only
1.5 times the obstacle height is found to be greatly influenced unless
it is placed farther than 10 obstacle heights downwind. The influence
upon a source elevated to 2.5 times the obstacle height is found to be
much less, however, the effect extends to sources as far downwind as 20
obstacle heights. No significant effect is found for source heights
that are greater than 3 times the obstacle height.
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Huber, A. H., W. H. Snyder, R. S. Thompson, and R. E. Lawson, Jr.,
1976: Stack Placement in the Lee of a Mountain Ridge. U.S. Environ-
mental Protection Agency, EPA-600/4-76-047, Research Triangle Park,
NC, Sept.
A wind tunnel study was conducted to examine the effects the
highly turbulent region in the lee of a two-dimensional mountain ridge.
Smoke visualization and hot film anemometry measurements showed that
the cavity size and shape were minimally affected by the thickness and
turbulence intensity of the approach, boundary layer flow. The size
of the region of strong circulation in the lee of the model ridge was
found to be strongly dependent upon the upwind terrain and the gross
topographic features (angles) of the downs lope. The largest cavity was
found to extend to two ridge heights in the vertical and to ten ridge
heights downwind.. A stack 2.5 times the height of the ridge is stated
to avoid the highly turbulent region of the cavity proper. It is
implied that a taller stack may be necessary to avoid all wake effects
since part of the plume can, in only a short distance, spread downward
into the wakes. The need for studies of the behavior of plumes from
sources placed downwind of the cavity region is stated since the tur-
bulence intensity downwind of the cavity was found to be still signi-
ficantly greater than that in the undisturbed flow.
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Huber, A. H. and W. H. Snyder, 1976: Building Wake Effects on Short
Stack Effluents. Third Symposium on Atmospheric Turbulence Diffusion
and Air Quality, Raleigh, NC, Oct. 19-22, pp 235-241.
A wind tunnel study was conducted to examine building wake effects
on effluents from stacks near a building whose width is twice its height.
Some discussion of the building influences on the plume dispersion is
presented. For those sources having an effective stack height less than
2.0 building heights, very significant effects upon measured ground
level concentrations were found. Visual observations of smoke were also
made in order to assess the building influence upon the stack emissions.
There was significant reduction in building effect for the most elevated
stack which was 2.5 times the building.
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Snyder, W. H. and R. E. Lawson, Jr., 1976: Determination of a Necessary
Height for a Stack Close to a Building--a Wind Tunnel Study. Atmospheric
Environment. 10_ 683-691.
Wind tunnel tests showed a stack 2.5 times the building height is
adequate for a building whose width perpendicular to the wind direction
is greater than its height, but unnecessary for a tall thin building.
Smoke was used for flow visualization and quantitative concentration
measurements of a tracer gas emitted with the stack effluent were made
downwind of the building. For a tall thin building, application of an
alternative to the 2.5 times rule (Briggs, 1973) was shown to be ade-
quate. Thus, it is concluded that a sufficient stack height in order to
not have the plume entrained into the wake of the building is equal to
the building height plus 1.5 times the building height or width, which-
ever is less.
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Frost, W. and A. M. Shahabi, 1977: A Field Study of Wind Over a Simulated
Block Building. NASA CR-2804 prepared by the Univ. of Tenn. Space Inst.,
Tullahoma, Tenn., March.
A field study of the wind over a building 2.4 m (deep) x 3.2 m
(high) x 26.8 m (long) is reported. The study was designed to provide a
fundamental understanding of mean wind and turbulence structure of the
wind field. Eight instrumented towers were placed in the region both
upwind and downwind of the building. Horizontal and vertical wind
sensors were placed at the 3, 6, 12, and 20 meter levels. Approximately
100 experimental runs have been conducted. Hand held smoke candles and
anemometers were used to define the extent of the region of recirculating
flow downwind from the building with its long side oriented perpen-
dicular to the flow. The downwind extent was about 12 +_ 2 building
heights. This is compared to values of 13-16 building heights reported
for similar two-dimensional laboratory tests. The smoke patterns
indicate that the wake extends to a height of approximately 1.5-2
building heights. The values of the velocity components at the 3 m
level were strongly influenced by the building, but at the 12 m (^3
building heights) level the influence was not apparent.
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International Atomic Energy Agency, 1977: Guideline for Atmospheric
Dispersion Estimates. Vienna Austria.
It is reported that the motion of effluents near bluff bodies, such
as buildings, is affected by distortion of the windfield. Stacks at
least twice the height of the tallest adjacent building are usually
necessary except when the discharges are insignificant. Because of the
great variety of possible terrain conditions, a generalized treatment of
the effects of features such as hills or valleys is stated as not
feasible, since the exact flows will be extremely site-dependent. The
use of fluid flow modeling is suggested as providing some help in esti-
mating the plume trajectory near hilly terrain.
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Robins, A. G. and I. P. Castro, 1977: A Wind Tunnel Investigation of
Plume Dispersion in the Vicinity of a Surface Mounted Cube-I. The
Flow Field, II. The Concentration Field. Atmospheric Environment,
17, 291-311.
Experiments investigating both the flow field and plume behavior
downstream of an isolated surface mounted cube in the Marchwood Engi-
neering Laboratory wind tunnel are reported. The wake air flow was found
to be strongly affected by upstream turbulence. For both a 0° and 45°
orientation of the building into the wind, the effective wake zone in a
turbulent boundary layer extended upwind to about five times the height
of the cube. The region of reversed flow extended downwind to 1.5
heights for wind angle, 9, of 0°, and to 2 heights for 8 of 45°. The
mean velocity deficit was reported to extend to twice the building
height for both the 0° and 45° orientation. A tracer gas was emitted
from a stack over the roof center. The stack extended from building
height to 2.5 times the building height. The influence of the building
was found to be detectable for 9=0° and a low stack emission rate;
however, for a ratio of emission velocity to wind speed of 3:1, the
influence was negligible for a stack height 2.5 times the building
height. For 9=45° the influence of the cube was detectable for all the
stack heights and emission velocity ratios. It is concluded that much
work remains to be done on the influence of nearby buildings on the
behavior of chimney plumes. Also, it is especially important to model
correctly the approach flow when undertaking wind tunnel investigations
of diffusion in the vicinity of isolated buildings.
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