United States
Environmental Protection
Agency
Atmospheric Research and Exposure
Assessment Laboratory
Research Triangle Park. NC 27711
EPA/600/S3-90/025 Aug. 1990
4>EPA Project Summary
Flow and Dispersion of
Pollutants within Two-
Dimensional Valleys: Summary
Report on Joint Soviet-American
Study
LH. Khurshudyan, W.H. Snyder, I.V. Nekrasov, R.E. Lawson Jr
R.S. Thompson, and F.A. Schiermeier ^wson, Jr/
Wind-tunnel experiments and a
theoretical model concerning the
flow structure and pollutant diffusion
over two-dimensional valleys of
varying aspect ratio are described
and compared. Three model valleys
were used, having small, medium
and steep slopes. Measurements of
mean and turbulent velocity fields
were made upstream, within and
downwind of each of these valleys
Concentration distributions were
measured downwind of tracer
sources placed at an array of
locations within each of the valleys
The data are displayed as maps of
terrain amplification factors, defined
as the ratios of maximum ground-
level concentrations in the presence
of the valleys to the maxima observed
from sources of the same height
located in flat terrain. Maps are also
provided showing the distance to
locations of the maximum ground-
level concentrations. The
concentration patterns are
interpreted in terms of the detailed
flow structure measured in the
valleys. These data were also
compared with results of a
mathematical model for treating flow
and dispersion over two-dimensional
complex terrain. This model used the
wind-tunnel measurements to
generate mean flow fields and eddy
diffusivities, and these were applied
in the numerical solution of the
diffusion equation. Measured
concentration fields were predicted
reasonably well by this model for the
valley of small slope and somewhat
Lion W6o f°r the valley of medi<-""
slope. Because flow separation was
observed within the steepest valley
the model was not applied in this
case.
This Project Summary was
developed by EPA's Atmospheric
Hesearch and Exposure Assessment
Laboratory, flesearcn Triangle Park
NC, to announce key findings of the
research project that is fully
documented in a separate report of
the same title (see Project Report
ordering information at back).
Introduction
This report presents results of the
Joint Soviet-American Work Program for
studying air flows and dispersion of
pollutants within valleys. The work was
conducted in the Fluid Modeling Facility
of the U.S. Environmental Protection
Agency (EPA), Research Triangle Park,
rMv_<.
Investigations of air pollution
transport and dispersion in the
atmosphere within valleys are essential
for the protection of air quality, because
industrial enterprises and other sources
of air pollution locate predominantly
within valleys. Although much effort has
already been expended in elucidating this
problem and establishing guidelines for
industry and air pollution control
organizations to follow in the prediction of
concentrations from sources located
within complex terrain, the problem is far
-------
from solved. Given the preponderance of
populations and industry located within
valleys, the lack of concerted research
efforts on flow structure and dispersion
within valleys is rather surprising,
particularly in comparison with the efforts
expended on flow and dispersion over
hills.
This study is a natural complement
to earlier work by Khurshudyan et al.
(1981), wherein similar measurements
were made of the flow structure and
dispersion of pollutants over two-
dimensional hills. An extensive data set
was collected in a wind tunnel on the flow
structure and concentration fields
resulting from sources placed within
three valleys with different width-to-depth
ratios. In addition to furthering basic
understanding of the physics, one of the
main purposes of the experimental study
was to test the performance of a diffusion
model for calculating maximum ground-
level concentrations (glcs) resulting from
elevated point sources placed within the
valleys.
The primary results of our study are
presented in terms of terrain amplification
factors (TAFs), defined as the ratios of
maximum ground-level concentrations in
the presence of the valleys to the
maximum glcs from sources of the same
height in the absence of the valleys (in
flat terrain). This definition does not
involve the locations of the maximum
glcs; these maxima will generally occur at
very different downwind distances from
the source in the two situations (with and
without the valley), but it is only the
values of the maxima which are
compared, wherever they occur. In the
U.S.S.R., the TAP is used directly in the
regulatory framework. The U.S. EPA, on
the other hand, uses a related "excess
concentration"; it is related to the TAP
through the additive factor of unity; that
is, a "zero" excess concentration is
equivalent to a TAP of 1.0, and a 40%
excess concentration is equivalent to a
TAP of 1.4.
Following the lead of Lawson et al.
(1989), we present the primary results as
contour plots of constant TAP. This
allows the further introduction of
"windows" of excess concentration. If a
source is far enough upstream of the
valley and the pollutant is released at low
level, the maximum glc will occur
upstream of the valley, so that the effect
of the valley will be negligible. If the
source is tall enough, the maximum glc
will occur far downstream of the valley,
so that, again, the effect of the valley will
be negligible. If pollutants are released
within the valley, however, the TAP will
generally exceed unity, and its value will
depend upon the source location within
the valley. Hence, a region or "window"
will exist such that pollutants emitted
within that window will result in
"excessive" glcs.
Lawson et al. (1989) have presented
"windows" of excess concentration for
typical shapes of two- and three-
dimensional hills that extended as far as
10 to 15 hill heights upstream and
downstream of the hills. The purpose of
the current study was to determine to
what extent valleys might influence
maximum glcs, that is, to establish values
and windows of excess concentration for
typical valley shapes that might be found
in the real world. The valleys chosen
were two-dimensional shapes with three
different aspect ratios, n = a/h = 3, 5,
and 8, where a is the valley half-width,
and h is the valley depth (height). The
valleys will hereafter be referred to
according to their aspect ratio; that is, as
valley 3, valley 5, and valley 8 for n = 3,
5 and 8, respectively. These valley
shapes were chosen to represent a fairly
typical range of realistic valley shapes.
The maximum slopes were 10° (valley 8),
16o (valley 5), and 26° (valley 3). As will
be shown later, the flow structure in these
valleys differed rather dramatically from
one to the next. In valley 8, the flow did
not separate, but nevertheless, the
influence of the valley on the TAFs was
significant. In valley 3, the flow clearly
separated on the upstream slope, and a
mean recirculation region was formed
inside the valley; this had important
influences on the TAFs. In valley 5, the
separation might be described as
incipient; the mean flow was downstream
everywhere, but instantaneous flow
reversals were commonly observed.
Thus, pollutants emitted within this flow
were frequently wafted back and forth
before being transported downstream,
and large TAFs were measured within
this valley.
Extensive measurements of the flow
structure within each of the three valleys
were made, and we attempt to interpret
the results (the TAP plots) in terms of the
flow structure.
Simultaneously with the measure-
ments in the wind tunnel, a theoretical
model (Elerlyand et al., 1975) was used to
calculate terrain amplification factors for
sources located within the valleys. This
model used wind-tunnel data on the flow
structure as input for numerical solution
ot the turbulent diffusion equation. As
will be seen later, the model provides
quite reasonable predictions of TAFs for
the valley of intermediate slope and
somewhat better predictions for t
gently-sloped valley. Because the moc
cannot handle separated flows, it was r
applied to the steep-sloped valley.
Apparatus, Instrumentation,
and Measurement Techniques
The model valleys were place
within the EPA Meteorological Wir
Tunnel, which has a test section 3.7
wide, 2.1 m high and 18.3 m long. Tf
approach flow was a simulate
atmospheric boundary layer, generate
using a fence and gravel roughnes
Extensive measurements of both the fla
terrain boundary layer and the flo
structure within the valleys were mac
using hot-wire and pulsed-wir
anemometry. Ethane gas, used as
tracer, was released from numerou
positions within each valley through
perforated hollow sphere to simulate
neutrally buoyant point source
Concentration measurements were mad
downstream using flame-ionizatio
detectors. More extensive description
of the experimental apparatus am
measurement techniques may be fount
in Snyder et al. 1990) or Khurshudyan e
a/. (1990).
A large number of concentratior
profiles (approximately 170) wen
measured. Primary stack heights were
Hs/h = 0.25, 0.50, 1.0, and 1.50. Primary
stack positions were at the upstrearr
edge (x/a = -1.0), center (x/a =0.0 anc
downstream edge (x/a = 1.0) of eacr
valley. Full surface concentration profiles
were measured with each of these stack
heights and locations within each valley
(and in flat terrain). Abbreviated surface
profiles, sufficient to determine the value
and location of the maximum ground-
level concentration were made at
intermediate stack heights and locations.
Lateral and vertical concentration profiles
were measured at only a few downwind
positions for a very limited number of
stack positions and heights.
Presentation and Discussion of
Experimental Results
Sufficient measurements were made
of the flat-terrain boundary-layer structure
and its dispersive characteristics to
ascertain that it was a reasonable
simulation of the neutral atmospheric
boundary layer. The free-stream wind
speed was 4ms-1.
The pulsed-wire anemometer proved
to be quite useful within the very highly
turbulent, separated flows within the
valleys, because it can sense flow
reversals. Probability density distributions
-------
f longitudinal velocity fluctuations were
onstructed from the pulsed-wire
leasurements. They showed that at the
nwest levels within valley 5, mean
elocities were quite small, but
istantaneous flow reversals were very
;ommon (up to 40% of the time). In
'alley 3, mean velocities were negative
)elow h/4, very close to zero at h/2, and
some reversals occurred even at the
/alley top h. In spite of the flow reversals
and very large turbulence intensities (up
:o 170%), for many practical purposes,
:he distributions were closely Gaussian in
:haracter.
Mean streamlines were calculated
from the mean velocity measurements
within the valleys. For valleys 5 and 8,
mass-consistent wind fields were
computed by using the model described
later. For valley 3, the mean velocity
profiles were simply integrated to find the
elevations of specific values of the
stream function. These streamline
patterns are displayed in Figure 1.
At first glance, the streamline pattern
over valley 8 is reminiscent of potential
flow; but closer examination reveals it is
asymmetrical, with the lower streamlines
being considerably closer to the surface
on the downwind slope than on the
upwind one. The streamline pattern over
valley 5 is clearly asymmetrical, and
because the streamlines diverge strongly
away from the surface, it is clear that the
velocity is reduced markedly at the valley
center; indeed, it appears that a
stagnation region exists in the valley
bottom. In valley 3, the streamline
pattern clearly shows a recirculation
region, with separation occurring a short
distance down the upwind slope and
reattachment occurring about halfway up
the downwind slope from the valley
center. The three valley shapes thus
result in three fundamentally different
flow patterns. These basic flow
structures are fairly typical and cover the
range of patterns to be observed at full
scale, albeit in neutral stratification.
These data have important
implications for the behavior of pollutants
released within these valleys. Two
primary features of two-dimensional
neutral flow affect glcs: the displacement
of the mean streamlines and the changes
in turbulence. The displacement of
streamlines determines how near to the
surface the centerline of a plume will
reach. The convergence and divergence
of the streamlines affect the plume width
(a) directly and (b) indirectly through their
distorting effects on the velocity
gradients, which in turn affect the
turbulence. The turbulence itself, of
course, spreads and diffuses the plume.
All of these effects can either increase or
decrease surface concentrations. Note
that, because the flow is two-dimensional,
the mean streamlines remain in vertical
planes. However, the longitudinal and
vertical turbulence intensities are greatly
increased through the flow distortions
and, because of the tendency of
turbulence to distribute its energy equally
in all directions, the lateral turbulence
intensities will also be greatly increased.
It is useful to examine in some detail
the flow structure within the three valleys
with a view toward anticipating the
behavior of pollutants released within
them. Consider first the mean streamline
pattern over valley 8 (Figure 1c).
Because it displays the smallest flow
distortions of the three valleys, we may
expect the; smallest changes (from flat
terrain) in maximum surface
concentrations, (i.e., TAFs closest to 1.0).
Because the mean streamlines begin to
diverge at the upstream edges of the
valleys, wo might expect maximum glcs
from sources placed there to be less than
those observed in flat terrain (TAFs less
than unity). On the upwind slope, the
mean streamlines diverge so that plumes
released there would be transported
farther from the surface. However,
because the turbulence is increased,
these plumes would be diffused more
rapidly to the surface. With these
counteracting tendencies, it is difficult to
speculate on the net effects except to
state that we would not expect the largest
TAFs to occur with sources located in
this position. For sources placed above
the valley center, it is easily seen that the
streamlines transport the plumes closer
to the surface and the enhanced
turbulence apidly diffuses a plume to the
surface. Hence, we may expect the
largest TAFs from sources placed in the
valley center. For sources placed at the
downwind odge of the valley, we again
observe the counteracting effects of
streamline divergence but increased
turbulence, and we expect the TAFs to
be near unity again. Note that enhanced
lateral diffusion will diminish surface
concentrations along the plume axis, but
increase the area of coverage'in the
lateral direction.
For valleys 5 and 3, we may expect
roughly the same behavior from sources
placed near the upstream and
downstream edges of the valleys, i.e.,
TAFs near unity. For low sources above
the center of valley 5, however, we
observe very small mean transport
speeds and very common flow reversals.
We may expect the plumes to be wafted
back and forth while being diffused
strongly in the lateral and vertical
directions before eventually being
transported downstream. Thus, we may
expect quite large TAFs for low sources
near the center of valley 5.
In valley 3, the streamline patterns
show a definite recirculation region that
extends to nearly 75% of the valley
depth. Plumes released on the
separation/reattachment streamline will
be transported directly to the surface.
We may thus expect very large TAFs
from such releases. Plumes released
well below the separation/reattachment
streamline, say in the reversed flow
region around h/4, would be transported
upstream in a more routine manner, with
the plume axis remaining nearly parallel
to the surface. Because of the very large
turbulence intensities and relatively low
transport speeds, we may still expect
large TAFs (but not as large as those
from the higher sources).
Figure 2 illustrates some typical
comparisons between surface
concentration profiles measured from
sources placed above the valley centers
and that from a source of the same
height in flat terrain. In all cases, the
stack height Hs was equal to one-half the
valley depth h. xs denotes the distance
from the source. The increased
concentrations caused by the valleys are
dramatic and the TAFs range from about
2.5 in valley 8 to about 12 in valley 3. As
the concentration increases, the distance
to the maximum decreases. The location
of the maximum for valley 3 was actually
slightly upwind of the source. For valley
5, the location of the maximum glc was
downstream, but very close -- about 2
stack heights away -- and the TAF is
about 6.
Measurements such as these were
made at an array of source locations and
heights in the vicinity of each of the three
valleys, and the TAFs were determined
for each location. Maps of these TAFs
are shown in Figure 3, where isopleths of
constant TAF have been drawn. The first
impression is that the patterns are
symmetrical about the vertical centerline,
but closer examination reveals some
asymmetry. Nevertheless, the near-
symmetry and the overall similarity in
shape amongst the three valleys is quite
surprising in view of the very different
flow patterns observed. In contrast, the
magnitudes of the maximum TAFs differ
widely, from 2.5 in valley 8 to 15 in valley
3. These differences, of course, reflect
the effects of the different flow structures.
Contours with TAF values of 1.4, 2,
4, etc., have been drawn where
-------
1.25
-1.25 -1
1.25
Figure 1. Streamline patterns derived from experimental measurements over the valleys. Note that the
vertical scales are exaggerated.
-------
A Valley 3
a Valley 5
O Valley 8
^— Flat terrain
I1 '"I" " I "" I1'" I""
2345678
x./h
Figure 2. Comparison of surface concentration profiles from sources placed within the
valleys with one from a source of the same height in flat terrain. Hs = h/2. Source
position is at the center of the valley (x/a =0).
appropriate. Note that these contours
form "windows" within which the
maximum glc exceeds the glc that occurs
in flat terrain by 40%, 100%, 300%, etc.
The longitudinal extent of the window of
40% excess concentration extends over
approximately 60% of the width of valley
8, 80% of the width of valley 5, and more
than 90% of the width of valley 3. The
vertical extent of the 40% window is 1.5,
2.0, and 2.5 valley heights above the
valley top for valleys 8, 5, and 3,
respectively.
Application of the data in Figure 3 is
straightforward. Let us consider a source
which is located in the center of a rather
broad valley, say one similar in shape to
valley 8; and the height of the source is
half the valley height. Figure 3c suggests
that the maximum glc would be about 2.5
times that expected from a source of the
same height but located in flat terrain.
On the other hand, if the valley were
considerably narrower, say close to
valley 5, Figure 3b suggests the
maximum glc would be about 7 times as
large as that from the same source in flat
terrain. Although precise interpolation of
these results for valleys intermediate in
shape and slope to those examined here
may be difficult, the results allow us to
place some useful limits on the effects of
valleys of intermediate shape and slope.
Figure 4 shows the loci of source
positions leading to the same locations of
maximum glc. These loci have been
identified by marking them with the
position of the maximum glc (in valley
heights from the centers of the valleys).
Note that the "undisturbed" or flat-terrain
loci (dotted lines) are simply parallel,
nearly straight, diagonal lines. Within the
valley, these loci are distorted, as shown
by the solid lines. The diagrams may be
used as follows: for any given source
position, we may plot that position on the
diagram, then follow the locus to the
ground; the intersection of that locus with
the ground is, of course, the location of
the maximum glc. Conversely, from a
knowledge of the location of the
maximum glc, we may use these
diagrams to determine the line along
which the source was positioned. These
loci become highly distorted near the
valley centers, and the steeper the valley,
the higher the distortion. As the distance
(both longitudinal and vertical) from the
valley center increases, these loci
gradually relax to their undisturbed or
flat-terrain values.
Numerical Model and
Comparisons with Experimental
Results
One of the main purposes of the
experimental study described above was
to test the applicability of a diffusion
model for the evaluation of maximum
glcs resulting from elevated continuous
point sources placed in a curvilinear
neutral atmospheric boundary layer. The
model used data from the wind-tunnel
measurements of wind velocities and
turbulence characteristics as input
-------
-7.5
-1.5
-1.0
-0.5
0.0
x/a
0.5
-4-
1.0
-\
1.5
-i.O
-0.5
0.0
x/a
—h
0.5
1.0
2.5
2.0
1.5
1.0
0.5
0.0
-0.5
-1.0
25
2.0
1.5
1.0
0.5
0.0
-0.5
-1.0
H-
1.0
H
1.5
-1.5
-1 0
-0.5
0.0
x/a
0.5
1.0
2.5
2.0
1.5
1.0
0.5
0.0
-0.5
-1.0
-h-
1.0
H
1.5
Figure 3. Contours of constant terrain amplification factor derived from experimental
measurements.
parameters to calculate two-dimensional,
mass-consistent flow fields. These flow
fields were then applied in the numerical
solution of the diffusion equation. Such a
model was previously developed by
Berlyand et al. (1975), primarily for
evaluation of pollutant dispersion in
complex terrain.
The present version of the model
does not incorporate a longitudinal
diffusion term, and therefore does not
calculate the spread of pollutants in
separated flows. Thus, no attempt was
made to apply the model to valley 3, and
calculations were made only for valleys 5
and 8 (and, of course, for flat terrain so
that TAFs could be computed).
Contour maps of constant TAP as
predicted by the model are shown in
Figure 5. These are to be compared with
the measurements shown in Figure 3.
The maps for valley 8 show generally
similar overall patterns, but differ in
-------
z/h
zlh
J--7.0
-7.5
zlh
-12
12
Figure 4. Distance in valley depths from the valley center to the location of maximum ground-level
concentration. Flat terrain values are indicated as dotted lines.
several details. The vertical extent of the
40%-excess window (the TAP = 1.4
contour) extends to about 1.5 h from the
measurements, but to only 1.25 h from
the model predictions. The horizontal
extent of the measured window is
somewhat larger than that of the model-
predicted window. The model-predicted
window is shifted somewhat upstream
and, whereas the resolution of the grid
used for the experimental measurements
was rather coarse, a hint of an upstream
shift is also observed there. The model
generally predicts larger TAFs to occur at
lower elevations, whereas the
measurements show elevated maxima.
Both predicted and observed TAFs were
generally less than unity when the source
was at the upstream or downstream edge
of the valley. Maximum TAF values are
quite close to one another. Generally
similar statements may be made when
comparing the calculated and observed
TAF maps for valley 5, but the
differences are somewhat larger.
Conclusions
The laboratory work provided a
reasonable simulation of the flow
structure and diffusion characteristics of
the neutral atmospheric boundary layer.
The model valleys were idealized in
shape, but cover the range of a majority
of valleys to be found at full scale, at
least in terms of the basic classes of flow
-------
'1 5
-1-°
-0.5
0.0
x/a
0.5
2.5
2.0
7.5
1.0
0.5
0.0
-0.5
-1.0
1.0
1.5
-1.5
-10
-0.5
0.0
x/a
-f-
0.5
1.0
H
1.5
Figure 5. Contours of constant terrain amplification factor derived from model calculations.
structure that may be observed. Valley 8
was rather gentle in slope, and the flow
over it may be characterized as relatively
smooth and well-behaved. Valley 5,
being steeper in slope, caused the flow to
separate intermittently, but not in the
mean. In valley 3, the steepest, the flow
clearly separated a short distance from
the upstream edge, and a recirculating
flow was formed within the valley.
Pollutants released at the same relative
locations within each of these valleys
behave very differently from one another,
and the resulting surface concentration
patterns are dramatically different.
The overall effects of the valleys on
surface concentrations are characterized
in terms of terrain amplification factors
(TAFs), defined as the ratios of maximum
ground-level concentrations from sources
located within the valleys to the maxima
that would exist from identical sources
located on flat terrain. Maps of these
TAF:s are provided for each valley. Also
provided are maps detailing the distances
to locations where these maximum
ground-level concentrations will occur.
These maps allow a practitioner to
quickly and easily assess the likely
impact of a source located in a valley and
to identify the location where that
maximum impact will occur.
A two-dimensional theoretical model
that uses a variational analysis technique
was applied to the wind-tunnel
measurements of the flow structure near
the valleys to produce mass-consistent
mean wind fields. Measurements of the
turbulent fluctuating velocities were also
used to calculate vertical and crosswind
eddy diffusivities. The diffusion equation
was then solved numerically to obtain
maximum ground-level concentrations
from elevated point sources of various
heights near valleys 8 and 5, as well as
over flat terrain. The calculated and
measured concentrations for flat terrain
showed good agreement. Comparison of
calculated and measured TAFs for valley
8 showed satisfactory agreement. Valley
5 exhibited more severe streamline
distortion and a stagnation region with
large fluctuating velocities near the
bottom of the valley, and therefore the
differences between calculated and
measured TAFs were significant in some
cases.
References
Berlyand M.E., Genikhovich E.L. and
Khurshudyan L.H. (1975) Use of the
results of modeling of an air stream in
wind tunnels for the calculation of air
pollution. Atmos. Diff. & Air Poll.,
Trudy GGO, Hydromet Press,
Leningrad, USSR (EPA Trans. TR-79-
0431)352,3-15.
Khurshudyan L.H., Snyder W.H. and
Nekrasov I.V. (1981) Flow and
-------
dispersion of pollutants over two-
dimensional hills: summary report on
joint Soviet- American study. EPA-
600/4-81-067, U.S. Environmental
Protection Agency, Research Triangle
Park, NC 143p.
Khurshudyan L.H., Snyder W.H.,
Nekrasov I.V., Lawson R.E. Jr.,
Thompson R.S. and Schiermeier F.A.
(1990) Flow and dispersion of
pollutants within two-dimensional
valleys: summary report on joint
Soviet-American study. EPA Report
(in review), U.S. Environmental
Protection Agency, Research Triangle
Park, NC 85p.
Lawson R.E. Jr., Snyder W.H. and
Thompson R.S. (1989) Estimation of
maximum surface concentrations from
sources near complex terrain in
neutral flow. Atmospheric
Environment 23, 321-31.
Snyder, W.H., Khurshudyan, L.H.,
Nekrasov, I.V., Lawson, R.E.Jr., and
Thompson, R.S. (1990) Flow and
dispersion of pollutants within two-
dimensional valleys. Atmospheric
Environment (submitted).
LH. Khurshudyan, is with Main Geophysical Observatory, Leningrad, U.S.S.R.,
I.V. Nekrasov, is with the Institute of Mechanics, State University of Moscow,
Moscow, U.S.S.R, R.E. Lawson, Jr., R.S. Thompson and FA. Schiermeier are
with Atmospheric Research and Exposure Assessment Laboratory, U. S.
Environmental Protection Agency , RTP, NC 27711.
William H. Snyder is the EPA Project Officer (see below).
The complete report, entitled "Flow and Dispersion of Pollutants within Two-
Dimensional Valleys: Summary Report on Joint Soviet-American Study,"
(Order No. PB 90-186362; Cost:, $17.00 subject to change) will be available
only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Atmospheric Research and Exposure Assessment Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
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