United States
Environmental Protection
Agency
Atmospheric Sciences J^T
Research Laboratory - AY
Research Triangle Park NC 27711 " *
Research and Development
EPA/600/S3-85/059 Sept. 1985
Project Summary
/1\
Windbreak Effectiveness for
Storage-Pile Fugitive-Dust
Control: A Wind Tunnel Study
Barbara J. Billman and S. P. S. Arya
Results of wind-tunnel experiments
to determine the optimal size and loca-
tion of porous windbreaks for con-
trolling fugitive-dust emissions from
storage piles in a simulated neutral at-
mospheric boundary layer are pre-
sented. Straight sections of windbreak
material were placed upwind of two
non-erodible, typically shaped piles and
were also placed on the top of one of
the piles. Wind speed, measured near
the pile surface at various locations
with heated thermistors, was isolated
here as the primary factor affecting par-
ticle uptake. Wind speed distributions
about the piles in the absence of any
windbreak and the flow structure
downwind of a three-dimensional
porous windbreak are presented. Rela-
tive wind speed reduction factors are
described and efficiencies based on the
relationship between wind speed and
particle uptake are given. The largest
and most solid windbreak caused the
greatest wind speed reduction, but
similar wind speed reductions were ob-
tained from several smaller wind-
breaks. A 50% porous windbreak of
height equal to the pile height and
length equal to the pile length at the
base, located one pile height from the
base of both piles was found to be quite
effective in reducing wind speeds over
much of the pile. Windbreaks placed on
the top of a flat-topped pile caused
large wind speed reductions on the pile
top, but small, if any, reductions on the
windward pile face. Windbreak effec-
tiveness decreased as the angle be-
tween the windbreak and the wind di-
rection decreased.
This Project Summary was devel-
oped by EPA's Atmospheric Sciences
Research Laboratory, Research Triangle
Park, NC, to announce key findings of
the research project that is fully docu-
mented in a separate report of the same
title (see Project Report ordering infor-
mation at back).
Introduction
Fugitive dust from sources such as
storage piles, materbls transfer points,
unpaved roads, and agricultural tilling
contribute significantly to total sus-
pended paniculate (TSP) levels in some
regions of the country. In addition to
limits on ambient concentrations of
TSP, radioactive paniculate is also regu-
lated. Early air pollution control efforts
emphasized controlling emissions from
stacks rather than fugitive-dust emis-
sions because the greater bulk of pollu-
tants came from stacks. Now, control
methods for fugitive-dust emissions are
also being tested.
Storage-pile fugitive-dust emission
rates depend upon the stored material's
bulk density, moisture content and par-
ticle size distribution, the storage pile
geometry, the wind velocity near the
pile surface and other parameters. How-
ever, particle uptake does not occur un-
less the wind speed is greater than a
given value, the threshold velocity,
which is dependent upon the type of
stored material, its moisture content
and particle size distribution. Several
empirical relationships between wind
speed and particle uptake rate are found
in the literature. Particle uptake appears
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to be related to un, where n is between
1 and 3.
The use of windbreaks for storage-
pile fugitive-dust control is based upon
the existence of a sheltered region
downstream of a windbreak. Wind tun-
nel and field experiments have shown
that porous windbreaks produce large
areas of reduced wind speed in their lee.
As expected, the location of the shel-
tered region shifts as the wind direction
varies from that of the windbreak nor-
mal.
In the present study, wind speed was
isolated as the primary factor affecting
particle uptake, although moisture con-
tent, particle size, and bulk density af-
fect fugitive-dust emissions as well.
Wind speed was measured near the pile
surface with and without windbreaks of
several sizes and porosities located var-
ious distances upwind or on the top of
two typically shaped storage piles. No
effort was made to simulate fugitive
dust emissions. Effect of wind direction
was also observed. The wind speed pat-
terns were analyzed to determine the
optimal windbreak porosity, height,
length, and location and to develop
windbreak design guidelines for
storage-pile fugitive-dust control.
Experimental Design and In-
strumentation
Experimental Design
The experiment was conducted in the
EPA Meteorological Wind Tunnel. A
neutrally stratified simulated atmos-
pheric boundary layer was generated
using a trip fence placed near the test
section entrance. Gravel roughness
composed of pebbles having typical di-
ameters of 10 mm covered the tunnel
floor downstream of the fence. The
boundary layer was characterized by a
depth of approximately 1 m, roughness
length (z0) of 0.12 mm, and friction ve-
locity (u.) of 0.048U0, where U0 = 4 m/s
is the free-stream speed.
Model size and free-stream wind
speed should ideally be determined
from matching the model and full-scale
Reynolds numbers Re. However, with
typical scale reductions, the model Re is
much less than the full-scale Re, but the
former is large enough for the flow
structure to be described in terms of
characteristic length and velocity
scales, independent of Re. Since atmos-
pheric flows are almost always aerody-
namically rough (for all wind speeds),
they are also Re-independent. Hence
wind tunnel velocities, normalized by
an appropriate scaling velocity, are
equivalent to normalized full-scale val-
ues, provided the relevant length scale
ratios are matched for geometric simi-
larity.
Windbreak effects on two typical, but
idealized, pile geometries are studied;
the results may be applicable to similar
full-scale piles. The piles modeled had
the same height (11 m) and side slopes
(37°), but different shapes, one a cone
and the other an oval, flat-topped pile.
The criterion of matching the ratio of the
pile height to the surface roughness
length was used to obtain the scaling
ratio between the model and prototype
of 1:100. The model surfaces were
roughened to satisfy the roughness
Reynolds number criterion for aerody-
namically rough surfaces.
Windbreaks were constructed of syn-
thetic materials of 50% and 65% poros-
ity, which are commercially available
and can be used in the field. Windbreaks
of three heights and two lengths were
placed at either of two distances upwind
of the conical pile base. One windbreak
was also placed at angles of 20° and 40°
from the position normal to the incident
flow. For the oval, flat-topped pile, the
longer axis of the pile was normal to the
airflow and parallel to the windbreak.
The same windbreak porosities and rel-
ative sizes and positions were used, but
additional tests were conducted with
two more windbreak heights and one
more length. The other windbreak loca-
tions tested were on the pile top, either
close to the centerline or at the up-
stream edge of the pile top parallel to
the pile's longer axis. Two heights and
two lengths were tested. For wind-
breaks in both positions the pile was ro-
tated 20° and 40° to simulate other wind
directions.
The final phase of the project was to
measure mean velocity and turbulence
intensities downstream of the less
porous windbreak oriented normal to
the airflow to determine whether re-
verse flow was present and to deter-
mine the regions of reduced mean flow
and enhanced turbulence.
Instrumentation
A hot-wire anemometer with a
boundary-layer type cross-wire probe
and a pulsed-wire anemometer were
used to measure mean flow and turbu-
lence intensity downstream of a wind-
break. The pulsed-wire anemometer is
used in regions where turbulence inten-
sity is very high or flow reversal occurs;
the pulsed-wire senses both wind speed
and direction.
Heated Fenwal thermistors projecting
out of the pile surface at various loca-
tions were used to measure wind
speeds. Thermistor anemometers oper-
ate under the same basic principle as do
hot-wire anemometers; that is, the heat
loss from the sensor is a function of
wind speed. The relationship between
wind speed and heat loss was deter-
mined experimentally from calibration.
Flow About Porous Windbreak
Relative wind speed deficit due to a
windbreak may be defined as
[UR(z) - U(z)]/UR(z), where UR(z) is the
reference speed at the location of the
windbreak but in its absence, and U(z) is
a speed at some distance downstream
of the windbreak. Lines of constant rela-
tive deficit are seen in Figure 1. Down-
stream distance and height were scaled
by the windbreak height h. Below
z = 1h, wind speeds were reduced at
least 50% from the upstream value at
the same height. The greatest reduc-
tions were observed for heights less
than z = 0.5h between approximately 4
and 8h downstream. In other words, the
maximum reduction did not occur im-
mediately downstream of the wind-
break, but occurred farther down-
stream. High turbulence intensity (the
ratio of the rms fluctuating longitudinal
velocity at a given location to the mean
wind speed at that location) was ob-
served in the low wind speed region,
although high values of the fluctuating
velocity extended downstream from the
top of the windbreak.
Flow About Storage Piles
For the conical pile, the areas of max-
imum wind speed were near the top of
the upwind face but toward the sides of
the pile. A high speed region was on the
upstream face, extending from near the
crest down both sides. The area of min-
imum wind speed was in the lee. For
normal incident flow to the oval pile, the
highest wind speeds were observed on
the windward face near the top of the
pile, extending down the sides, similar
to the case with the conical pile. Again,
the lowest speeds were observed in the
lee; but a secondary minimum also oc-
curred on the top of the pile.
Windbreak Effects on Flow
About Storage Piles
Initial guidance on the desired size of
the windbreak and its location was ob-
tained from an examination of the ob-
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5
4.5
4
3.5
3
2.5
A
1.5
1
.5
0
5
4.5
4
3.5
3
2.5
2
1.5
1
.5
-1.5
1.5
4.5
7.5
10.5 12 13.5 15
x/h
Figure 1. Relative wind speed deficit downstream of a porous windbreak.
served flow and sheltered region be-
hind the windbreak in the absence oi
the pile. Since height and width of the
sheltered region were directly related to
windbreak height and length, wind-
breaks placed upstream of the pile hav-
ing dimensions less than the pile height
or length were expected to be less effi-
cient, as were smaller windbreaks
placed on the pile top. With a wind-
break, the wind speed at a given loca-
tion on the pile surface is some fraction
of that in the unprotected case. The rela-
tive amount by which the wind speed is
reduced is called the wind speed reduc-
tion factor RJ and, in percent, is defined
as
tions extended to the pile sides. Wind-
breaks of height one half the pile height
caused smaller reductions near the top
of the pile and even caused wind speed
increases in part of the lee of the conical
pile and on the top of the oval pile.
Windbreaks of height greater than the
pile caused significantly higher reduc-
tion only on the top of the flat-topped
pile. Higher windbreaks also tended to
be more effective when located farther
from the pile. Windbreak effectiveness
decreased with increasing angle of flow
from the normal.
20
Ri = (uo,i - Ui
40
i x 100
(1)
where Uj and u0/i are wind speeds at the
i-th location on the pile for the cases
with and without the windbreak, respec-
tively.
Contours of constant wind speed re-
duction resulting from windbreaks of
different porosity, but height equal to
the pile height and length equal to the
length of the pile top (0.6B), located one
pile height from the base of the pile are
shown in Figure 2. The lower porosity
windbreak gave greater wind speed re-
ductions; reductions greater than 40%
were observed for the 50% porous
windbreak, but were not for the 65%
porous windbreak. For a windbreak as
long as the pile base, the high reduc-
direction
40
20
20
20
20
40
Figure 2.
20
Wind speed reduction factor for
the windbreak of height 1.0H
and length 0.6B placed 1 H from
the oval, flat-topped pile base
with porosity 65% (solid line)
and 50% (dashed line).
For windbreaks placed on the top of
the oval, flat-topped pile, large reduc-
tion factors (up to 65%) on the pile top
were observed. The location and extent
of the area with significant wind speed
reduction depended upon windbreak
size, location, and angle of the incident
flow. This suggests that fugitive-dust
emissions on the top of the pile may be
controlled locally through the use of a
windbreak.
Relation to Particle Uptake
In terms of fugitive-dust emissions,
the windbreak efficiency E may be
defined as E = — (Q/Q0), where Q and
Qo are the storage-pile fugitive-dust
emission rates with and without the
windbreak, respectively. Since only
surface wind speeds have been meas-
ured here, assumed relationships be-
tween wind speed and emissions are
used to calculate efficiencies. To a first
approximation, it was assumed here
that the reference wind speed is suffi-
ciently high that wind speeds every-
where, with and without a windbreak,
exceed the threshold speed. An effici-
ency En can be defined based upon a
given power-law relation between wind
speed and particle uptake, Q«un (where
n is between 1 and 3). In effect, these
efficjences are 1 — (uVu7n), where u^
and u0n are the area-averaged values
over the pile surface with and without a
windbreak, respectively. A better defi-
nition of efficiency would include a
threshold wind speed, but the relation-
ship between threshold speed and
particle type, size and moisture content
is not well understood. Efficiencies Ei
and E3 are calculated here, using values
of n = 1 and 3.
E! for the windbreaks placed up-
stream of the conical and larger piles
with normal incident flow are given in
Tables 1 and 2, respectively. In general,
a windbreak was more effective (higher
ET) when placed upstream of the conical
pile, as compared to a windbreak of the
same relative size placed upstream of
the larger, oval-shaped pile. Trends in
E1 with changes in height, length, loca-
tion and porosity of the windbreak were
similar for both piles. In general, a wind-
break at least as high as the pile is desir-
able. The 1.5H height was slightly more
effective than the 1.0H height with the
oval, flat-topped pile, reflecting in-
creased wind speed reductions on the
pile top for the highest windbreak. Effi-
ciencies were higher for the less porous
windbreak material. Except for the
windbreaks of height one half the pile
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Table 1. Efficiency (Erf for the Various Windbreaks Placed Upstream of the Conical Pile
65% porous windbreak 50% porous windbreak
position:
length:
height
0.5H
1.0H
1.5H
1H
1.0D
34
48
47
1.5D
32
45
44
3H
1.0D
28
53
55
1.5D
30
52
54
1H
1.0D
46
66
64
1.5D
45
67
65
3H
1.0D
36
65
71
1.5D
36
71
77
Table 2.
Efficiency (Erf for the Various Windbreaks Placed Upstream of the Oval, Flat-
Topped Pile
65% porous windbreak
50% porous windbreak
position:
height
0.5H
0.75H
1.0H
1.25H
1.5H
1H
0.6B
15
—
27
—
33
1.0B
16
—
34
—
39
3H
0.6B
13
—
28
—
38
7.06
—
—
37
—
—
0.6B
18
—
34
—
44
1H
I.OB
20
41
53
56
58
3H
1.5B
21
—
51
—
59
0.6B
15
—
31
—
43
I.OB
17
34
49
57
62
tered region. These results suggest that
fugitive-dust emissions may be locally
controlled with windbreaks placed on
the top of a relatively level storage pile.
In particular, portable windbreaks may
be quite practical since they could be
positioned to protect active areas of the
pile.
Wind speed was isolated here as the
major factor affecting storage-pile
fugitive-dust emissions, but storage-
pile moisture content, type of material
stored and threshold wind speed also
affect emissions. A clearer understand-
ing of the relationship of wind speed
and threshold speed to fugitive-dust
emissions would allow for better analy-
sis of the data presented. Additional
field measurements of fugitive dust
from storage piles with and without
windbreaks would be helpful for com-
parison to the efficiencies and design
guidelines presented here.
height (0.5H), efficiency was lower
when the windbreak was not as long as
the pile base length.
Trends in the values of E3, the effi-
ciency based upon the u3 relation to
dust uptake, with windbreak height,
length, location and porosity were
found to be similar to those for EI, ex-
cept the values of E3 were considerably
larger than those of E1 (see Figure 3).
Although the highest efficiencies (E3)
of 99% and 96% corresponded to the
50% porous material of height 1.5 times
the pile height (H), length 1.5 times the
base diameter of the conical pile and
equal to the base length of the oval, flat-
topped pile, respectively, located 3H
from the base of the piles, the efficien-
cies of the more economical windbreak
of the same porosity, height equal to the
pile height and length equal to the pile
base length were only slightly lower
(97% and 89%, respectively). Clearly,
the latter size would be preferable on
the basis of cost effectiveness. Any loca-
tion between 1H and 3H from the base
of the pile could be chosen depending
on the convenience.
Conclusions
This wind tunnel study has shown
that windbreaks normal to the wind di-
rection placed upwind of a conical and a
larger, oval, flat-topped storage pile re-
duce wind speeds near the surface of
the pile and hence suggestreductions in
fugitive-dust emissions. Of the wind-
breaks tested for each pile, the largest
50% porous windbreak placed 3H from
the pile appears to be best in terms of
greatest wind speed reductions and ef-
fectiveness for fugitive-dust control.
However, all the 50% porous wind-
breaks at least as high as the pile and as
long as the pile base had similar overall
effects. Windbreaks of height and/or
length less than that of the pile were
clearly less effective. Optimal wind-
break location appears to be related to
windbreak height, particularly for the
conical pile; the higher the windbreak,
the farther it should be located upwind
of the pile. However, locations farther
than 3H were not examined.
Windbreak length and position are
even more important in determining ef-
fectiveness when the airflow is not nor-
mal to a windbreak. With a windbreak of
height and length equal to the pile di-
mensions, fairly high wind speed reduc-
tions resulted when the windbreak was
placed upwind normal to the flow and
also at an angle of 20° to the normal, but
very little reduction occurred at an angle
of 40°.
Windbreaks placed on the top of the
oval, flat-topped pile caused large areas
of significant wind speed reductions on
the pile top, both downwind and up-
wind of the windbreak, but very small
reductions to the high wind speeds on
the windward face. The area of greatest
reduction was not immediately down-
wind of the windbreak, but displaced
farther downstream. Changes in wind
direction shifted the location of the shel-
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porosity
A 50%
D 50%
O 55%
O 65%
700
porosity
D 50%
A 50%
O 65%
O 55%
Figure 3.
.5 1 1.5 2 0 .5 1 1.5
windbreak height/pile height windbreak height/pile height
Efficiency (E3 vs. height for windbreaks placed 3H from the pile base: (a) conical pile, (b) oval, flat-topped pile.
Barbara J. Billman and S. P. S. Arya are with Department of Marine, Earth and
Atmospheric Sciences, North Carolina State University, Raleigh, NC 27695-
8208.
William H. Snyder is the EPA Project Officer (see below).
The complete report, entitled "Windbreak Effectiveness for Storage-Pile Fugitive-
Dust Control: A Wind Tunnel Study," (Order No. PB 85-243 848/AS; Cost:
$16.95, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Atmospheric Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
•&U. S. GOVERNMENT PRINTING OFFICE: 1985/559-111/20691
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