United States Office of Air Quality EPA-450/4-81-003
Environmental Protection Planning and Standards July 1981
Agency Research Triangle Park NC 27711
Air
c/ERA Guideline for Use of
Fluid Modeling to Determine
Good Engineering Practice
Stack Height
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EPA 450/4-81-003
GUIDELINE FOR USE OF FLUID MODELING
TO DETERMINE GOOD ENGINEERING PRACTICE STACK HEIGHT
June 1981
UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
Office of Air, Noise, and Radiation
Office of Air Quality Planning and Standards
Research Triangle Park, North Carolina 27711
U,3 trwiroryvv-rital Protection Agency
Re,.}-.:.) ", Library
230 3c\;-.;, Dearborn Street
Chicago, iiiincis 60604
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U,S. Environmental Protection Agency
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ACKNOWLEDGMENT
This guideline was prepared by Alan Huber, formerly of the Monitoring
and Data Analysis Division, Office of Air Quality Planning and Standards,
Environmental Protection Agency, now assigned to the Agency's Meteorology
and Assessment Division. Appreciation is extended to Dr. William Snyder
of the Meteorology and Assessment Division, Environmental Sciences
Research Laboratory, for his helpful comments and discussion concerning
fluid modeling. His fluid modeling guideline document, referenced
herein, serves well in establishing EPA standards to be followed in the
conduct of such studies. Changes to the June 1979 draft were based
partially on comments and suggestions received during the period of
public comment and on EPA's assessment of its own demonstration studies
conducted by staff at the Agency's Fluid Modeling Facility. Special
appreciation is extended to Robert E. Lawson, Jr., of the Meteorology
and Assessment Division, Environmental Sciences Research Laboratory, for
taking the responsibility of conducting and reporting a demonstration
study. Overall, the changes to the June 1979 draft should simplify the
study design and provide a framework for both consistent reports and
reviews.
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TABLE OF CONTENTS
Page
1.0 INTRODUCTION 1
2.0 BACKGROUND 3
3.0 BASIC CONCEPTS 5
3.1 Dynamic Similarity Criteria 7
3.2 Boundary Layer Conditions 13
3.3 Surface Roughness, Terrain,
and Building Scaling 14
3.4 Plume Rise 15
3.5 Concentration Measurements 18
4.0 REQUIREMENTS FOR A FLUID MODEL DEMONSTRATION 21
4.1 Preliminary Design 22
4.1.1 Model Surface and Its Boundary Layer ... 22
4.1.2 Plume Rise 29
4.1.3 Atmospheric Dispersion Comparability ... 31
4.2 Determination of GEP Stack Height 37
4.2.1 Demonstration of Adverse Effects 38
4.2.2 GEP Stack Height 41
5.0 REPORT CHECKLIST 43
6.0 REFERENCES 47
m
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1.0 INTRODUCTION
This guideline contains specifications for the use of fluid model-
ing to determine Good Engineering Practice (GEP) stack heiyht. The
guidance is intended for use by the U. S. Environmental Protection
Agency (EPA), by State and local air pollution control agencies, and by
industries and their consultants in the design and final review of a fluid
modeling study determination of GEP stack height. The Agency issues
guidelines in association with regulations in order to make clear any
requirements for data and to present criteria the Agency will use in
evaluating the adequacy of that data. The specifications in this
guideline are necessary to assure consistency among studies. It is very
important for both those conducting the fluid modeling study and those
reviewing the results to share a common set of criteria for reference.
The aim of fluid modeling is to produce an accurate representation
of the atmosphere using the flow of air or water in a test facility,
e.g., wind tunnel or water channel. Certain similarity criteria must be
considered if fluid modeling studies are to accurately reproduce atmospheric
phenomena. A separate guideline entitled, "Guideline for Fluid Modeling
of Atmospheric Diffusion," (Snyder, 1981), reviews the fundamental
principles and practical applications of fluid modeling. The aim of
that guideline is to establish the capabilities and limitations of fluid
modeling, and to establish EPA standards for the conduct of fluid modeling
studies. This guideline is based on Snyder's state-of-the-art review.
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2.0 BACKGROUND
As required by Section 123 of the Clean Air Act Amendments of
1977, the Administrator has proposed regulations (Sections 1 and 18
of 40 CFR Part 51) 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 in
Section 123 of the Clean Air Act Ammendments of 1977 as "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
as a result of atmospheric downwash, eddies and wakes which may be created by
the source itself, nearby structures or nearby terrain obstacles."
The scientific literature, in general, indicates that a case-
specific review is integral to assuring the prevention of adverse aero-
dynamic effects in the immediate vicinity of a given source. However,
the literature also identifies a general formulation that establishes a
minimum height necessary to prevent significant effects of nearby
structures. The GEP formulation is a reasonable working rule, defined
as:
H = H + 1.5L (1)
where: H = GEP stack height
H = Height of the structure or nearby structure
L = Lesser dimension (height or width) of the structure or
nearby structure.
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The basis of the formulation and a summary of extensive scientific
literature on the subject can be found in the "Guideline for Determination
of Good Engineering Practice Stack Height" (EPA, 1980).
Proposed regulations (40 CFR Part 51) to implement Section 123 of
the 1977 Clean Air Act Amendments allow the stack heights near structures
as determined by Equation 1 to be used in some cases as the maximum
creditable stack height which may be used in establishing a source's
emission limitation for a State Implementation Plan. The GEP creditable
stack height, based on nearby terrain features, must be determined on a
case-by-case basis through the use of appropriate field or fluid modeling
studies. Field or fluid modeling studies may also be used by the source
operator to show that a stack height greater than determined by Equation 1
is needed to prevent excessive pollutant concentrations. This guideline
is appropriate when fluid modeling is used to determine GEP stack height.
An excessive concentration, for the purpose of determining GEP stack
height, is defined in the regulation as a maximum ground-level concentration
monitored or modeled in the presence of nearby structures or terrain
obstacles that is 40 percent or more, in excess of the maximum ground-level
concentration, monitored or modeled for the same orientation and stack
parameters in the absence of downwash, wake, or eddy effects produced by
nearby structures or terrain.
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3.0 BASIC CONCEPTS
The basic concepts for designing a fluid modeling study are out-
lined in the following subsections. The construction of a fluid model
requires that the flow in the test facility (e.g., wind tunnel) be
appropriately fixed and along with the surface roughness, terrain,
and/or buildings, scaled to accurately reproduce atmospheric phenomena.
The stack and plume from the source must also be similarly scaled if
dispersion patterns in the fluid model are to simulate those in the
field. The fluid model encompasses the entire situation within the four
walls of the test facility that is designed to accurately simulate
atmospheric flow in the field. Consideration of each of the concepts
outlined in the following subsections leads to requirements for data and
the reporting of that data as given later in Section 4. Specific
references to requirements given in Section 4 are underlined.
A detailed formulation and discussion of the fundamental principles
for fluid modeling of atmospheric phenomena is presented by Snyder
(1981). A summary of the important criteria is presented here. Certain
similarity criteria must be considered if fluid modeling is to accurately
reproduce atmospheric phenomena. The dynamics of the flow in the fluid
model must accurately simulate those in the field. The effects of
surface conditions in the field upstream of the modeled area must be
accounted for in the fluid model by developing appropriate boundary
layer conditions. The necessary surface roughness, terrain, and buildings
are included in the construction of the fluid model. The plume trajectory
in the fluid model must be similar to that in the field if air quality
impact is to be evaluated.
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The purpose of the fluid modeling study for determining GEP stack
height is to demonstrate the stack height needed to avoid excessive
concentrations caused by the effects from nearby structures or terrain
obstacles as specified by stack height regulations. GEP stack height
is appropriately determined for the situation under atmospheric con-
ditions that result from those surface influences of highest extent.
Above some minimal reference wind speed, e.g., 3 m/s, the flow
pattern near the structure or terrain obstacle in the field is independent
of wind speed, as is reasoned in the discussion presented in Section 3.1
on Reynolds number independence. The greatest effect on the plume
should occur when plume rise near the source is lowest. For most sources,
even those with a relatively high exit velocity, high wind speeds found
to occur occasionally at most locations will result in significantly
reduced plume rise and thus the greatest potential for ground-level
concentrations in excess of those in the absence of structure or
terrain obstacle influences.
The wind speed that will result in the determination of greatest
GEP stack height is seen for all foreseeable situations to exceed 6 m/s.
The atmosphere is characterized by a generally neutral state of stability
when the surface wind speed at a height of 10m is greater than 6 m/s
(Turner, 1970). Thus, the critical conditions of stability for de-
termining GEP stack height are expected to be associated with a neutral
(adiabatic) atmosphere. Specific guidance for fluid modeling of
an adiabatic atmosphere is given herein. Guidance for modeling
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a nonadiabatic atmosphere is not provided; the need to model these situ-
ations requires case-by-case consideration.
To define GEP stack height for a specific stack, measurements in
the wake of and in the absence of either the structure or terrain
obstacle are needed to assess the increase in maximum concentrations.
The concentration increase must be assessed to determine whether the in-
crease constitutes an excessive concentration. Concentrations in the
wake of the structure or terrain obstacle are considered excessive if
the maximum ground-level concentrations are at least 40 percent greater
than the maximum in the absence of their influences. Wind-tunnel modeling
is ideally suited for this type of determination since the model structure
or terrain feature being studied can be easily removed to assess its
effect. More importantly, a properly designed wind-tunnel study can
account for the aerodynamically induced influences affecting the dispersion
of the stack effluent.
3.1 Dynamic Similarity Criteria
To rigorously model the dynamic behavior of atmospheric flow, five
nondimensional parameters must be matched between the model and the
field. These parameters, as discussed by Snyder (1981), are:
1. Froude number, Fr = UR/ / gL6TR/T0;
2. Ross by number, Ro = UR/L£2R;
3. Reynolds number, Re = URL/v;
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4. Peclet number, Pe =
5. Reynolds-Schmidt number, Re-Sc = UDL/a.
K
where:
UR = reference velocity;
ftR = reference angular velocity;
L = reference length;
6TR = temperature deviation from adiabatic atmosphere;
T = temperature of adiabatic atmosphere;
g = gravitational constant;
v = kinematic viscosity (momentum diffusivity);
K = thermal diffusivity;
a = molecular diffusivity.
The Froude number represents the ratio of inertial forces to
buoyancy forces on a local air parcel. Calculation of the Froude
numbers should be based on the measured temperature profiles. A large
value of the Froude number implies that buoyancy forces are smdll relative
to inertial forces. Thus, atmospheric flows having large Froude numbers
are considered neutral (adiabatic). A truly adiabatic atmosphere
has an infinite Froude number. Isothermal flow must be maintained in
the test facility tp_ adequately model ar^ adiabatic atmospheric flo w.
This similarity criterion for fluid modeling of adiabatic atmospheric
flow can be easily met by insuring that air in the room containing the
facility and the fluid temperature in the test facility are equal. This
is especially necessary when the flow speed through the test facility is
slow. Steps 2a and 3a of Section 4.1.3 are required to satisfy this
similarity criterion.
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The Rossby number represents the ratio of the inertia! forces
to the Coriolis force on a local air parcel. Coriolis force results
in the wind vector changing direction with increasing height above the
surface. If the Rossby number is large, the Coriolis force is relatively
small and thus does not have a significant effect on a dispersing pollutant.
Snyder suggests that the Rossby number is sufficiently large at downwind
distances (L) less than about 5 km to ignore Coriolis force for modeling
dispersion in adiabatic atmospheric flow over flat terrain. Nc in-
formation is available to assess the effect of Coriolis force in regions
of complex terrain. This implies that fluid modeling should be limited
to areas within 5 km of the source, since Coriolis force cannot presently
be simulated. Mathematical models of atmospheric dispersion in current
use do not account for Coriolis forces. Therefore, while fluid
modeling should be_ limited tp^ areas within 5_ krn^ of_ the source, its use
for modeling larger areas may as^ for mathematical models be_ similarly
justified, when necessary. The Rossby number criterion js^ not a^ critical
modeling parameter, although vt poses a^ limitation on_ the use of_ fluid
modeling. There are no special data requirements to satisfy this similarity
criterion.
The Reynolds number represents the ratio of the inertial forces
to the frictional forces on a local air parcel. When the modeling
medium is air, the reference velocity must be increased by the same
amount as the reference length is reduced in order to match the Reynolds
number. In water, the reference velocity need only be increased by 1/15
the reduction in the reference length, since the kinematic viscosity of
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water is 1/15 that of air. However, using water as the medium requires
much more energy for equal rates of flow. This physical limitation
generally results in water tunnels having to be smaller in size and
operated with lower flow rates than wind tunnels. The reference
length in atmospheric dispersion problems must be modeled at a reduced
scale of several orders of magnitude, making an equivalent increase in
the reference velocity impractical. Thus if strict adherence to the
Reynolds number criterion were required, no atmospheric flows could be
modeled.
Various arguments to justify the use of smaller Reynolds numbers in
fluid modeling compared to those in the atmosphere are found in the
literature. The best argument appears to be the principle of Reynolds
number independence. This principle is based upon the hypothesis that
in the absence of buoyancy and Coriolis effects, the pattern of tur-
bulent flow is similar at all sufficiently high Reynolds numbers. If the
Reynolds number is large, frictions! forces are relatively small com-
pared to the inertia! forces. For Reynolds number independence to hold,
the frictional forces must remain relatively small and have little
effect on the overall flow as the Reynolds number is decreased. A large
amount of experimental evidence now exists to support this principle.
For atmospheric flows, Reynolds number independence appears to apply
except in the very smallest scale of the turbulent flow very close to
the ground or other physical boundary. In effect, the reference length L
(scale of flow structure examined) is small where the frictional forces
are important. Flow very near structures or terrain features may not be
Reynolds number independent.
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Practice indicates that sufficiently large Reynolds numbers are
attainable at least for modeling the flow over sharp-edged geometrical
structures or terrain features in ordinary meteorological wind tunnels.
However, more work must be done to determine if simulation of flow over
more streamlined surfaces can be sufficiently modeled. Frictional
forces have very little effect if the general flow is detached from the
surface or other physical boundary. Flow over streamlined surfaces is
less susceptible to detachment and thus is more sensitive to the value
of its Reynolds number. Reynolds numbers in atmospheric flow generally
are sufficiently large for the independence principle to hold. Fortu-
nately, flow similarity is generally observed at the lower values of
Reynolds number attainable in fluid modeling, provided the flow is
locally detached or if the area of study is sufficiently above the
surface. This has led modelers in some situations to force flow de-
tachment by adding roughness to the model structures or terrain features.
The Reynolds number criterion is_ a_ critical modeling parameter. Step 4_
lH Section 4.2.1 requires a_ test for Reynolds number independence.
The Peclet number is most easily discussed by writing it as the
product of the Reynolds Number and Prandtl Number:
URL
Pe = JJ-£= Re-Pr.
The Reynolds-Schmidt Number can similarily be written as the product of
the Reynolds Number and the Schmidt Number:
Re-Sc = URL v = Re-Sc.
v a
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Both the Prandtl number and the Schmidt number are properties of the
fluid. The Prandtl number is the ratio of the momentum diffusivity to
the thermal diffusivity. The Schmidt number is the ratio of the mo-
mentum diffusivity to molecular diffusivity. These numbers are somewhat
different between air and other fluids. This would seem to preclude
using any model medium other than air in simulations of atmospheric
flow. A high Reynolds number is necessary in the wind tunnel flow to
match the Peclet number or Reynolds-Schmidt number found in the field.
Arguments similar to those constructed for Reynolds number independence
are used to justify the neglect of the Peclet number and Reynolds-
Schmidt number as modeling criteria, provided the Reynolds number is
sufficiently large. Both heat and mass are regarded as passive
quantities in connection with most environmental atmospheric dispersion
problems. Thus, if the Reynolds number of the main structure of the
flow is sufficiently large, advection and the larger scale turbulent
motions are totally responsible for the transport and dispersion of a
passive pollutant. That is, molecular or thermal diffusion acts mostly
to smooth out the very small-scale discontinuities of concentration or
temperature. Molecular or_ thermal diffusion J_s_ assumed t£ contribute
negligibly tp_ the dispersion of_ the source plume within the simulated
turbulent atmospheric boundary layer, provided the Reynolds number js_
large enough with the Peclet number and Reynolds-Schmidt number being
themselves unimportant.
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3.2 Boundary Layer Conditions
The effects of upstream surface conditions on the velocity of the
wind result in a variation with height described generally by some
theoretical distribution such as a logarithmic or power law profile.
The profile is characterized by the depth of the boundary layer and a
representative surface roughness length. Turbulence intensity of the
wind naturally decreases with height above the surface roughness. The
profiles of mean velocity and turbulence intensity are very significant
characteristics that should be very closely matched in the model.
Measurement of vertical profiles of Reynolds stress throughout the
region of interest is especially useful in characterizing the surface
friction velocity u*, which is a parameter used in representing the
velocity near the surface by a logarithmic profile. Measured profiles
in the field will not likely be available at most sites where 6EP stack
height is to be determined. In Section 4, general modeling criteria are
3b. 3c_ jm Section 4.1.3 requires mean velocity, turbulence intensity, and
Reynolds stress profiles tp^ be_ measured at_ several positions^ throughout
the model.
Consideration of additional flow characteristics of the atmospheric
boundary layer would be desirable. However, specific guidance is not
possible. The purpose of specifying necessary modeling criteria for the
boundary layer is to first insure that dispersion throughout the modeled
flow correctly provides dispersion patterns comparable to those given by
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recommended air quality modeling techniques as described in the Guideline
on Air Quality Models (EPA, 1978). Concern for the similarity of
additional flow characteristics is not necessary if the model boundary
layer dispersive characteristics are documented through measurements in
the test facility to fall between estimated values for Pasquill-Gifford
stability category C and D as prescribed by Turner (1970). Step 4 in
Section 4.1.3 requires documentation of_ dispersion from the source.
A documentation and test of comparability of the fluid model
boundary layer conditions to flow over flat terrain in the absence of
any buildings can be done simply. Problems, however, arise in doing the
same for flow over complex terrain or over urban areas where local
differences near the surface result from different surface features.
Because of these differences one cannot establish that the fluid model
boundary is nondeveloping or that the dispersive characteristics can be
represented by a general categorization. In order to evaluate the fluid
model boundary layer conditions, it is necessary to first document the
flow in absence of the complex terrain or urban structures as prescribed
above. Differences found for flow over the complex terrain or urban
area can then be related to the increased surface roughness. Section
4.2.1 requires measurements sufficient tp_ document such differences.
3.3 Surface Roughness, Terrain, and Building Scaling
Minute geometric details of terrain or structures do not signif-
icantly affect atmospheric flow. Thus, such detail need not be
considered in a fluid modeling simulation. Objects about the same size
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as the characteristic surface roughness length need not be produced in
geometrical form but an equivalent roughness must be established. For
example, gravel can be added to the surface of the terrain or buildings
to establish an equivalent roughness.
Major terrain features and building structures must be scaled
without geometric distortion. The amount of reduction in scale is
limited by the requirement for the flow to be Reynolds number independent.
Discussion and some guidance on selecting the proper model surface
roughness, terrain height and building height are given by Snyder (1981).
The fluid modeler's decisions must be based on a number of interacting
concerns, including the size of the area to be modeled, the necessary
boundary layer depth, the desired diffusion characteristics, and Reynolds
number independence. Often, the fluid modeler's experience is the best
guide. Data requirements and criteria which EPA will use to evaluate
the resulting study are given in Section 4. It is recognized that each
fluid modeler may have somewhat different approaches to selecting these
design parameters. This is acceptable if the results meet the report
requirements. Section 4.1.1 outlines the requirements for modeling the
surface and its boundary layer.
3.4 Plume Rise
The atmospheric conditions that are considered here in determining
GEP stack height are characterized as neutral stability (adiabatic) with
high mean wind speed. Under such conditions, plume rise near the source
where its rise is dominated by momentum flux will be small while its
rise farther downwind may be largely due to buoyancy flux. A thorough
review of the issues relating to fluid modeling of plume rise is presented
by Snyder (Section 3.1, 1981).
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In general, plume rise near the source in an adiabatic atmosphere
has proven to be well-described for most conventional sources by the
Briggs (1975) formulation,
2
3
BT
4H
H
+
L^'
B
Hs
3
M
gD2ws(pa-Ps)|
4p IPH
^Ma s
r 12
X
Hs
(2)
H U'
= iT
for gi = 1/3 + U/W^» e2 = 0-6 where Lm is a momentum length scale and LD
5 HI O
is a buoyancy length scale, defined as
(3)
Lm _ T
H ~ "2.
5
'psWs
U^
a
11/2
1 D
H
s
L . (w "\ 3
TT= 4
s
1
Fr^"
s.
D "s
H U
s I J
. . . fu ^3
1
~ 4
1
fr'i
a
D s
He lu 1
SJ
(4)
where:
p = stack effluent density
pa = ambient air density
a
W = stack effluent exit speed
D = stack exit diameter
HS = stack height
U = mean wind speed at height of stack
x = streanwise coordinate
q = acceleration due to qravity
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Frs = Ws/[gD(pa-ps)/ps]1/2
Fra = Ws/[gD(pa-ps)pa]1/2
The first term in Equation 2 represents the contribution due to
source momentum and the second term represents the contribution from
source buoyancy. Close to the stack, the initial momentum term will be
important, whereas, the buoyancy term in most cases will ultimately
dominate. Matching the parameters in the above plume rise equation
should insure comparable plume rise in the fluid model so long as the
plume is not downwashed into the wake of the stack, buildings, or
elevated terrain.
A GEP stack height determination must examine the effect of the
nearby structure or terrain obstacle on the plume from the source. GEP
stack height is limited to the height necessary to avoid excessive
concentrations as explained in Section 2. Immediate plume rise near the
source may in some situations significantly effect the stack height
necessary to avoid the most adverse effects. The model study must be
designed to demonstrate plume rise near the source to be comparable to
the estimate by Equation 2 in the absence of buildings or terrain. This
can be satisfied by correctly modeling the momentum length scale as
presented in Section 4.1,2. EPA has based its definition of "excessive
concentrations" on model results for sources not having significant
plume rise downwind from the stack and thus the demonstration will be
consistent. The model study design is considerably simplified by not
having to model the buoyancy length scale which generally controls the
resulting plume rise further downwind of the source. Also the results
of the fluid model study will be generally more reliable.
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When stack downwash occurs, special consideration must be given to
the flow around the stack. It is essential to assure that the flow
within the boundary layer around the stack is turbulent. Common practice
in fluid modeling is to use a trip wire or fence or other surface
roughness to force the boundary layer flow to be turbulent. This is not
necessary for modeling rectangular stacks since their sharp corners
force flow separation. In all cases, it is necessary to assure that the
stack effluent exhaust is fully turbulent. Data requirements and
criteria that EPA will use tp_ evaluate the representativeness of_ the
plume rise jji the fluid model are presented ir\_ Section 4.1.2.
3.5 Concentration Measurements
Concentrations measured in a fluid modeling study should be related
to those in the field through the nondimensional concentration, C = xUH2/Q
as presented by Snyder (Section 3.5, 1981), where
X = mass concentration of pollutant (gm/m3),
U = reference wind speed (m/s),
H = characteristic length (m), and
Q = pollutant emission rate (gm/s).
The sampling time for measurements taken as part of the fluid
modeling study must be long enough to provide steady-state averages.
Fluid modeling is designed to correspond to conditions in the field for
which the wind direction is steady. It is essential that the maximum
ground-level concentrations be shown to represent the steady-state
average values since they are crucial in the demonstration of excessive
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concentration. Data requirements and criteria which must be_ considered
ijT^ establishing this fact are presented ij^ Section 4^.
In absence of effects of building and/or terrain, the pattern of
concentrations in the fluid model should be comparable to those estimated
by mathematical models recommended by EPA. Steady-state average concentratioi
measured in the fluid model should thus correspond to one-hour average
concentrations in the field. In those situations where persistence of
the wind direction can be assumed, fluid modeling can be used for estimating
average concentrations for longer periods of time. Fluid modeling
studies can also be used for estimating average concentrations for
periods having variability in the wind direction by including a separate
examination of the flow for several directions. Concentrations can then
be estimated by considering the frequency of wind direction.
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20
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4.0 REQUIREMENTS FOR A FLUID MODEL DEMONSTRATION
Section 3 presented a summary of the basic design concepts used
in developing a fluid modeling study that is comparable to field
conditions. Elements necessary for determining a GEP stack height
are specified in this section. Specific guidance on requirements
for data and the reporting of that data are given. Fluid modeling
studies can be adapted to meet detailed specifications since essential
characteristics can be controlled. Atmospheric flow is extremely complex
in that profiles of its characteristics can vary with time and space.
In general, profiles in the field rarely are available at the sites
where a GEP stack height is to be determined. Therefore, it is only
necessary for the modeling study to be designed to meet the general
atmospheric conditions, given here as comparable to those used in air
quality models recommended by EPA. Where detailed field information
describing the situation is available, the modeling study should be
designed to best assimilate it. For these situations, the fluid model
should be shown to be comparable to the field.
The requirements given here are based on the general guidance
by Snyder (1981). Readers and users of this guideline should be
familiar with the presentation by Snyder. Deviation from his general
guidance occurs in this guideline in a few instances where the objectives
were judged to be met without additional detail.
Section 4.1 presents important criteria that should be considered
prior to the construction of the fluid model for the actual situation.
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The model scale should first be determined. Then a test for atmospheric
dispersion comparability should be conducted as specified. Section 4.2
specifies the procedure and reporting requirements for determining GEP
stack height. The step-by-step procedure leads simply to a satisfactory
study provided sufficient preliminary consideration is given to the
criteria presented in Section 4.1.
Specific reporting requirements presented here should be followed
as outlined. All supplementary data taken as part of the study should
also be incorporated into the report. A separate appendix describing
the fluid modeling facility and instrumentation used in the conduct of
the study should be attached. Normal operating conditions and associated
parameters should be described. A daily log should be kept during the
conduct of the study since the Agency may wish to conduct an audit and
review. Since on-site visits and demonstration of repeatability of some
measurements may be requested as part of quality assurance procedures,
a proposed plan of study must be sumbitted to the reviewing Agency.
4.1 Preliminary Design
4.1.1 Model Surface and its Boundary Layer
The size of all building structures and the general topography in
the vicinity of the source should be examined and the area to be deter-
mined. A roughly cubical building or other major structure, or a three-
dimensional hill upstream of the source should be included if its height
exceeds l/20th of the distance from the source. An obstruction whose
crosswind dimension is large compared to its height (width greater than
10 times its height) should be included if its height is greater than
22
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l/30th of its distance upstream. For tall obstructions (height greater
than width), the width replaces the height scale in the above determination
of the critical distances. If possible, ridges even farther upstream
should be included. In areas having undulating terrain, the hill or ridge
height is defined as the elevation difference between its peak and local
trough. A detailed topographic map and discussion concerning the
selection of the size of the modeled area should be presented in the
study report.
Additional parameters and criteria should also be considered in the
selection of the scale of the modeled area. At this stage of experimental
design, the fluid modeler should select design parameters that can be shown
to satisfy necessary requirements. The fluid modeler's experience is likely
the best guide in planning the study.
1. The buildings, other structures, and/or terrain should be im-
mersed in an appropriate boundary layer that can be characterized as
representing atmospheric dispersion between that for Pasquill-Gifford
category C and D over flat terrain (Turner, 1970). The depth of the
model boundary layer, 6, should be scaled to represent 600 m above the
general level of terrain in the field, independent of surface roughness
and wind speed. The depth of the model boundary layer is not critical
so long as the boundary layer up to the level of the stack and its plume
are scaled appropriately. The design wind speed will not be considered
excessive so long as the speed is less than the speed that is exceeded
less than 2 percent of the time (i.e., 98th percentile wind speed). This
should be based on frequency distributions from at least one year of
wind records representative of the source location. A frequency distribution
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based on categories of specific design wind directions would only be
appropriate if on-site meteorology is used. Wind speeds greater than the
98th percentile speed could be justified if air quality violations are a
problem at higher wind speeds. In most cases, the wind records must be
extrapolated to estimate the wind speed at stack height. The reviewing
agency should determine the availability of appropriate wind records
and their appropriate extrapolation to stack height.
2. The surface roughness length z0 and the friction velocity u*
should be derived from the mean velocity profile:
_U_= 2.5 ln-4^ , - (5)
L
*
in the range, 1.5li < z < 1 . 5li + 100 m, where z is the height, h is
r ~ - r r
the general height of the surface roughness elements, and d is the
displacement height (neglected for z0<0.2 m, full scale). Simiu and
Scanlan (1978) suggest that reasonable values of d in cities may be
estimated using the formula
d=H~-z0/k, (6)
where H is the general roof-top level and k is the von Karman constant
(0.4). Values of the surface roughness length, z0, for various types
of surfaces are presented in Table 1 as a guide for a comparison.
Actual values over urban areas with tall buildings or near elevated
terrain may be substantially larger. Values of the friction velocity,
u*, are dependent on the value of z0. Values for u* as suggested by
Counihan (1975) are presented in Figure 1 , as a guide for a comparison.
24
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Table 1. Values of Surface Roughness Length (ZQ) for Various Types
of Surface (from Simiu and Scanlan, 1978).
Type of Surface o
(cm)
Sand 0.01 - 0.1
Sea Surface 0.003a - 0.5b
Snow Surface 0.1 - 0.6
Mown Grass H).01m) 0.01 - 1
Low Grass, Steppe 1 - 4
Fallow Field 2 - 3
High Grass 4 - 10
Palmetto 10 - 30
Pine Forest (Mean height of trees: 15m;
one tree per 10m2; zd=12m) 90 - 100
Outskirts of Towns, Suburbs 20 - 40
Centers of Towns 35 - 45
Centers of Large Cities 60 - 80
aWind speed at 10m above surface = 1.5m/sec.
Wind speed at 10m above surface > 15m/sec.
25
-------
E
6
o
i- -Q rd >
O -r- S_
i- -O 3
S- (O O
M-
O> "O
O) -C Ol
(J -t-> _C
(C IO
M- E to
S- !- Q
13
(/) o
N
-O -~
c ^: un
re +-> r-^ *-*
CT) O^t CTl
" sz i r~.
Q.
^2E§
S- O S-
M-
O -r- X
CL 2 s- 0)
, C
O * fO -r-
13 O ID S-
r- o C OJ
S- r- =5 2
fO O) O O
> > JD Q.
O)
cn
26
-------
The vertical profile of -uw (i.e., the correlation between the fluctuating
velocity in the streamwise and vertical directions) may also be used
to estimate the value of u* as demonstrated in the example of Figure 2.
The value of u* is equal to the value of a -uw at the surface which should
be based on the entire profile.
3. The mean velocity profile through the entire depth of the
boundary layer should be represented by a power law U/U^ = (z/6)p. The
power law index p is dependent on the value of z . Values for p as
suggested by Counihan (1975) and Irwin (1979) are presented in Figure 1,
as a guide for a comparison.
4. The surface of the model should be covered with roughness of
size e such that eu*/v _> 20, as suggested by Snyder (1981). Similarily,
the step size in "stepped" terrain models should be of order e and
surfaces of buildings or other structures should be covered with roughness
of size e. Compromises may be appropriate for surfaces having sharp
edges. For sharp-edged surfaces, the flow is separated and likely not
significantly affected by roughness on its surface. Protuberances in
the terrain, buildings, or other structures less than the size e need
not be reproduced in detail in the model. Similarly, details in the flow
and dispersion pattern are not reproduced for scales less than the size
£.
5. The flow over significant elevated terrain, buildings or other
structures nearby the source is Reynolds number independent. For design
purposes, a minimum Reynolds number U,,L/v greater than 11,000 is taken
here as sufficient without demonstration for sharp-edged obstacles. The
27
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1.0 -
0.8 -
16
14
0.2
V
£
w.
1^=18171/5
a STATION 9.8m
* STATION 13
0 STATION 16
STATION 19
STATION 22
o STATION 24
0.5
1.0
-uw /u 2
1.5
Figure 2. Variation of shear stress with height measured at various
downwind positions in a wind tunnel boundary layer (adiabatic
flow). Data from Zoric and Sandborn (1972), (as in Snyder 1981)
28
-------
reference velocity U,, is the mean velocity upstream at the height of the
obstacle and the reference length L is its lesser dimension (height or
width). A test of Reynolds number independence should be conducted when
significant effects of flow over terrain or smooth-shaped obstacles are
being considered.
6. Blockage of flow (i.e., the ratio of cross-sectional area of a
model to the cross-sectional area of the test section) is limited to a
percent for an ordinary wind tunnel and to 10 percent in a tunnel with
a properly adjusted ceiling.
4.1.2 Plume Rise
The Briggs (1975) formulation as presented in Section 3.4 is
adopted here to provide an estimate of the plume center!ine height for
the source in the field. The GEP demonstration requires only that the
stack effluent density, exit speed, and diameter be appropriately
scaled to model the momentum length scale as discussed in Section 3.4.
Vertical profiles of concentration through the plume center!ine can be
used to provide a measurement of the elevated center!ine height in the
fluid model, where reflection from the surface, and influences from
terrain, buildings or other structures are not significant. The measured
elevated plume centerline height should be comparable to estimates as
discussed in Section 3.4. In all cases the plume height in the model
should be representative of plume rise in the field near the stack.
However, the plume height farther downwind of the source will not be
representative of plume rise in the field for highly buoyant sources.
29
-------
The exhaust from stacks is usually fully turbulent in the field.
The effluent Reynolds number cannot be matched to assure similarity. It
is sufficient, however, for the fluid model effluent Reynolds number to
exceed a critical value following the arguments of Reynolds number
independence. The fluid model stack effluent Reynolds number, W D/v,
should be guided (in order of decreasing "correctness") by the following:
(a) Fix the effluent Reynolds number to be as large as possible,
preferably greater than 15,000.
(b) If it is necessary to reduce the effluent Reynolds number
below 2,000, trip the flow to ensure a fully turbulent
exhaust. A smoke visualized effluent should be used to
demonstrate a fully turbulent exhaust.
(c) If it is desired to reduce the effluent Reynolds number
below 300, it will be necessary to do some experimentation to
determine under what conditions the plume will simulate the
behavior of a plume in the field.
Plume rise must be fixed by matching each of the following ratios:
Ws ps
___
V Pa Hs
which results in matching the momentum length scale. To model situation
with stack effluent downwash around the lip of the stack, the flow within
the boundary layer around the stack must be turbulent.
30
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4.1.3 Atmospheric Dispersion Comparability
In the absence of buildings, other surface structures, or large
roughness and/or elevated terrain, dispersion in the fluid model must
show comparability to that described for the atmosphere by the basic
Gaussian plume distribution (Turner, 1970). Concentration measurements
for this test of comparability must be compared with values representative
of field estimates given between estimates for Pasquill-Gifford category
C and D. The procedure for demonstrating this comparability is outlined
below. The purpose of this test is to provide an evaluation of the
model flow in absence of buildings, other surface structures or large
roughness, and/or elevated terrain. This test will insure that each
study shares some common ground and demonstrates comparability to
recommended modeling techniques for atmospheric dispersion over flat
terrain (EPA, 1978).
Step 1:
(a) Select model scale and the model flow velocity. In
choosing the scale, consideration should be given to all
criteria as outlined in Sections 4.1.1 and 4.1.2 The flow
velocity should be matched at the height of the proposed
stack.
(b) Select the position where the model stack will be placed.
(c) Select the method for providing a fully developed and
appropriate boundary layer at and downwind of the stack.
(d) Report a detailed description of the fluid model.
31
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Step 2:
(a) Measure the mean temperature 9 (°K) near the model surface and
at several positions within the freestream flow. Additional
profiles are necessary if operating conditions change. These
measurements are not necessary if model freestream speeds
exceed 3 m/s and the facility is temperature controlled during
the study.
(b) Take vertical profiles of the mean velocity U (m/s), and
the longitudinal turbulence intensity(u^yu, vertical
turbulence intensity(wz )^/u, and -uw (m2/s) near the position
where the stack will be placed, downwind at the end of the
planned study area, and midway between these two positions
(3 profiles each).
(c) Take lateral profiles of the mean velocity and longitudinal
turbulence intensity along the model surface and two elevated
profiles bracketing the range of plume heights evaluated in
the study near the position where the stack will be placed and
near the end of the planned study area (4-6 profiles each).
Step 3:
(a) Report and evaluate the temperature profiles. The profile of
mean temperature should be uniform. A deviation from a uniform
profile would indicate that air within the facility building
is not well mixed.
(b) Report and evaluate the velocity profiles. Report the vertical
profiles of mean velocity on log-linear scaled paper and
estimate the values for the effective surface roughness length
z and the friction velocity u* at each position, per
Equation 5. Estimate these values by determining the best
fit to the data representing the lowest 100 m, full scale,
above the height of the surface roughness elements. Replot
the profiles of mean velocity on linear scaled paper
and estimate the power law index p. The model values of z ,
u*, and p should be consistent with guidance presented in Table 1
and Figure 1, representing atmospheric flow over flat terrain
with z <0.2 m and 6 = 600m. Report the profiles of turbulence
intensity. Figure 3 is presented for consideration to be used
as a guide. Values of turbulence intensity representative of
conditions in Figure 3 fo z >0.2 m may indicate the model
flow is too turbulent. The best test lies with the evaluation
of concentration measurements as discussed below. The profiles
of mean velocity and profiles of turbulence intensity should
all be similar throughout the study area. Significant differences
32
-------
0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40
VJ/L
Figure 3. Variation of longitudinal turbulence intensity with height
under adiabatic conditions (from Snyder, 1981).
33
-------
in either the downwind direction and/or the lateral direction
may indicate a deficiency in the model design or in the facility
operation. No guidance can be presently specified for deciding
how much deviation is unacceptable. These profiles should be
used only to provide a qualitative assessment.
(c) Report and evaluate the profiles of -uvv after dividing by its
estimated value at the surface. The value at the surface is
equal to the surface friction velocity squared u*2, which
should be comparable to the estimate determined from the
velocity profile. These profiles should be used only to
provide a qualitative assessment.
Step 4:
(a) Position a model stack, so that the top of the model stack is
at a height representing 100 m above the ground. It is desirable
at each demonstration of comparability to use the same height.
A 100 m high stack was selected because it is believed to be
generally representative of the height of stacks for which GEP
demonstrations are conducted. A 200 m high stack should be used
for situations where GEP stack height is above 170 m. Design
the model stack so that its internal diameter is equal to
0.05 times the stack height. Fix the flow rate of a nonbuoyant
stack exhaust containing a tracer so that the exhaust velocity
is 1.5 times the mean velocity at stack top. This should allow
concentration measurements of the tracer to be taken in absence
of either plume rise or stack downwash. Concentration measurements
for this situation are required to demonstrate comparability to
atmospheric dispersion between that estimated using Pasquill-
Gifford dispersion parameters for category C and D as presented
in Turner (1970).
(b) Take vertical and lateral profiles of concentration through the
plume center!ine near the quarter intervals between the source
and the end of the planned study area (3 profiles each). At
least one of these profiles should clearly show the elevated
plume centerline height in order to provide an evaluation of
the represenative plume rise near the source. Otherwise, an
additional profile is necessary. Take a ground-level longitudinal
profile of concentration downwind along the surface ground-level
centerline to the end of the study area (1 profile). Determination
of the surface ground-level centerline should be supported
by several ground-level concentrations in the lateral.
34
-------
(c) Convert model concentrations to equivalent field values with
the form xUs/Q (m~2). Plot each vertical and lateral profile
of concentration measurement separately along with plume
estimates for both Pasquill-Gifford category C and D (Turner,
1970). Use the mean wind speed at the height of the top of
the stack U as the reference wind speed. The distribution
of measured values should fall between the two estimated
distributions. Estimated plume distributions incorporating
estimated dispersion parameters may be presented. Distinct
elevated center!ine concentrations should fall between the
estimates.
Plot the ground-level longitudinal profile of concentration
measurement along with plume estimates for both Pasquill-
Gifford category C and D (Turner, 1970). The distribution of
measured values should fall between the two estimated distri-
butions with an additional allowance where these two distri-
butions overlap as presented in Figure 4. The dashed lines
in Figure 4 allow a factor of two differences in the over-
lapping region. The concentrations should fall between the
estimates at least downwind to the distance of maximum
ground-level concentration. It is critical that the ground-
level concentration measurement here and the above vertical
and lateral concentration measurement not be representative of
estimates for more stable situations than category D.
Representation by categories more unstable than C are not
considered critical since a determination of the GEP stack
height will likely be less than that resulting under more
stable conditions since the more turbulent atmospheric flow
should somewhat overshadow the local building/terrain effect.
35
-------
2x10 5
5x10'6
CSI
E
cf
a
X2x10'6
5x10'7
2x10'7
0.5
DISTANCE, km
Figure 4. Ground-level concentration with distance for a 100m high
plume, estimated with Pasquill-Gifford dispersion parameters
for stability category C and D (Turner, 1970).
36
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4.2 Determination of GEP Stack Height
Requirements and procedures for evaluating the model boundary layer
characteristics and dispersion for the actual situation are specified
below. The results of the previous section establish fluid model comparability
to atmospheric dispersion as estimated by recommended mathematical
models in the absence of buildings, other structures or large roughness,
and/or elevated terrain. Differences between the model boundary layer
characteristics and dispersion for the standard situation established in
the previous section and the actual situation analyzed below should be
related to real expected differences in the field due to the effects of
buildings, other structures or large roughness, and/or elevated terrain.
37
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4.2.1 Demonstration of Adverse Effects
The procedure for demonstration and documentation of the adverse
effects of buildings and/or elevated terrain nearby the source is outlined
below.
Step 1: Place the model topography into the test facility.
(a) For modeling the situation of a few isolated buildings in
flat terrain, this is a simple matter. The buildings are
simply immersed in the boundary layer designed for satisfying
the requirements of Section 4.1.3.
(b) Additional complexity arises for a model of flow over a
general urban area and/or elevated terrain. In addition
to constructing the model of buildings and/or elevated
terrain, surface roughness used in covering the model and
general roughness elements upwind of the model to provide
appropriate boundary layer characteristics for the situation
may be different from those used in the atmospheric dis-
persion comparability test (Section 4.1.3.).
(c) Report a detailed description of the fluid model.
Step 2:
(a) Take profiles of mean velocity and longitudinal turbulence
intensity as in Step 2 (b) Section 4.1.3 (3 profiles each).
(b) Take profiles of mean velocity and longitudinal turbulence
intensity as in Step 2 (c) Section 4.1.3 (4-6 profiles each).
(note) There is no need to repeat measurements that would have
values identical to those satisfying requirements in Section 4.1.3.
(i.e., measurement in the same boundary layer beyond the influence
of the building and/or terrain).
Step 3:
(a) Follow Step 3 (b) Section 4.1.3 to determine the model
values of z , u*, and p. They should be consistent with
guidance presented in Table 1 and Figure 1. Specific
guidance for flow over areas of elevated terrain is not
available since little data correlating the flow character-
istics to the height and separation distances of terrain
38
-------
features has been taken. In general, z in areas with elevated
terrain should be much larger than the largest values
found in Table 1. Significant differences may be found
at each location due to differences in the local surface
roughness. A discussion of such differences should be pre-
sented in the study report.
(b) Follow Step 3 (c) Section 4.1.3.
Step 4: Test for Reynolds number independence.
(a) For sharp-edged obstacles having a Reynolds number U,,L/v
greater than 11,000, no demonstration test is required.
(b) Where the effects of flow over elevated terrain or smooth-
shaped obstacles are being evaluated, a Reynolds number test
is required. A full evaluation of Reynolds number inde-
pendence would require a demanding research project for each
situation. The simple test required here should, however,
be sufficient enough to provide a critical evaluation.
Position a small source emitting a tracer at the site of
the GEP stack in question at a height equal to the building
or elevated terrain whose effects are in question. The
source should be nonbuoyant and have no plume rise. Take
a longitudinal surface-level profile of concentration along
the downwind direction. Repeat the profile after at least
doubling the freestream wind speed U^. Take a vertical profile
of the mean velocity at the site of the stack for this new
situation. Plot and compare the two profiles of concentration
xLI/Q (m~2) using the freestream wind speed as the reference
speed. Differences in concentration should not be greater than
10 percent. Reconsideration of the model design is necessary
where greater differences are observed.
Step 5: An evaluation of the plume from the stack in question must be
made. In general, the fluid modeler should first examine the plume
through a visualization technique, i.e., photographs of smoke exhaust.
Then a decision can be made as to the height for which GEP credit can be
justified. Photographs and/or measured data taken as part of this
process must be included in this report. Full documentation as outlined
below is required for the actual determination of GEP stack height.
(a) Take vertical and lateral profiles of concentration through
the plume centerline at positions one-fourth and one-half of
the distance between the source and the end of the study area.
Also, do the same at the end of the study area and at the
position of maximum ground-level concentrations. The value of
39
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maximum ground-level concentration must be unquestionably
determined. This requires a longitudinal surface-level
profile along the plume centerline, supported by 2 to 4 partial
lateral profiles including one across the position of maximum
ground-level concentration.
In some situations, it may be necessary to model at a scale
such that the likely maximum ground-level concentration falls
downwind beyond the modeled area. This is very undesirable,
and should be avoided whenever possible. Before such an
approach is planned, the agency should first review a proposal,
discussing its necessity. The diffusive characteristics
for a limited region beyond the modeled area may be
obtained by extrapolating the measured values, only for
situations of flow over generally homogeneous terrain and/or
uniform urban environments. For these situations, additional
vertical and lateral profiles of concentration at the position
three-fourths between the source and the end of the study area
should be made in lieu of measurements at the maximum. Extrap-
olation should be limited to distances equivalent to one-half
the distance between the source and end of the modeled area.
Ground-level concentration profiles for several stack
heights having their maximum value falling within the
modeled area should be made. These profiles can be used
to extrapolate a maximum value for higher stack heights,
and support the value obtained by extrapolating the measured
vertical and lateral profiles of concentration.
(b) Convert model concentrations to equivalent field values with
form xUs/Q(m"2). Take the mean wind speed at the height of
the top of the stack U as the reference wind speed. Report
each vertical profile of concentration measurement. In absence
of reflection from the surface, the plume centerline can be
estimated as the vertical position of the maximum concen-
tration. At least one value must be compared to the esti-
mated plume rise as discussed in Section 4.1.2. Additional
vertical profiles must be measured if the above required
profiles do not provide sufficient information. Report
each vertical and lateral profile of concentration measure-
ments separately along with estimated plume distributions
incorporating estimated dispersion parameters.
In areas of elevated terrain downwind from the source, such
estimates may be difficult and perhaps meaningless. In
these instances a discussion relating the plume behavior to
anticipated effects of the terrain is needed. Plot the longi-
tudinal and lateral profiles of concentration measurements.
40
-------
The maximum must be unquestionably determined. Two repeated
measurements at the positions of the maximum should be taken
and reported as support that the concentration does, in fact,
represent the steady-state average. Differences in the
three concentrations should not be greater than 10 percent
of their values.
In some situations of flow over generally homogeneous terrain
and/or uniform urban environments, the maximum value may be
obtained by extrapolation as discussed above. For these
instances, it is necessary that the appropriate dispersion
parameters can be derived from the vertical and lateral
profiles of concentration measurements. The maximum ground-
level concentration can then be estimated by inserting
the derived dispersion parameters into the Gaussian plume
formula (Turner, 1970). Extrapolation of measured longitudinal
ground-level profiles should support the estimated maximum
ground-level concentration. Approval for such a study plan
must be granted by the reviewing agency prior to the study to
allow agency experts to provide a critical assessment.
4.2.2 GEP Stack Height
The stack height for which full documentation has been provided in
the previous sub-section is GEP if the maximum surface-level concen-
tration is 40 percent or more in excess of the maximum in the absence of
downwash, wake, or eddy effects produced by nearby structures or terrain.
The procedure for validating the proposed GEP stack height is presented
below.
Step 1:
Remove the building(s) or elevated terrain in question. This
is a simple matter in the case of buildings nearby the stack.
The situation near elevated terrain is complicated since
removal of the terrain feature in question may result in an
unrealistic discontinuity in the topography. A similar difficulty
arises where a high plateau is upwind of the source. In
such instances it may be necessary to remove all upwind
terrain and replace its area with appropriate surface roughness.
The surface roughness elements must be shown to result in an
41
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Step 2:
appropriate zo and u*. This requires -that a vertical profile
of mean velocity, longitudinal turbulence intensity, and
shear stress be measured upwind of the source at the position
where the elevated terrain feature was located. Estimate z0
and u* guided by Step 3 (a,b) Section 4.2.1. The boundary
layer should be appropriately characterized as with the
actual topography.
Determine the maximum ground-level concentration. Document
fully as required by Step 5, Section 4.2.1.
Step 3:
The proposed stack height is creditable as GEP if the maximum
ground-level concentration determined in Step 5, Section 4.2.1
is at least 40 percent in excess of the maximum ground-level
concentration determined in Step 2 above. Discussion relating
the increased maximum ground-level concentration measured
in the presence of the building(s) or terrain in question
to anticipated effects due to downwash, wakes, or eddies
should be presented in the report.
42
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5.0 REPORT CHECKLIST
The fluid modeling study report should include the five items that
are outlined below. The report should completely document the design
and operation of the model study. Three tests should be conducted. The
data collected for these tests must allow for conclusions to be drawn
concerning the atmospheric conditions simulated by the fluid model, and
the cause of increased maximum ground-level concentrations for a stack
in the presence of nearby building(s) and/or terrain. The height of the
stack examined in the study is creditable as GEP if the maximum ground-
level concentration is at least 40 percent greater in the presence of
the nearby building(s) and/or terrain than that measured in their absence.
The report should include at least the following:
1. A detailed topographic map and discussion concerning the
selection of the size of the modeled area and meteorological parameters.
2. Discussion of model design and similarity criteria.
3. An evaluation of the test facility in the absence of building(s),
other surface structures, or large roughness and/or elevated terrain
including:
(a) a detailed description of the fluid model including features
of the scale model, surface roughness, velocity profile, and method used
to provide the fully developed boundary layer,
(b) one representative vertical profile of mean temperature if
model free-stream speed is less than 3 m/s,
43
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(c) three vertical profiles of mean velocity,
(d) three vertical profiles of longitudinal and vertical
turbulence intensity,
(e) three vertical profiles of uw,
(f) four lateral profiles of mean velocity,
(g) four to six lateral profiles of longitudinal turbulence
intensity,
(h) effective surface roughness length z , friction velocity
u*, and velocity power law index p, determined by evaluating the mean
velocity profiles and the shear stress profile,
(i) three vertical and lateral profiles of concentration
through the elevated centerline of the plume,
(j) one ground-level longitudinal profile of concentration
downwind along the plume centerline,
(k) evaluation of comparability of measured concentrations to
plume estimates for Pasquill-Gifford category C and D (Turner, 1970),
(1) evaluation of the measured elevated centerline of the plume.
4. Documentation for the GEP stack height test in the presence of
building(s), other surface structures, or large roughness and/or elevated
terrain should include:
44
-------
(a) detailed description of the fluid model including features
of the scale model, surface roughness, velocity profile, and the method
used to provide the fully developed boundary layer,
(b) three vertical profiles of mean velocity,
(c) three vertical profiles of longitudinal and vertical
turbulence intensity,
(d) three vertical profile of uw ,
(e) effective surface roughness length ZQ, friction velocity
u*, and velocity power law index p, determined by evaluating the mean
velocity profiles and the uw profi1es,
(f) possible test for Reynolds number independence,
(g) four vertical and lateral profiles of concentraion
through the elevated center!ine of the plume including profiles at the
position of maximum ground-level concentration,
(h) one ground-level longitudinal profile of concentration
downwind along the plume centerline at ground-level,
(i) two to four lateral ground-level profiles including one at
the position of maximum ground-level concentration,
(j) discussion supporting the unquestionable determination of
the maximum ground-level concentration,
(k) evaluation of the measured elevated centerline of the plume.
45
-------
5. Documentation for the GEP stack height test in the absence of
building(s) or elevated terrain considered in justifying the stack
height should include:
(a) the same as steps (g), (h), (i), (j), and (k) above,
(b) discussion relating the increased maximum ground-level
concentration measured in the presence of the building(s) or elevated
terrain in question to anticipated effects due to downwash, wakes, or
eddies,
6. A separate section or appendix describing the fluid modeling
facility, instrumentation used in the conduct of the study, and their
normal operating conditions and associated parameters.
46
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6.0 REFERENCES
Briggs, G. A., 1975: Plume Rise Predictions. In Lectures on Air Pollution
and Environmental Impact Analysis, American Meteorological Society,
Boston, Massachusetts.
Counihan, J., 1975: Adiabatic Atmospheric Boundary Layers: A Review
and Analysis of Data from the Period 1880-1972. Atmos. Envir., 9_, (10),
pp. 871-905.
Environmental Protection Agency, 1978: Guideline on Air Quality Models.
EPA-450/2-78-027, Research Triangle Park, North Carolina.
Environmental Protection Agency, 1980: Guideline for Determination of
Good Engineering Practice Stack Height (Technical Support Document
for Stack Height Regulations), EPA-450/4-80-023, Research Triangle Park,
North Carolina.
Irwin, J. S., 1979: A Theoretical Variation of the Wind Profile Power-
Law Exponent as a Function of Surface Roughness and Stability. Atmos.
Envir., 13., (1), pp. 191-194.
Simiu, E. and R. H. Scanlan, 1978: Mind Effects on Structures. John Wiley
and Sons, New York, New York, 458 p.
Snyder, William H., 1981: Guideline for Fluid Modeling of Atmospheric
Diffusion. EPA-600/8-81-009, Research Triangle Park, North Carolina.
Turner, D. B., 1970: Workbook of Atmospheric Dispersion Estimates.
Publication No. AP-26, Office of Air Programs, Environmental Protection
Agency, Research Triangle Park, North Carolina.
Zoric, D. L. and V. A. Sandborn, 1972: Similarity of Large Reynolds
Number Boundary Layers. Boundary Layer Meteorol., 2_, pp. 326-33.
-------
TECHNICAL REPORT DATA
(P!eare read in>tru<.tion; on tlu reverse before comf'irting)
1. REPORT NO. 1 2.
EPA-450/4-81-003 |
4. TITLE AND SUBTITLE
Guideline for Use of Fluid Modeling to Determine Good
Engineering Practice Stack Height
7. AUTHOR(S)
9. PERFORMING ORGANIZATION NAME AND ADDRESS
U.S. Environmental Protection Agency
Office of Air Quality Planning and Standards
Research Triangle Park, North Carolina 27711
12. SPONSORING AGENCY NAME AND ADDRESS
3, H£CfP'E:MT'S ACCESSION NO.
I
5 REPORT DATE
July 1981
6 PERFORMING ORGANIZATION CODE
8. PERFORMING ORGANIZATION REPORT NO.
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
13. TYPE OF REPORT AND PERIOD COVERED
14. SPONSORING AGENCY CODE
15. SUPPLEMENTARY NOTES
16. ABSTRACT
Guidelines for developing and reviewing fluid modeling studies for
determining good engineering practice stack height. Includes review of appropriate
fluid modeling theory and a specific report requirement checklist.
17. KEY WORDS AND DOCUMENT ANALYSIS
a. DESCRIPTORS |b. I DENTI F IERS/OPEN ENDED TERMS C. COSATI Field/Group
Air Pollution j Air Pollu
Good Engineering Practice Stack Height
Physical Modeling
Fluid Modeling
I
18. DISTRIBUTION STATEMENT 19 SCCU^.fv
tion Control 13-B
CLASS , r>r^ f~'fc->rtj 21 NC. OF PAGES
Unlimited 20 SFCUR.TY';LA:?3 Tmspajie' '22 PRICE
unclassified
^
EPA Form 2220-1 (Rev. 4-77) PREVIOUS EDITION i s OBSOLETE
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