WINDBREAK EFFECTIVENESS FOR STORAGE-PILE FUGITIVE-DUST CONTROL
A Wind Tunnel Study
by
Barbara J. Billman
and
S.P.S. Arya
Department of Marine, Earth and Atmospheric Sciences
North Carolina State University
Raleigh, NC 27695-8208
Cooperative Agreement Number CR-811973
Project Officer
William H. Snyder
Meteorology and Assessment Division
Atmospheric Sciences Research Laboratory
Research Triangle Park, North Carolina 27711
ATMOSPHERIC SCIENCES RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
RESEARCH TRIANGLE PARK, NORTH CAROLINA 27711
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DISCLAIMER
This report has been reviewed by the Atmospheric Sciences Research
Laboratory, U.S. Environmental Protection Agency, and approved for
publication. Approval does not signify that the contents necessarily
reflect the views and policies of the U.S. Environmental Protection
Agency, nor does mention of trade names or commercial products constitute
endorsement or recommendation for use.
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ABSTRACT
Results of wind-tunnel experiments to determine the optimal
size and location of porous windbreaks for controlling fugitive-dust
emissions from storage piles in a simulated neutral atmospheric boundary
layer are presented. 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 particle 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. Relative 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
obtained from several smaller windbreaks. 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 effectiveness decreased as the angle
between the windbreak and the wind direction decreased.
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CONTENTS
DISCLAIMER if
ABSTRACT 1 i i i
FIGURES v
TABLES vi i i
SYMBOLS i x
ACKNOWLEDGEMENTS xi i
1. INTRODUCTION 1
2. LITERATURE REVIEW 4
2.1 Fugitive-Dust Emission Rates 4
2.2 Flow About Windbreaks 7
2.3 Windbreaks for Storage-Pile Fugitive-Dust Control 10
3. EXPERIMENTAL DESIGN AND INSTRUMENTATION 13
3.1 Experimental Design 13
3.2 Instrumentation 22
4. FLOW ABOUT A POROUS WINDBREAK 41
5. FLOW ABOUT STORAGE PILES 51
5.1 Conical Pile 51
5.2 Oval, Flat-topped Pile 53
6. WINDBREAK EFFECTS ON FLOW ABOUT STORAGE PILES 58
6.1 Windbreaks Upstream of the Pile 58
6.1.1 Relation to flow structure 58
6.1.2 Results 59
6.2 Windbreaks on the Pile Top 75
7. FURTHER ANALYSES AND DISCUSSION OF RESULTS 80
7.1 Relation to Particle Uptake 80
7.2 Comparison to Previous Studies 88
8. CONCLUSIONS 90
9. REFERENCES 93
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FIGURES
Number Page
2.1 Sketches of streamlines about windbreaks: (a) solid,
(b) porous 8
3.1 Boundary-layer velocity profile: (a) linear,
(b) semi-logarithmic 14
3.2 Sketch of oval, flat-topped pile 16
3.3 Geometry of conical pile and windbreak 20
3.4 Example hot-wire anemometer calibration plots 25
3.5 Sketch of a pulsed-wire anemometer probe , 27
3.6 Typical signals observed from various stages of the pulsed-wire
anemometer 29
3.7 Example pulsed-wire anemometer calibration plot 31
3.8 Resistance-temperature thermistor curves 33
3.9 Thermistor circuit 34
3.10 Dissipation factor vs. wind speed for Fenwal GB31J1
thermi stors 34
3.11 Top view of conical pile. Stars: Thermistor positions on pile.
Dots: Effective thermistor positions due to pile rotation 37
3.12 Top view of oval, flat-topped pile. Dots: Thermistor positions.37
3.13 Comparison of thermistor and single-wire wind speed
measurements 38
4.1 "Streamlines" and normalized velocity vectors downstream of a
porous windbreak 42
4.2 Relative wind speed deficit downstream of a porous windbreak... 44
4.3 Turbulence intensity downstream of a porous windbreak 46
4.4 Normalized r.m.s. longitudinal velocity fluctuation downstream
of a porous windbreak 47
4.5 Profiles of normalized wind speed downstream of a porous
wi ndbreak \ 48
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Number Page
4.6 Profiles of normalized longitudinal velocity fluctuation
downstream of a porous windbreak 50
5.1 u/Up about conical pile for no windbreak case... 52
5.2 u/Up about oval, flat-topped pile for no windbreak case 54
5.3 Corrected u/ur about oval, flat-topped pile for no windbreak
case 55
5.4 Corrected u/ur about oval, flat-topped pile at an angle to
incident flow for no windbreak case: (a) 20°, (b) 40° 57
6.1 Superposition of conical pile positions and undisturbed
"streamlines" downstream of a porous windbreak of height:
(a) 0.5H, (b) l.OH, (c) 1.5H 60
6.2 Superposition of oval, flat-topped pile positions and
undisturbed "streamlines" downstream of a porous windbreak
height: (a) 0.5H, (b) l.OH, (c) 1.5H 61
6.3 Wind speed reduction factor for the 65% porous windbreak of
height 0.5H and length l.OD placed 1H from the conical pile
base 63
6.4 Wind speed reduction factor for the 65% porous windbreak of
height 0.5H and length l.OB placed 1H from the oval, flat-
topped pile base 63
6.5 Wind speed reduction factor for the 50% porous windbreak of
height 1.5H placed 1H from the oval, flat-topped pile base with
length 0.6B (solid line) and l.OB (dashed line) 67
6.6 Wind speed reduction factor for the windbreak of height l.OH
and length l.OD placed 1H from the conical pile base with
porosity 65% (solid line) and 50% (dashed line) 69
6.7 Wind speed reduction factor for the windbreak of height l.OH
and length 0.6B placed 1H from the oval, flat-topped pile base
with porosity 65% (solid line) and 50% (dashed line) 69
6.8 Wind speed reduction factor for the 50%.porous windbreak of
height 1.5H and length 1.5D placed 1H (solid line) and 3H
(dashed line) from the conical pile base 71
6.9 Wind speed reduction factor for the 50% porous windbreak of
height 1.5H and length 0.6B placed 1H (solid line) and 3H
(dashed line) from the oval, flat-topped pile base 71
VI
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Number Page
6.10 Wind speed reduction factor for the 50% porous windbreak of
height l.OH and length l.OD placed 1H from the conical pile
base oriented (a) normal, (b) 20°, and (c) 40° to the flow
direction 74
6.11 Sketch of windbreak positions with respect to thermistor
positions on the top of the oval, flat-topped pile 76
6.12 Wind speed reduction factor for the 65% porous windbreak
placed near the pile leading edge: (a) height 0.27H and length
0.5T, (b) height 0.14H and length 0.5T, (c) height 0.27H and
1 ength 0.16T 77
6.13 Wind speed reduction factor for the 65% porous windbreak of
height 0.27H and length 0.5T placed near the pile centerline.
Pile is: (a) normal, (b) 40° to the incident flow 78
7.1 Efficiency (£3) vs. height for windbreaks placed 3H from the
pile base: (a) conical pile, (b) oval, flat-topped pile 86
vn
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TABLES
Number Page
3.1 Combinations of windbreak parameters used with the conical
pile i 21
3.2 Combinations of windbreak parameters used with the oval,
f 1 at-topped pile 23
6.1 umax/ur for the various windbreaks placed upstream of the
conical pile 72
6.2 umax/ur for the various windbreaks placed upstream of the
oval, flat-topped pile 72
7.1 Efficiency (E^) for the various windbreaks placed upstream
of the conical pile 83
7.2 Efficiency (E^) for the various windbreaks placed upstream
of the oval, flat-topped pile 83
7.3 Efficiency (£3) for the various windbreaks placed upstream
of the conical pile 85
7.4 Efficiency (£3) for the various windbreaks placed upstream
of the oval, flat-topped pile 85
V117
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SYMBOLS
a constant
A surface area
b constant
B oval, flat-topped pile base length [L]
c constant
Cp pressure drop coefficient
C constant
d distance between pulsed wire and sensor wire [L]
D conical pile base diameter [L]
E voltage [volt] or windbreak efficiency or effectiveness factor
EF emission factor [M/L2T]
f percentage of time that unobstructed wind speed exceeds 5.4 m/s
at mean pile height
F constant
h windbreak height
H pile height
i electric current [amp]
k von Karman's constant
K thermistor dissipation factor [ML^/T^e]
L windbreak length or a characteristic length [L]
n exponent 1 or 3
p nunber of days with > 0.25 mm precipitation per year
P static pressure [H/LJ2] or distance between windbreak and pile
upstream base [L]
PE Thorntwaite's precipitation-evaporation index
Q storage-pile eVosion rate [M/T]
IX
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R resistance [ohm] or wind speed reduction factor
s pile surface area [L.2]
S silt content
t
t time or time-of-flight [T]
T oval, flat-topped pile top length [L] or temperature [e]
u wind speed [L/T]
u1 longitudinal component of velocity fluctuation [L/T]
u friction velocity [L/T]
U mean wind speed or a characteristic wind speed [L/T]
V voltage [volt]
w oval, flat-topped pile base width [L]
w1 vertical component of velocity fluctuation [L/T]
x Cartesian coordinate (streanwise) [L]
y Cartesian coordinate (lateral) [L]
z Cartesian coordinate (vertical) [L]
z0 surface roughness length [L]
a exponent in King's law
2 thermistor constant [0]
Y angle between direction normal to the pulsed-wire plane and the
instantaneous velocity vector
5 boundary-layer height [L]
p air density [M/L-3]
Pb bulk density [M/L3]
Subscripts and special symbols
( )0 no windbreak case or free-stream value
\
( )} value based on linear relation between wind speed and particle
uptake
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( )3 value based on cubic relation between wind speed and particle
uptake
( )a ambient value
( )i specific pile location value or index
( )max maximum value
( )r reference value
( )R reference value
( )t threshold value
( )y thermistor value
( ) area-averaged value
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ACKNOWLEDGEMENTS
The authors wish to acknowledge the advice and help given by
Dr. W.H. Snyder and his staff at. the EPA Fluid Modeling Facility. The
cooperation and help from Messrs. W.B. Kuykendal and D.L. Harmon
(EPA-AEERL) is appreciated. Mr. W.R. Pendergrass (Oak Ridge Associated
Universities) provided the pulsed-wire anemometer. Mr. R. Lawrence
(KPN, International) provided the commercial windbreak material.
xn
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1. INTRODUCTION
Total suspended particulate (TSP) levels in many regions of the
country do not meet the National Ambient Air Quality Standard; fugitive
dust from sources such as storage piles, materials transfer points,
unpaved roads, and agricultural tilling contribute significantly to TSP
levels in some regions. Wind erosion causes an estimated 30% of
storage-pile fugitive-dust emissions; load-in and load-out processes
(30%) and vehicular traffic (40%) cause the remainder (Cowherd et al.,
1974).
Many states regulate fugitive-dust emissions by forbidding visible
fugitive dust beyond the property line surrounding the source; some
states have more strict storage-pile fugitive-dust regulations (e.g.
Bureau of National Affairs, 1982 and 1983). Although limiting emissions
from a fugitive dust source may not be required, a given industrial
facility may use the Environmental Protection Agency (EPA) "bubble"
policy to their benefit by controlling fugitive dust rather than the
more costly process of upgrading the particulate controls on their
stacks to make them more efficient.
Radioactive particulate is also regulated. EPA's "Environmental
Radiation Protection Standards for Nuclear Power Operations" (40 CFR
190) provides limits for radiation doses received by the public. The
Nuclear Regulatory Commission (NRC) amended 10 CFR 20, requiring that
NRC licensees comply with the EPA regulations. Criterion 8 of that
amendment states that to control dusting from tailings, dry tailings
shall be wetted or chemically stabilized, unless the "tailings are
effectively sheltered from the wind, such as may be the case where they
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are disposed of below grade and the tailings surface is not exposed to
wind. . ."
Storage piles, in addition to tailings, are found at mines, mineral
processing plants, coal-fired electric generating plants, within the
iron and steel industry, etc. and contain coal, coke, limestone,
aggregate, sand, etc.
Early air pollution control efforts emphasized controlling
emissions from stacks rather than fugitive emission sources because the
greater bulk of pollutants came from stacks. Now that efficient control
methods for particulate matter from stacks are available, control
methods for fugitive dust sources are being tested. The EPA Air &
Energy Engineering Research Laboratory (formerly Industrial
Environmental Research Laboratory) requested that the EPA Atmospheric
Sciences Research Laboratory (formerly Environmental Sciences Research
Laboratory) conduct a wind tunnel study to assess windbreak
effectiveness for the control of fugitive dust from storage piles.
This study was undertaken as part of a cooperative agreement between
the EPA and North Carolina State University.
Since wind direction is variable, encircling a storage-pile with a
windbreak will shield it from winds in all directions. However, this
may not be practical, particularly for an active pile, or economically
feasible due to size. A straight windbreak placed to protect a pile
from the prevailing winds may be more practical. A windbreak may also
be placed on top of some piles to protect certain areas, say where
material is removed from or added to the pile. In the present study,
wind speed is isolated as the primary factor affecting particle uptake,
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although moisture content, particle size, and bulk density affect
fugitive-dust emissions as well. Wind speed was measured near the pile
surface with and without windbreaks of several sizes and porosities
located various 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 patterns
were analyzed to determine the optimal windbreak porosity, height,
length, and location and to develop windbreak design guidelines for
storage-pile fugitive-dust control.
The following sections consist of a literature review, a
description of the experimental design, instrumentation, and the flows
about a representative windbreak and about the two storage piles. Wind
speed patterns for various windbreak cases are presented and effects of
the windbreak parameters are noted. Finally, the wind speed
distributions are related to particle uptake.
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2. LITERATURE REVIEW
Reviews of three topics are presented as background material to
the present study. The relationship between wind speed and fugitive-
dust emission rate is examined first. Second, the structure of the
flow about a windbreak placed on a horizontal surface is examined.
Finally, a review of the literature dealing with windbreaks for storage-
pile fugitive-dust control is presented.
2.1 FUGITIVE-DUST EMISSION RATES
Mechanical forces by such implements as bulldozers acting on the
pile surface and by falling material impacting the surface, freezing
and thawing, etc., create particles that may become airborne. Storage-
pile fugitive-dust emission rates depend upon the stored material's
bulk density, moisture content and particle size distribution, the
storage pile geometry, the wind velocity near the pile surface and
other parameters. However, particle uptake does not occur unless 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 relationships between wind speed and particle uptake rate
are found in the literature. Bagnold (1941) suggested that the particle
uptake rate is proportional to the cube of the wind speed. Gillette
(1978a), in a wind tunnel test of the effects of sandblasting, wind
speed, soil crusting, and soil surface texture on wind erosion, showed
that the soil particle flux is proportional to the cube of the friction
velocity (u ), where u is determined from the mean velocity profile
* \ *
over a horizontal surface,
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u* z
U= In > (2.1)
K ZQ
where U is wind speed at height z, z0 is the surface roughness length,
and k is von Karman's constant (~0.4). Blackwood and Wachter (1978)
suggested that the storage pile emission rate, Q (mg/s), may be
expressed as
Q = (cu3 pb2 s°-345)/(PE)2, (2.2)
where u is wind speed (m/s), pb is bulk density (g/cm3), s is pile
surface area (cm^), PE = Thorntwaite's precipitation-evaporation index
(Thornthwaite, 1931), and c is a constant. Axetell (1978) suggested an
emission factor of 1.6u Ib/acre-hr, where u is in m/s. Finally, wind
erosion emissions from active storage piles may also be estimated from
the EPA (1983) emission factor
EF = 1.9 [S/1.5] [(365-p)/235] [f/15], (2.3)
where EF is the TSP emission factor (kg/day/hectare), S is silt content
(% of particles < 75 ym in diameter), p is number of days with > 0.25 mm
precipitation per year, and f is the percentage of time that the
unobstructed wind speed exceeds 5.4 m/s at the mean pile height. This
equation is based on sampling emissions from sand and gravel storage
piles and hence gives less accurate estimates when applied to other
stored materials. It implies that no emissions occur when the
unobstructed wind speed is less than 5.4 m/s.
Field tests with portable, open-floored wind tunnels indicated that
threshold speeds, given in terms of threshold friction velocity (u )^,
are typically 0.2 to 2 m/s depending upon the type of material
(Gillette, 1978b; Gillette et al., 1980; and Cowherd et al., 1979). In
V
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other field tests, threshold speeds of about 10 m/s at a height of 15 cm
above a coal pile surface were estimated based upon the onset of visible
particle uptake (Cowherd, 1982; Cuscino et al., 1983). Extrapolating
these speeds to a 10 m reference height from the velocity profile
(Equation 2.1) implies that very high mean wind speeds (e.g. 20 m/s)
are needed for erosion at the surface (z=0) to commence. Hence Cowherd
(1982) suggested that strong wind gusts, not the mean wind, cause
erosion, although it was noted that wind speeds 15 cm above a storage
pile surface may approach the 10 m reference speed (Soo et al., 1981).
In the above relationships for particle uptake, emission rate is
independent of time. However, unless an unlimited supply of erodible
particles is present, erosion will be time dependent. Erosion rate has
been observed to decrease with time (e.g. Cowherd et al., 1979).
Cowherd (1982) suggested that erosion rate is proportional to the
amount of erodible material remaining and that a given storage pile has
an "erosion potential" equal to the total quantity of erodible material
present on the surface prior to erosion. The erosion potential for
coal increased with wind speed. An emission factor was then defined to
be dependent upon the frequency of disturbance to the pile surface and
the erosion potential at the expected wind gust speed.
A more comprehensive review of storage pile wind erosion emission
factors may be found in Currier and Meal (1984).
To summarize, particle uptake, hence storage pile emission rate,
clearly depends upon the ambient wind speed. Previous studies indicate
that uptake is highly sensitive to wind speed, being related either
V
directly or to the cube of the speed, the threshold velocity, or only
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occurring during high wind gusts. If the wind speed over a given area
of a storage pile is reduced, then fugitive dust emissions from that
area will also be reduced. The current study isolates the single
variable, wind speed, as the primary factor affecting emission rates
and attempts to evaluate the effectiveness of windbreaks in reducing
particle uptake. A complete assessment, of course, would require an
analysis of all factors, including material type, particle size
distribution, moisture content, etc.
2.2 FLOW ABOUT WINDBREAKS
Rows of trees or hedges have been used to protect homes and
agricultural fields from high winds for many years (e.g. Van Eimern,
et al., 1964). Wind tunnel and field experiments have shown that
windbreaks produce large areas of reduced wind speed in their lee.
Speeds near the surface are typically reduced to half the upstream
value for a distance of 8 to 12 windbreak heights downstream of a
porous windbreak (e.g. Raine and Stevenson, 1977). Wind speeds are
also reduced upstream of a porous windbreak. Perera (1981) observed
reductions of 50% one windbreak height upstream.
Figure 2.1 shows typical streamline patterns for flow about solid
and porous windbreaks. The streamline patterns are not drawn to scale;
Caborn (1957) and Naegeli (1953) found that a "dense" barrier caused a
sheltered region up to distances of 10-15h downwind and for a (50%)
porous barrier, 20-25h. The general flow features are of interest here.
Recirculation regions are evident both upwind and downwind of the solid
windbreak; they are regions of low velocity and high turbulence
intensity (Figure 2\l(a)). Air incident on the porous windbreak flows
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(a)
//////// //////////////////Y////7/77/
(b)
Figure 2.1 Sketches of streamlines about windbreaks: (a) solid,
(b) porous.
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over and through the windbreak. A region of reduced wind speed is
evident in Figure 2.1(b); the upwind recirculation region is eliminated.
In wind tunnel experiments, the low wind speed area moved downstream as
porosity (ratio of open to totaf cross-sectional area) increased and, in
general, at a given downwind distance, wind speed was less with a lower
porosity windbreak (Raine and Stevenson, 1977). With the use of pulsed-
wire anemometers which detect wind direction, other wind tunnel tests
showed that as porosity increased, the downwind recirculation region
became smaller and moved downstream (Perera, 1981).
Raine and Stevenson (1977) made several further observations.
Greater wind speed reductions and turbulence intensity (u'/U, where u1
is the r.m.s. longitudinal velocity fluctuation) enhancements were
observed with decreasing porosity. But since the turbulent fluctuation
at a given location varied much less with porosity than did the mean
wind speed, the higher turbulence intensities were due to the lower U.
The maximum in u1 was located just above and extended downstream from
the top of the windbreak. In the region bounded by the surface, the
windbreak, and a line connecting the top of the windbreak to the surface
just downstream of the velocity minimum (x~8h, where h is windbreak
height), the turbulent fluctuation u1 was less than that observed in
the absence of the windbreak, but above and downstream of this region,
u' was greater, indicating the diffusion of turbulence generated in the
windbreak-induced shear layer. Greater magnitudes and larger areas of
wind speed reduction occurred with smoother upstream terrain and lower
turbulence in the approach flow.
Perera (1981)valso compared hot-wire (HWA) and pulsed-wire (PWA)
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anemometer measurements downwind of a solid barrier. Velocity measured
with the HWA was greater than that measured with the PWA for heights
less than about 1.8h at a downwind distance of 5h, but at higher
heights, the PUA measurements we're greater. Longitudinal turbulence
intensity from PWA measurements were greater than the HWA measurements
at the downwind distance 5h for heights up to 4h, where they converged.
Effect of thermal stability was observed in a few field and wind
tunnel studies. Seginer (1975a) observed that during unstable
conditions, wind speed at 2=0.25h beyond x=2.5h decreased with
increasing stability. Between the porous windbreak and x=2.5h,
stability was not important. Ogawa and Diosey (1980) observed that
the length of the recirculating zone downstream of a solid fence was a
maximum for near-neutral stability. It decreased rapidly as stability
increased because the stable stratification inhibited changes in the
flow caused by the fence. For unstable stratification, the length of
the recirculating zone decreased slowly with increasing instability.
Jacobs (1984) noted a faster recovery of wind speed with downwind
distance for unstable stratification.
As expected, the location of the sheltered region shifts as the
wind direction varies from that of the windbreak normal (Gandemer,
1981; Seginer, 1975b; Mulhearn and Bradley, 1977). Measurements along
the windbreak normal showed that the minimum wind speed increases and
its location moves toward the windbreak as the angle between the wind
direction and the windbreak normal increased (Seginer, 1975a,b).
2.3 WINDBREAKS FOR STORAGE-PILE FUGITIVE-DUST CONTROL
\
Very little information on the use of windbreaks as a storage-pile
10
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fugitive-dust control method is contained in the literature. Low
efficiencies were reported by Bohn et al. (1978) and Jutze et al.
(1977), 30% and "very low," respectively; although these were only
estimates. No guidelines on windbreak design and use are available.
Results of two laboratory investigations have been reported. Results
of a water flume test indicated that the windbreak height should be at
least 1.4H, where H is the pile height and that the lower two-thirds of
the windbreak be 20% porous and the remainder, 50% porous (Davies,
1980). However, the author did not state whether more than one
windbreak height was used, nor which porosities were tested. Details
of windbreak length with respect to the pile length and/or width and
windbreak position were not given, nor were the details of the
atmospheric boundary layer simulated. Results from a wind tunnel study
using a two-dimensional windbreak and pile are reported by Soo et al.
(1981) and Cai et al. (1983). Combinations of two windbreak heights
(0.25H and 0.5H), two porosities (solid and 33% porous), and five
positions (0, 1, 2, 3, and 4 pile heights from the pile base) were
tested. The wind tunnel speed and model pile size were chosen to match
the full-scale Reynolds number (Re=UL/v, where U and L are the
characteristic wind speed and length, and v is kinematic viscosity),
based on a pile height of 3.05 m and an ambient wind speed of 1.14 m/s.
A 33% porous windbreak of height 0.5H, placed the optimal distance of
3H from the pile base reduced the wind speed by 50% just above the
surface at the top of the windward face of the pile (Soo et al., 1981
and Cai et al., 1983). The position of 2.5H was optimal for a 33%
porous windbreak of height 0.25H (Soo et al., 1981). Lower wind speeds
11
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at the top of the windward face of the pile were observed with solid
barriers of both heights at all the positions (Soo et al., 1981). In
their study the atmospheric boundary layer was not simulated, although
the flow was turbulent. '
12
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3. EXPERIMENTAL DESIGN AND INSTRUMENTATION
Some details of the wind tunnel boundary layer, the model piles
and windbreaks and the experimental plan are given in the following
subsections. '
3.1 EXPERIMENTAL DESIGN
The experiment was conducted in the EPA Fluid Modeling Facility
(FMF) Meteorological Wind Tunnel (MWT), a low-speed, open-return tunnel
having a test section 2.1 m high x 3.7 m wide x 18.3 m long. A complete
description of the wind tunnel and operating characteristics can be
found in Snyder (1979). Simulating the lower part of the atmospheric
boundary layer (ABL) and determining model size and free-stream wind
speed are necessary to assure that wind tunnel results will be valid
for the full-scale case under investigation. A neutrally stratified
simulated ABL was generated using a 153 mm high trip fence placed near
the test section entrance. Gravel roughness composed of pebbles having
typical diameters of 10 mm covered the tunnel floor downstream of the
fence. The tunnel ceiling height was adjusted to achieve a zero
longitudinal pressure gradient in the free stream. The velocity profile
in the surface layer is described by Equation 2.1 and is shown in
Figure 3.1. The tunnel free-stream speed was 4 m/s. Roughness length
(z0) and friction velocity (u ) were determined from the log-wind law.
The boundary layer was characterized by a depth of approximately 1 m, a
z0 of 0.12 mm, and a u of 0.048U0, where U0 is the free-stream speed.
Further details of the boundary layer are given by Castro and Snyder
(1982).
\
Model size and free-stream wind speed should be determined from
13
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14
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matching the model and full-scale Reynolds numbers. However, with
typical scale reductions, the model Re is typically 3 to 4 orders of
magnitude less than the full-scale Re. Very high wind speeds in the
tunnel are generally required to-match full-scale Re, but are often
impractical. Fortunately for wind tunnel modeling, "geometrically
similar flows are similar at all sufficiently high Reynolds numbers"
(Townsend, 1956). That is, for Re greater than some minimum value, the
turbulent flow structure described in terms of characteristic length
and velocity scales is independent of Re. Since atmospheric flows are
almost always aerodynamically rough (for all wind speeds), they are
also Re-independent. Hence wind tunnel velocities, normalized by a
reference speed, are equivalent to normalized full-scale values.
No two storage piles have the same shape and size, and active
piles have constantly changing dimensions. For purposes of the present
study, windbreak effects on two typical, but idealized, pile geometries
are studied, with the results being generally applicable to many full-
scale piles. Based on a survey of several coal piles at electric
generating plants, typical piles are 11 m high, have slopes of 37°
and range from conical to flat-topped in shape. The two piles modeled
were an 11 m high conical pile with 37° slopes (base diameter 29.2 m)
and an oval, flat-topped pile of height 11 m, 37° slopes, and base
dimensions 63 m by 78 m (Figure 3.2). The larger pile is an elongated
frustum. It is the lower part of a cone of base diameter 63.0 m and
height 23.7 m, which is cut-off at 11.0 m, with a 15.0 m extension
placed between the two halves of the frustum.
The model piles had to be small enough to be within the surface
15
-------
OJ
•c
OJ
c.
C-
c
4->
I
o
l«~
o
.c
o
4->
01
_i*
to
CM
•
CO
01
t_
3
CT>
16
-------
layer but large enough to facilitate measurements and construction of
windbreaks of height the same order as the pile height. A full-scale
neutrally stratified ABL is typically 600 m high and has a surface
layer depth of 100 m (Counihan, -1975). Since typical pile and windbreak
heights are well within the surface layer, matching the ratio of the
pile height to the boundary layer depth was not considered important.
Instead, Jensen's (1958) criterion of matching the ratio of the model
height to the surface roughness length (z0) was used to obtain the
relevant model length scale. Model piles 110 mm high (1:100 scale)
resulted (Re=2.9 x 10 , based on UQ and the pile height). The model
surfaces were roughened to make them aerodynamically rough with
roughness elements of size greater than approximately 400v/U0 (roughness
Reynolds number criterion) (Snyder, 1981). The pile could not be
roughened with the same gravel as that covering the floor of the tunnel
because the 10 mm gravel was considered too large for the pile size.
Gravel having diameter less than 4 mm was used instead. With a free-
stream speed of 4 m/s, the roughness Reynolds number criterion discussed
above was met with the 4 mm gravel. The terrain surrounding actual
piles is also usually rougher than the pile surface.
Finally, the windbreak material was chosen from 50%, 60%, and 70%
porous synthetic material that is commercially available for use in
windbreaks. The openings of 25 mm x 12 mm and 25 mm x 25 mm in the 60%
and 70% porous screens were considered too large for the overall
windbreak height (110mm) for direct use in the wind tunnel. Hence, a
more uniform windbreak material having the same aerodynamic drag or
pressure-drop coefficient as the more porous commercial screens was
17
-------
desired. Caput el al. (1973) used the pressure-drop coefficient cp
to account for the porosity and size and shape of the windbreak
openings. The pressure drop coefficient is defined as
cp = AP/(0.5pU2), (3.1)
where AP is the static pressure drop across the material, p is air
density, and U is reference wind speed, here, the wind speed upstream
of the windbreak.
The pressure drop coefficients of the three windbreak materials and
a 16 x 18 mesh nylon screen (65% porous) were determined through tests
in a smaller wind tunnel (1 m x 1 m x 4 m). The material was placed 1 m
from the entrance to the test section and fully covered the cross
section. The pressure drop was measured with pitot-static tubes placed
upstream and downstream of the material, both connected to an MKS
Baratron (Type 170M-6B) capacitance manometer. The output was digitized
at a rate of 200 Hz over the 60 s sampling time, then processed on the
EPA FMF Digital Equipment Corporation (DEC) PDP-11/40 minicomputer.
The pressure drop coefficients for the 50%, 60%, and 70% porous
materials and the mesh screen were found to be 5.5, 2.2, 1.4, and 1.8,
respectively, independent of wind speed for U > 2 m/s. The mesh screen
was therefore used to represent the high porosity material as its Cp
was midway between that of the 60% and 70% screens. Note that cp for
the 50% porous windbreak material is nearly three times that for the
mesh screen.
Given the boundary layer, the model piles, and windbreak materials,
the experimental procedure may be described. It consisted of three
\
major parts:
18
-------
(1) Measurement of surface wind speed patterns on a conical
storage pile with and without a windbreak located upwind,
(2) Measurement of surface wind speed patterns on an oval,
flat-topped pile with and without a windbreak located upwind or on the
top of the pile, and
(3) Measurement of vertical profiles of velocity downstream
of a porous windbreak over horizontal terrain.
The windbreak and conical pile set-up is shown in Figure 3.3.
Each windbreak had vertical metal supports at both ends, which were
tacked to the floor and to which guy wires were attached. The windbreak
material was folded underneath the gravel-covered plywood sheets on the
tunnel floor. Windbreaks of 50% and 65% porosity; heights 0.5H, l.OH,
and 1.5H, where H is the pile height; lengths l.OD and 1.5D, where D is
pile base diameter; and positions 1H and 3H from the upstream pile base
were tested. All combinations of the parameters result in six
windbreaks of each porosity placed at two different distances from the
conical pile, i.e. a total of 24 cases (Table 3.1). Upon completion of
these tests, one windbreak was placed at angles of 20° and 40° from the
position normal to the incident flow.
For the oval, flat-topped pile, one of the windbreak orientations
was similar to that shown in Figure 3.3; the longer axis of the pile
being parallel to the windbreak. The same windbreak porosities and
relative sizes and positions were used (the windbreak length is given
in terms of the pile base length (B)) except that not all of the 24
cases were tested. A few other relative sizes were tested: heights
0.75H and 1.25H, and length 0.6B (length of flat top, see Figure 3.2).
19
-------
QJ
c
•r-
a
T3
C
fO
-------
TABLE 3.1 COMBINATIONS OF WINDBREAK PARAMETERS USED WITH THE CONICAL
PILE
porosity (%)
50
50
50
50
50*
50
50
50
50
50
50
50
65
65
65
65
65
65
65
65
65
65
65
65
height (h/H)
0.5
0.5
0.5
0.5
1.0
1.0
1.0
1.0
1.5
1.5
1.5
1.5
0.5
0.5
0.5
0.5
1.0
1.0
1.0
1.0
1.5
1.5
1.5
1.5
length (L/D)
1.0
1.0
1.5
1.5
1.0
1.0
1.5
1.5
1.0
1.0
1.5
1.5
1.0
1.0
1.5
1.5
1.0
1.0
1.5
1.5
1.0
1.0
1.5
1.5
position (P/H)
1
3
1
3
1
3
1
3
1
3
1
3
1
3
1
3
1
3
1
3
1
3
1
3
*This windbreak was also placed 20° and 40° from the
position normal to the incident flow.
21
-------
The other windbreak orientation was to place a windbreak on the pile
top to reduce wind speeds in active regions of the pile. The windbreaks
were placed either close to the centerline or at the upstream edge of
the pile top parallel to the pile's longer axis. Two heights, 0.14H
i
and 0.27H, and two lengths, 0.16T and 0.5T, where T is the pile top
length, were tested. For windbreaks in both positions the pile was
rotated 20° and 40° to simulate other wind directions. The
specifications for all the windbreaks used with the oval, flat-topped
pile are given in Table 3.2.
The final phase of the project was to measure velocity downstream
of the less porous windbreak oriented normal to the air flow to
determine whether reverse flow was present and to relate the flow behind
the windbreak to that over a pile placed in the lee of the windbreak.
The windbreak chosen was 50% porous, 112 mm high and 1180 mm long
(aspect ratio 10.5). Vertical profiles of velocity were measured with
hot-wire and pulsed-wire anemometers along its centerline at downstream
distances of 1, 5, 8, and 12 windbreak heights.
3.2 INSTRUMENTATION
Velocity profiles were measured at selected locations over the pile
surface and flat tunnel floor with hot-wire and pulsed-wire anemometers
and wind speeds were measured at a number of points over and close to
the pile surface with fixed thermistors. Hot-wire and pulsed-wire
probes were attached to the MWT instrument carraige which can position
the probe to within ±1 mm and may be operated by remote control outside
the tunnel. The Cartesian coordinate system is oriented such that the
x, y, and z axes are longitudinal (positive downstream), lateral and
22
-------
TABLE 3.2 COMBINATIONS OF WINDBREAK PARAMETERS USED WITH THE OVAL,
FLAT-TOPPED PILE
porosity (%) height (h/H) length (L/B) position (P/HJ
50 0.5 ' 0.6 1
50 0.5 0.6 3
50 0.5 1.0 1
50 0.5 1.0 3
50 0.5 1.5 1
50 0.75 1.0 1
50 0.75 1.0 3
50 1.0 0.6 1
50 1.0 0.6 3
50 1.0 1.0 1
50 1.0 1.0 3
50 1.0 1.5 1
50 1.25 1.0 1
50 1.25 1.0 3
50 1.5 0.6 1
50 1.5 0.6 3
50 1.5 1.0 1
50 1.5 1.0 3
50 1.5 1.5 1
65 0.5 0.6 1
65 0.5 0.6 3
65 0.5 1.0 1
65 1.0 0.6 1
65 1.0 0.6 3
65 1.0 1.0 1
65 -1.0 1.0 3
65 1.5 0.6 1
65 1.5 0.6 3
65 1.5 1.0 1
length (L/T) location
65*
65*
65
65
65
0.14
0.14
0.27
0.27
0.27
0.5
0.5
0.16
0.5
0.5
upstream edge
center! ine
upstream edge
upstream edge
center! ine
*These windbreaks were also placed 20° and 40° from the
position normal to the incident flow.
23
-------
vertical, respectively. The corresponding velocity components are u,
v, and w. Theory and operation of the three anemometers will be briefly
discussed.
Hot-wire anemometers are commonly used in turbulent, wind tunnel
flows. A TSI, Inc. constant temperature hot-wire anemometer (model
1053B) with a TSI boundary-layer cross-wire probe (model 1243) was used
to obtain vertical profiles in the undisturbed (no pile, no windbreak)
boundary layer and downstream of a windbreak. Mean velocity, angle of
flow, r.m.s. turbulence velocities and turbulence intensities in two
directions (longitudinal and vertical) and Reynolds stress were
obtained. Yaw response corrections developed by Lawson and Britter
(1983) were applied to the turbulence intensity and stress measurements.
The anemometer has been extensively used by other investigators at the
EPA FMF (e.g., Castro and Snyder, 1982; Pendergrass and Arya, 1984).
The probe was calibrated in the free stream against a standard pitot-
static tube each day. The hot-wire output was fit to a King's law form
by the hot-wire calibration routine CALLPA:
£2 = aUa + b, (3.2)
where E is output voltage, U is wind speed, and a, b and « are
constants. CALLPA gives the best-fit a, b, and a. An example of the
computer output is shown in Figure 3.4. The hot-wire output was
digitized to 12 bit precision and processed at a rate of 500 Hz on the
PDP-11/40 with the program HOT. 90 s samples provided reasonably
repeatable results.
The pulsed-wire anemometer is used to measure mean and fluctuating'
velocities in regions where turbulence intensity is very high or flow
24
-------
,_" I I I I
£! O«nc!piff»M §
— L'I t\i —. -«f\t *
.-*. OO —— OO LJ
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ft. . ' ' rj
t- CO
=^_ (Db'.cri —UT— d
0-
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CCli.
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'— l-OOujOO=»0> >.
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25
-------
reversal occurs. Measurements with the pulsed-wire anemometer were
taken downstream of a windbreak since flow visualization indicated
intermittent flow reversal.
A pulsed-wire probe consists of a pulsed wire normal to and between
two parallel sensor wires (Figure 3.5). The pulsed wire is activated
by a voltage pulse, momentarily raising the pulsed-wire temperature to
several hundred degrees Celcius and heating the air surrounding the
wire. The heat, acting as a tracer, is advected by the wind with the
velocity occurring at that instant. Depending on the instantaneous
flow direction, one of the two sensor wires (operated as resistance
thermometers) senses the tracer. Thus, the basic measurement is of the
time-of-flight of the heat tracer from the pulsed wire to either of the
sensor wires. Ideally, the time-of-flight t is
t = d/(Ucosy), (3.3)
where d is the distance between the pulsed wire and sensor wire, U is
the magnitude of the velocity vector and Y is the angle between the
direction normal to the probe plane and the instantaneous velocity
vector (Figure 3.5). The probe plane is the plane parallel to all
three wires. In other words, given the separation distance between the
wires, the velocity component normal to the probe plane is calculated
from the time-of-flight. If the velocity at the time of pulsing is
close to zero or if the angle of flow (Y) is greater than about 70°
(based on the sensor wire length and d), the heat tracer will not be
sensed at the sensor wires. In such cases, the velocity is recorded as
zero. For operation in turbulent flow the wire is pulsed many times
per second over a ^given time period to obtain repeatable mean and
26
-------
c
i_
CL
o
o
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c
i
•C
QJ
3
CL
00
in
•
op
OJ
(_
3
O
27
-------
r.m.s. fluctuating velocity components.
The basic electronic components, in addition to the sensor wires,
are a differential amplifier, a differentiator, and two comparators.
The polarity of the output from the differential amplifier indicates
which sensor wire received the tracer. The signal is then
differentiated to provide a more clear indication of the time the heat
tracer was sensed since the tracer diffuses as it is advected. One
comparator is set to be triggered by a positive sensor wire signal
(positive velocity) and the other by a negative signal (negative
velocity). A 12 bit binary counter begins when the pulse is fired and
stops when either of the comparators is triggered. Hence the time-of-
flight is measured and the flow direction is known. The other major
electronic component is a circuit to eliminate the spike on the sensor
wire circuit which occurs when the pulse is fired. The sensor signals
at various stages of the electrical operation may be observed on an
oscilloscope. Typical signals are shown in Figure 3.6; the signal with
the spike, the signal less spike, the differentiated signal, and the
time-of-flight signal. The time-of-flight signal indicates when the
counter stops. Further details on the theory of the pulsed-wire
anemometer are found in Bradbury and Castro (1971).
A PELA Flow Instruments, Ltd. pulsed-wire anemometer was calibrated
in the tunnel free stream against a standard pitot-static tube. The
probe plane was oriented perpendicular to the velocity component of
interest, U. The FMF pulsed-wire calibration computer program PWCAL
fits the wind speed, U, and time-of-flight, t, data to the relation
'* U = C/t + F/t2 (3.4)
28
-------
00
•O
lU
4J
C
(1)
H
0)
-o
x-x
O
(0
c
oo
•rH
co
o
I
UJ
O
0)
S
c_
Ol
O
in
O
in
c
CD
o
•r-
Q.
^
VO
*
CO
O)
i_
3
Ol
29
-------
where C and F are constants. An example of the fit Is shown in Figure
3.7. After one sensor wire was calibrated, the probe was rotated 180°
to calibrate the other sensor wire. Data were collected and processed
by the program PWAHOT on the FMF' DEC PDP-11/44. A sampling rate of
20 Hz over 3 minutes gave reasonably repeatable results.
The last instrument to be described is the thermistor which was
used to measure wind speeds near the pile surface at various locations.
We have considered wind speed near the surface as the basic parameter
affecting particle uptake. Unfortunately, it is difficult to measure
the wind speed near a roughened, sloping surface, particularly when the
wind direction at the measurement point is unknown and difficult to
determine. Pitot-static tubes and hot-wire anemometers require certain
orientation with respect to the wind for accurate results. Heated
thermistor beads were used here for several reasons. First, several
thermistors could be mounted on the pile surface, eliminating the
tedious task of moving a hot-wire from location to location, orienting
it properly and placing it the same distance from the surface for each
sample. Once the thermistor was fixed at the pile surface, errors
resulting from differences of thermistor orientation and height, and
effects of local roughness elements were minimized by comparing wind
speed at a specific point on the pile for the cases with and without a
windbreak.
The resistance-temperature relationship for thermistors is
expressed as
R= R'exp{3[(T)-l - (I')-*]}. (3.5)
where R and R' are the resistances at temperatures T and T1,
30
-------
TRIG LEVELS* 1. I. Srf
U = 17£H.:)5/T
5.
l/T
0.00:53
0. 00 • 4?
0.10:33
0.00112
0.00095
0.00079
o.oocsa
0.00034
U IME"i
U.-fQ
3 . fi ?Ci
3 ^22
2! 850
2.310
1.830
1.27C
0.630
SSE
L 1C--
4 . HS3
H 3" ^
3i-i23
2.323
2.3:u
1.6i6
1.237
0.57J
= O.OC'499
L " - V ' 1' .
-C". 372
C.6G5
C.2S5
-C. C<2!
3. 53o
-2.553
6.309
S =0.9
v.s
.0002
.OOO't
.QOC5 .OD09 .00!
l/T C1/KICR25EC2
.0015
Figure 3.7 Example pulsed-wire anemometer calibration plot.
31
-------
respectively, and 6 is a constant dependent on the thermistor material.
All the thermistors (Fenwal GB31J1) were calibrated in a hot oil bath-.
Calibration curves for two of those calibrated and the Fenwal rated
curve, R(25°C) = 1000 ± 200 n, & = 3442 ± 90 K, are shown in Figure 3.8.
Fenwal specified the thermistor time constant of 4 s in still air at
25°C. The thermistor diameters were about 1 mm and their lengths were
about 1.5 mm.
The electric circuit consisted of a regulated power supply of
constant voltage E and thermistor-resistor pairs in parallel with the
power supply (Figure 3.9). The voltage across each series resistor is
the output voltage (V-j). Voltages V-j and E were digitized at a rate of
50 Hz and processed on the POP 11/40 minicomputer with the computer
program THE. A one minute sampling tine gave reasonably repeatable
results.
Thermistor anemometers operate under the same basic principle as do
hot-wire anemometers, that is, the heat loss from the sensor is a
function of wind speed. Rasmussen (1962) has shown that
i2R = K(TT - TJ, (3.6)
where i is current through the thermistor, R is thermistor resistance,
Tj and Ta are thermistor and ambient temperature, respectively, and K
is the dissipation factor. K is a function of the wind speed and the
properties of the fluid surrounding the thermistor, and was determined
experimentally as follows. The thermistor was placed in the center of
the smaller wind tunnel with the probe support body and the thermistor
oriented vertically (the same orientation with respect to the air flow
as for a thermistor1 mounted on the model pile). At each wind speed, E
32
-------
10
a
z •
o
z
UJ
o
I
z
o
o
oc
.1 -
.01
Rated calibration
Thermistor #1 calibration
Thermistor #2 calibration
R(25<>C) __§__
1000 0 3442 K
883 n 3509 K
1193 0 3495 K
.0025 .0026 .0027 .0023 .0029 .003 ,0:31 .0032 .0033 .:C3^ .0035 .CC3j
1/TEMPERfiTUPE (K-'-l
Figure 3.8 Resistance-temperature thermistor curves.
33
-------
_C
L_ *"**
Figure 3.9 Thermistor circuit
O
o
.1
K(U) = 0.97 ua27
t __!_ . t. II
1
U (m/s)
10
Figure 3.10 Dissipation factor vs. wind speed for Fenwal GB31J1
thermistors. Triangles: original data. Squares and circles:
additional data>
34
-------
and V-j were measured. From those, i, R and Tj were calculated. Ta was
measured with a YSI model 4320 temperature sensor. Hence, K at each
wind speed was calculated. By varying the wind speed from 0.3 m/s to
8 m/s, the functional relationship of K (mW/°C) to u (m/s) was found to
be (Figure 3.10)
K = 0.97u0.27. (3.7)
Hence, thermistor output is related to wind speed. It was originally
assumed that this relationship was valid for all the thermistors used
in this project. Later, two other thermistors were similarly
calibrated. The curves of thermistor output (K) vs. tunnel wind speed
were similar to that of the original curve (refer to Figure 3.10); the
maximum differences in wind speed were ±10%. The original calibration
tended to slightly overestimate wind speed as compared to the data from
the other thermistors.
Tests of thermistor sensitivity to orientation were also conducted
in the smaller wind tunnel. With the probe and support in the vertical
position, two tests were conducted. First, the probe was rotated about
the vertical axis, sampling the output for several orientations at a
given wind speed. The maximum difference in measured wind speed from
the tunnel speed was 5%. Second, the wind speed calibration was made
with the probe oriented vertically as described above, but rotated 90°,
to observe effects of the leads exposed between the thermistor and the
support. Measured speeds were within the scatter of the calibrations
for the probe in the original orientation. Hence, orientation about
the vertical axis was not significant. Yaw sensitivity was tested by
orienting the probe* and support in the horizontal, normal to the air
35
-------
flow, and rotating the support in the xy-plane to angles up to ±30°.
The maximum difference in measured wind speed from the tunnel speed was
8%.
The thermistors were mounted normal to and about 2 to 3 mm above
the pile surfaces; close enough to the surface that the flow presumably
parallels the surface, and where wind speed may be assumed to be
directly related to the surface shear stress. Nine thermistors were
mounted at different elevations on the conical pile in the arrangement
shown in Figure 3.11. 81 thermistors were mounted on the oval, flat-
topped pile in the arrangement shown in Figure 3.12.
Thermistor and hot-wire anemometer (TSI Model 1210, end-flow
single-wire) measurements were compared to further substantiate
thermistor use as an anemometer and the use of Eq. 3.7. For the pile
oriented in the position shown in Figure 3.11 with air flow from the
left, wind speeds were measured by the middle thermistor on the windward
side of the pile. Wind speeds were calculated with the calibration
equation (Eq. 3.7) for several wind tunnel tachometer settings. The
pile was rotated approximately 45° and the hot-wire was placed at the
same relative position as the thermistor. Wind speeds were measured at
the same tachometer settings. The results showed good agreement (Figure
3.13). However, slight differences in height above the surface,
individual roughness elements, and thermistor and hot-wire orientation
could cause larger differences in the velocities measured by the two
probes. The same may be expected for velocity measurements using
different thermistor probes in apparently similar settings.
As discussed above, the maximum error in wind speed due to
36
-------
Figure 3.11 Top view of conical
pile. Stars: Thermistor positions
on pile. Dots: Effective thermistor
positions due to pile rotation.
Figure 3.12 Top view of oval,
flat-topped pile. Dots: Thermistor
positions.
37
-------
3.5 ••
± 2.5 -
LU
CC
I
LU
O
~ 1.5 +
tn
1 1.5 2 2.5 3 3.5
U, THERMISTOR W/S:
Figure 3.13 Comparison of thermistor and single-wire wind speed
measurements.
38
-------
thermistor orientation was ±8%. Possible systematic errors resulting
from the application of the calibration curve (Eq. 3.7) to all the
thermistors could have been eliminated if each thermistor had been
calibrated; however, due to time constraints, each probe could not be
calibrated. Effects of individual roughness elements and thermistor
height above the surface were other possible sources of systematic
error. For the purposes of the present study, wind speed was used more
as a relative, than absolute, measure, i.e. wind speed with and without
a windbreak or wind speed with one windbreak and that with another were
compared. The error indicated above is for absolute wind speed, but
relative wind speed was of more importance here; its error was expected
to be even less.
In addition, thermistors cannot detect flow reversal, i.e. they
respond to wind speed and not wind velocity. If reverse flow were
steady, the magnitude of the measured wind speed would be fairly
accurate. On the lee side of the piles, with unsteady flow, the
measured average mean wind speed will be higher than the actual (vector-
averaged) speed.
Wind speed distributions were measured on the piles in the absence
of any windbreak at least once every three days and frequently once a
day to determine system repeatability. The peak-to-peak difference at
a given position was ±10%, an r.m.s. error of ±2%, indicating quite
good repeatability.
For the conical pile, each run consisted of measuring the wind
speed with the nine thermistors, rotating the pile 30°, measuring the
wind speed at thesexnine positions, etc., through 360°, resulting in
39
-------
108 data points per run (refer to Figure 3.11). For the oval, flat-
topped pile, each run consisted of measuring the wind speed at the 81
positions shown in Figure 3.12.
40
-------
4. FLOW ABOUT A POROUS WINDBREAK
Mean and fluctuating longitudinal velocity components were measured
downstream of a representative windbreak (50% porous, height (h)=112 mm,
t
aspect ratio=10.5) with hot-wire (HWA) and pulsed-wire (PWA) anemometers
to quantitatively describe the flow structure in the sheltered area, to
determine if flow reversal occurred, and to further explain results of
the major part of the project. Due to the finite size of the windbreak,
the flow was expected to be three-dimensional. Vertical profiles were
taken at distances of 1, 5, 8, and 12h from the windbreak. The results
from the two anemometers may also be compared.
"Streamlines" for two-dimensional, steady flow were obtained from
the FMF computer program STRFUN which calculates the height at which a
given stream function value (fy) occurred (41 = /udz). Since the flow
was likely three-dimensional, the calculated "streamlines" are not
necessarily the true streamlines; knowledge of the three velocity
components are required to calculate the true streamlines. However,
along the centerline not too far from and close to the windbreak, the
flow is expected to be nearly two-dimensional. "Streamlines" derived
from the pulsed-wire data are shown in Figure 4.1. The PWA data was
used because it was expected to be more accurate since the PWA senses
flow reversal. Note the similarity to Figure 2.1(b). A region of low
wind speed near the surface between 2h and 8h from the windbreak is
evident, with the air flowing up and over this region. Flow
visualization indicated intermittent flow reversal (unsteady flow) near
the surface in this region. Effects of the windbreak are noticeable up
•\
to z=4h, with upward deflection apparent as far downstream as x=6h.
41
-------
in in in in
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For steady flows, velocity vectors are parallel to streamlines.
Since the flow angle in the xz-plane was given by the hot-wire output,
the normalized velocity vectors (U/llR(h)) are also shown in Figure 4.1
(UR(h) is the wind speed at the Windbreak height, but in its absence).
The flow generally parallels the "streamlines" except in the region
below the windbreak height at x=lh, suggesting that the flow is three-
dimensional, i.e. that some flow in the lateral direction exists, even
along the center!ine. Vortices may be formed as the air flows round
the sides of the windbreak. The hot-wire observed angles are not
nearly as large as those implied by the "streamlines" (derived from PWA
data) because of differences in wind speed profiles measured by the two
instruments. The angles clearly show upward flow at x=lh and downward
flow at x=8h.
Relative wind speed deficit may be defined as [l)R(z)-U(z)3/UR(z)
where UR(Z) is the reference speed (measured with the HWA) at the
location of the windbreak but in its absence and U(z) is a speed
(measured with the PWA) at some distance downstream of the windbreak.
Again, PWA data are presented as they are expected to be more accurate.
Lines of constant relative deficit are seen in Figure 4.2. Downstream
distance and height are scaled by the windbreak height h. A large
region of wind speed reduction was observed downstream of the windbreak.
The height of the region increased from z=1.4h at x=lh to z=3h at
x=12h. Below z=lh, wind speeds were reduced at least 50% from the
upstream value at the same height. The greatest deficits were observed
for heights less than z=0.5h between approximately 4 and 8h downstream.
In other words, the maximum deficit did not occur immediately downstream
43
-------
c
o
O)
•a
0)
Qi
CM
-------
of the windbreak, but occured farther downstream. A strong gradient is
clearly evident just above and downstream of the windbreak.
Lines of constant turbulence intensity are shown in Figure 4.3,
where turbulence intensity is the ratio of the r.m.s. fluctuating
longitudinal velocity (u1, measured with the PWA) at a given location
to the mean wind speed at that location. The maximum was observed near
x=8h below z=0.5h. Intensities of greater than 30% were observed for
heights less than 1.5h between x=2 and 12h and at x=lh just above the
windbreak height. A pulsed-wire must be used here because with
intensities greater than 25-30% the errors with hot-wire data become
large.
The fluctuating velocity component may also be described in
absolute terms. Figure 4.4 again shows the distribution of u', but
normalized by UR(|I). The presence of a maximum between z^l.ZBh and
1.5h suggests that greater turbulence is generated in the shear layer
separating from the top of the windbreak. The turbulence diffuses with
downstream distance. The normalized r.m.s. velocity fluctuations were
quite low (5-7%) just downstream of the windbreak.
Figure 4.5 shows vertical profiles of the longitudinal mean
velocity component (measured with both anemometers) normalized by UR(h)
as a function of downwind distance. These are compared with the
reference wind speed profile. Comparison of the HWA and PUA results
show significant differences only in the region of high turbulence
intensity. The HWA tended to overestimate the wind speed in the high
turbulence region. No flow reversals in the mean occurred, but
instantaneous reversals were indicated by the PWA (when the upwind
45
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48
-------
sensor wire sensed the tracer) and used to calculate the average over
the sample time. The HWA time-averaged mean is not a vector average
since the flow direction is not sensed. Hence, the PWA wind speeds
were smaller than the HHA indicated wind speeds. Again, the profiles
show significant reductions in speeds near the surface at x=5h and 8h.
The reductions from the reference values are quite clear.
Figure 4.6 shows vertical profiles of the r.m.s. longitudinal
fluctuating velocity component normalized by U^(h). Again, results
from the HWA and PWA downstream of the windbreak and the HWA reference
profile are shown. The PWA results were shown in Figure 4.4 in a
different format. The dashed lines separate the region in which the
r.m.s. fluctuations exceed 10% of the reference value from those in
which the fluctuations are less than or nearly equal to the reference
value. Increased turbulence is quite apparent at x=lh just above the
windbreak height. The maximum fluctuation at each downwind distance is
approximately twice the reference value at the same height. The
r.m.s. fluctuations in the lower region are approximately half the
reference r.m.s. fluctuations at x=lh. Vertical diffusion of the
increased turbulence in the shear layer is again apparent. PWA values
tended to be higher than HWA values for heights greater than the
windbreak height and slightly less than HWA values for lower heights.
The greatest differences were in the region of the highest r.m.s.
fluctuations.
49
-------
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5. FLOW ABOUT STORAGE PILES
The flow about the two storage piles will be described. It is
important to locate the areas of high wind speed since that is where
particle uptake is most likely to occur. It is also important to locate
the areas of low wind speed; the areas where wind erosion is least
likely to occur. The potential fugitive-dust emissions from
uncontrolled (no windbreak protection) piles are related to the wind
speed distributions shown here.
5.1 CONICAL PILE
Figure 5.1 shows the top view of the pile with contours of
normalized wind speed, u/ur, where u is the wind speed at the pile
surface measured with the thermistor and ur is the wind speed at the
equivalent full-scale height of 10 m in the absence of the pile. 10 m
was chosen as the reference height rather than 11 m, the pile height,
because 10 m is the standard height to which "surface" wind speed
measurements are usually referenced in the meteorological literature.
Mote that the air flow in the figure is from the left. The areas of
maximum wind speed are near the top of the upwind face but toward the
sides of the pile. A high speed region (u/ur > 0.8) is on the upstream
face, extending from near the crest down both sides. The area of
minimum wind speed is in the lee near the top of the pile with regions
of low wind speed extending down the pile on both sides of the
centerline. High speeds along the pile sides are expected because the
flow is accelerating round the pile. The flow separates on the lee
side, resulting in « region of low-speed recirculating flow.
51
-------
Figure 5.1 u/ur about conical pile for no windbreak case,
52
-------
5.2 OVAL, FLAT-TOPPED PILE
Flows about the pile oriented with its longer axis normal to the
air flow, then at 20° and 40° from the original position are described.
Figure 5.2 shows the top view of the pile with contours of normalized
wind speed. This was the pile orientation during all the windbreak
tests, except for the 20° and 40° pile orientations. The overall
pattern, as indicated by the isotachs, is fairly symmetric except for
some asymmetry of flow in the lee. Since the thermistors were located
in a symmetrical pattern on the pile, a point-to-point comparison of
measured wind speed about the axis parallel to the wind direction was
made. Several significant differences were observed. Upon completion
of the runs with the windbreaks, wind speeds were measured with the
pile oriented 180° to its original position. Again, the overall pattern
was similar to that shown in Figure 5.2, but point-to-point comparisons
about the line of symmetry showed several significant differences. The
differences likely resulted, in part, from the sources of error in the
thermistor measurements discussed in section 3.2. Further, with nine
times as many thermistors on this pile, as compared to the conical
pile, errors from applying the calibration equation (Eq. 3.7) to all
the thermistors could be greater than that discussed earlier.
Considering these uncertainties, the wind speed pattern was corrected
based upon the data from the 0° and 180° pile orientations, assuming
flow symmetry around the pile surface.
The corrected pattern is seen in Figure 5.3. 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
53
-------
flow
0.2
Figure 5.2 u/ur about oval, flat-topped pile for no windbreak
case.
54
-------
.4
_flow
direction
'bogt
"at-topped p,,. for
no
55
-------
Again, the lowest speeds were observed in the lee; but a secondary
minimum also occurred on the top of the pile.
Thermistor derived wind speed values for the cases with a windbreak
t
were also corrected. The correction factor for each thermistor was the
percentage difference between the value measured with the pile in the
0° position and the corrected value, both in the absence of any
windbreak. The data for all the windbreak cases and for all the pile
orientations were corrected, using the same correction factors which
ranged in magnitude from 0% to 50%, with 82% of the correction factors
being within ±20%.
The normalized wind speed patterns for the pile oriented at 20°
and 40° from its original position normal to the air flow are shown in
Figure 5.4. The band of high speed near the top of the upstream face
was observed for both orientations. The lobes of high speed extending
toward the base on the sides shifted with pile orientation.
56
-------
o
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10 H
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6. WINDBREAK EFFECTS ON FLOW ABOUT STORAGE PILES
The use of windbreaks for storage-pile fugitive-dust control is
based upon the existence of a sheltered region downstream of a
windbreak. Effects of windbreak height, length, porosity and position
i
on wind speed about the piles are discussed for the case of a windbreak
placed upstream of the piles and on the top of the flat-topped pile.
Relative wind speed reduction and the observed maximum wind speed are
used to assess the relative effectiveness of the various windbreaks.
6.1 WINDBREAK UPSTREAM OF THE PILE
Before the results are presented, general windbreak effects are
hypothesized based upon flow structure and pile location in the
sheltered region.
6.1.1 Relation to flow structure
Since height and width of the sheltered region are directly
related to windbreak height and length, windbreaks placed upstream of
the pile having dimensions less than the pile height or length are
expected to be less efficient as are smaller windbreaks placed on the
pile top. Wind speed reductions downstream of a windbreak were greatest
near the surface and decreased with height (Figure 4.2). Although some
reductions were observed at heights greater than the windbreak height,
little, if any, reductions were observed at z=2h. Therefore, windbreaks
of height <0.5H cannot be expected to have much effect on reducing the
high wind speeds occurring near the tops of the piles; the effect may
be greater when the pile is farther from the windbreak since the height
of the sheltered region increases with downwind distance. Similarly,
\
windbreaks of length less than the pile base length are expected to be
58
-------
less effective since the pile is not "fully within" the sheltered
region.
Figures 6.1 and 6.2 show a superposition of the conical and
oval, flat-topped piles' cross-sections at the 1H and 3H positions for
the three windbreak heights onto the "streamlines" downstream of a
windbreak of length 10.5 times its height (Figure 4.1). Streamlines in
the pile's presence are not the same as those shown; the location of
the low wind speed region with respect to the pile is of interest here.
The upstream face of both piles is in the same relative position, but
the difference in pile size is quite apparent. For the 0.5H high
windbreak, the upstream face is in a lower wind speed area when the
pile is in the 1H rather than the 3H position. For the l.OH and 1.5H
high windbreaks, however, the upstream pile face is in a lower wind
speed region when the pile is in the 3H position. The pile may be in
an even lower wind speed region at 6H for the 1.5H high windbreak.
6.1.2 Results
Windbreak effects on surface wind speed patterns are discussed in
this subsection. Windbreaks were placed upstream of both piles, normal
to the wind direction, and at an angle to the wind direction with the
conical pile. The cases with a windbreak normal to the air flow are
discussed first. Figures showing normalized wind speed for many of the
windbreak cases are not included because the change in wind speed due
to a windbreak is of more interest. With a windbreak, the wind speed
at a given location on the pile surface is some fraction of that in the
unprotected case. The relative amount by which the wind speed is
\
reduced is called the wind speed reduction factor R^ and, in percent,
59
-------
e
"O •*-*•
QJ O
to C
-o ^
c
J3
c .
o o
O
D.
r— CT>
•t— -i—
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jcr
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O O
O O
O t-
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t- o.
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3
«/> <*-
VO OJ
t_
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u_ -a
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60
-------
. tf>
. . o
/
y
.e
x
. . CD
. . to
o
-O
I C
*• o
(O O
> CL
O
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4J QJ
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l^ =
OJ
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M/z
M/z
61
-------
is defined as
Ri = K,i - Ui)/u0,i x 100' (6-l)
where u-j and u0>-j are wind speeds at the i-th location on the pile for
the cases with and without the windbreak, respectively. R-j is zero
when the windbreak causes no change in the wind speed, and 100% when
the wind speed is reduced to zero. R-j of 25% means that the wind speed
resulting from the windbreak's presence is 25% less than the wind speed
in the unprotected case. It is important to remember that the combined
effects of windbreak height, length, porosity, and position, as well as
turbulence in the approach flow, and pile shape and surface roughness
determine the wind speed distribution over the pile surface. Some of
these factors will be considered here.
An optimum windbreak size (height and length) exists since a very
small windbreak is expected to be ineffective and a very large windbreak
may be effective in reducing wind speeds but may turn out to be too
expensive. Contours of constant wind speed reduction factor R}
resulting from the 65% porous screen of height 0.5H, located 1H from
the pile base and length l.OD (Figure 6.3) and l.OB (Figure 6.4) are
shown. Referring also to Figures 5.1 and 5.2, in the areas of maximum
wind speed of the no windbreak cases, these windbreaks reduced the wind
speed by approximately 20%. For both piles, relatively large reductions
were observed over approximately the lower three-fourths of the upstream
face, with the greatest reductions apparent near the piles' centerlines.
Regions of wind speed increase (negative reduction factor) were observed
on the lee side of the conical pile and on the top, upstream half of
the larger pile. The increase in speed observed on the top of the
62
-------
direction"
.40
40
Figure 6.3 Wind speed reduction factor for the 65% porous windbreak
of height 0.5H and length l.OD placed 1H from the conical pile base.
flow
direction^
Figure 6.4 Wind speed reduction factor for the 65% porous windbreak
of height 0.5H and length l.OB placed 1H from the oval, flat-topped
pile base.
63
-------
larger pile may be due to a combination of air deflection about the
windbreak and the pile. In terms of fugitive dust control, these
increases are probably not significant for the conical pile because the
wind speeds will be relatively Tow in this region unless the reference
speed (ur) is very high, but the increase could be significant for the
larger pile, again depending on ur. This general pattern of high wind
speed reduction in the lower portion of the upstream face and increase
in the conical pile upper lee region and the top upstream half of the
ovals flat-topped pile was typical of all the windbreaks of height
0.5H, but not of the higher windbreaks.
Slight asymmetry is apparent in the lee of the larger pile in
Figure 6.4; this was also observed in the presence of other windbreaks.
Since the wind speeds are low in the lee, even a small change in wind
speed appears as a large relative change in the reduction factor.
For higher windbreaks (heights l.OH and 1.5H) upstream of the
conical pile, the area of greatest wind speed reduction was the upper
part of the windward side, with typical reductions of at least 50%.
Areas of negative reduction factors (up to --15%) were observed with
windbreaks of height l.OH located 3H from the conical pile; these
areas were significantly smaller than those observed with lower
windbreaks (Figure 6.3). Wind speeds were reduced everywhere for the
other higher windbreaks with reduction factors ranging from 15% to 60%
on the lee side. The reduction pattern for windbreaks of the same
porosity, length, and position did not differ significantly when the
windbreak height was increased from l.OH to 1.5H, except for the least
porous windbreak located farther (3H) from the pile. In that case, the
64
-------
highest windbreak caused the greatest reductions over much of the pile.
The reductions in the lee near the pile top also were higher for the
1.5H high windbreak.
For higher windbreaks (heights l.OH and 1.5H) upstream of the
oval, flat-topped pile, wind speeds were reduced everywhere. Increasing
the height from l.OH to 1.5H gave greater reductions on the pile top,
but the general pattern of wind speed reduction depended upon the
windbreak location. In general9 a windbreak of height l.OH located 1H
upwind of the pile base gave similar wind speed reductions (R-j) on the
top part of the upstream face and on the pile top. Increasing the
height to 1.5H caused greater reductions only on the pile top. A
windbreak of height l.OH in the 3H position caused higher reductions on
the upstream face than on the pile top. Increasing the height increased
the reductions on the top to approximately the same values as those on
the upstream face. Relative reductions of 30% to 60% occurred over
most of the top part of the upstream sloping face when windbreaks of
heights l.OH and 1.5H were close to the pile (the 1H position); for
windbreaks of both heights at the 3H position, the corresponding range
of reduction factors was 45% to 75%. The increase in wind speed
reduction on the top of the pile with windbreak height increasing from
l.OH to 1.5H was greater for windbreaks located 3H from the pile base
(e.g., for 50% porous windbreaks at the closer position, the reduction
increased from approximately 50% to 70%, but at the farther position,
it increased from approximately 30% to 60%).
To summarize, windbreak height is clearly important, as
hypothesized in section 6.1.1. The 0.5H high windbreak is not as
65
-------
effective as higher windbreaks (l.OH and 1.5H), because the forner
reduces wind speeds over a smaller portion of the pile. Higher
windbreaks reduce wind speeds over a larger part of the pile's surface
and cause greater reductions in the regions of high wind speed observed
in the no windbreak case. In general, with increasing windbreak height
the wind speed reductions increase also and cover a greater portion of
the pile, but with a conical pile, the windbreak as high as the pile is
nearly as effective as a higher one.
Increasing length is expected to effect greater reductions at the
pile sides. This was observed with the 50% porous windbreaks, although
the difference was slight as the length was increased from l.OD to 1.5D
and from l.OB to 1.5B with the conical and larger piles, respectively.
Windbreaks of length less than the pile base were also placed upstream
of the oval, flat-topped pile. The length was equal to the maximum
length of the pile at the top. Very high speeds were observed on the
upstream sloping face near the pile sides with these windbreaks.
Longer windbreaks located upwind of the pile base caused much higher
reductions in wind speeds along the sloping sides and slightly higher
reductions at the sides of the flat pile top. The reduction factors
for the two 50% porous windbreaks of height 1.5H at the 1H position are
shown in Figure 6.5. The effect of length at the sides is quite
evident. Length becomes even more important if the incident wind is
not normal to the windbreak (see later discussion in this subsection).
Porosity was another windbreak parameter under investigation. For
a given windbreak height, length, and position, greater wind speed
reductions occured for the less porous (50%) windbreak, particularly on
66
-------
flow
.40
20
20
40'
40
20
0
40
60
SSSrc
67
-------
the windward face and top of the piles. The reduction factors for the
two different porosity windbreaks of height l.OH and length l.OD located
1H from the conical pile and the two windbreaks of height l.OH and
length 0.6B located 1H from the oval, flat-topped pile are shown in
Figures 6.6 and 6.7, respectively. In both cases the distributions are
quite similar in shape, with larger reductions in the 50% porous case.
The last windbreak parameter studied was the distance between the
windbreak and pile. As discussed earlier, the effect of windbreak
position appears to be related to windbreak height. Windbreaks of
height 0.5H caused greater wind speed reduction in the lower part of
the windward face and towards the sides when placed 1H, rather than 3H,
from the piles' bases, although slightly higher reductions were observed
along the top of the slope of the oval, flat-topped pile when the 0.5H
high windbreak was at the 3H position. Windbreaks of height l.OH
placed 3H from either pile caused greater wind speed reduction on the
windward face, but for the conical pile, less reduction in the lee.
Windbreaks of height 1.5H placed 3H from either pile caused greater
wind speed reductions on the windward face than if the windbreak were
at the 1H position. However, for windbreaks of height l.OH and 1.5H
with the oval, flat-topped pile, the reduction on the pile top was less
when the windbreak was in the 3H position. For both piles, the
reductions near the center!ine along the pile slope were low near the
pile base and high towards the top of the slope with windbreaks greater
than 0.5H high at the 1H position. In the 3H position, slightly higher
reductions occurred near the top of the piles on the windward face and
much higher reductions occurred in the lower portion such that the
68 .
-------
flOW
direction
60
Figure 6.6 Wind speed reduction factor for the windbreak of height
l.OH and length l.OD placed 1H from the conical pile base with porosity
65% (solid line) and 50% (dashed line).
flow
direction"
Figure 6.7 Wind speed reduction factor for the windbreak of height
l.OH and length 0.6B placed 1H from the oval, flat-topped pile base
with porosity 65%x(solid line) and 50% (dashed line).
69
-------
reductions tended to be uniform along the slope. Figures 6.8 and 6.9
show the wind speed reduction distributions for the two windbreak
locations (1H and 3H) for the 1.5H, high 50% porous windbreaks of length
1.5D placed upstream of the conical pile, and length 0.6B placed
upstream of the larger pile. Clearly, larger areas of high reduction
occurred for the 3H location with the conical pile. Higher reductions
occurred over much of the upstream face of the larger pile whereas
lower reductions occurred on the top for the 3H case. Hence, the
overall effect would depend upon the relative magnitudes of the local
effects.
Patterns of wind speed reduction factors clearly show effects of
windbreak height, length, porosity and position. The maximum wind
speed, independent of position, may also be used to assess relative
effectiveness of the various windbreaks since it is related to the
maximum particle emission rate. Values of maximum wind speed normalized
by the wind speed at the equivalent full-scale height of 10 m in the
absence of the pile (ur) for the windbreak cases are given in Tables 6.1
and 6.2 for the conical and oval, flat-topped piles, respectively.
Note that the ratio umax/ur is 1.16 and 1.12 for the unprotected (no
windbreak) cases. In general, higher maximum wind speeds were observed
for windbreaks upstream of the oval, flat-topped pile than for the
corresponding windbreak with the conical pile, although many trends
were the same for both piles.
All the windbreaks reduced maximum wind speed, which is desired
for fugitive dust control. Windbreaks of height 0.5H caused much
higher umax/ur than, did the higher windbreaks. Differences in umax/ur
70
-------
flow
direction"
6Q
80
X60
20
60
80
Figure 6.8 Wind speed reduction factor for the 50% porous windbreak of
height 1.5H and length 1.5D placed 1H (solid line) and 3H (dashed line)
from the conical pile base.
flow
direction
•40
20
6Q
20
Figure 6.9 Wind speed reduction factor for the 50% porous windbreak of
height 1.5H and length 0.6B placed 1H (solid line) and 3H (dashed line)
from the oval, flatv-topped pile base.
71
-------
TABLE 6.1 umax/ur FOR THE VARIOUS WINDBREAKS PLACED UPSTREAM OF THE
CONICAL PILE
65% porous windbreak
50% porous windbreak
position: 1H
length: l.OD
height
0.5H
l.OH
1.5H
0.91
0.55
0.56
1.5D
0.93
0.59
0.60
3H
l.OD
0.91
0.54
0.50
1.5D
0.94
0.56
0.52
1H
l.OD
0.90
0.31
0.39
1.5D
0.93
0.34
0.42
3H
l.OD
0.82
0.37
0.25
1.5D
0.86
0.27
0.17
TABLE 6.2 umax/ur FOR THE VARIOUS WINDBREAKS PLACED UPSTREAM OF THE
OVAL, FLAT-TOPPED PILE
position:
length: 0.
height
0.5 H
0.75H
1.0 H
1.25H
1.5 H
0.
0.
0.
65% porous
1H
68
98
85
78
l.OB
0.96
0.70
0.69
windbreak
3H
0.
0.
0.
0.
6B l.OB
97
84 0.68
79
50% porous windbreak
1H 3H
0.6B
0.95
0.73
0.63
l.OB
0.92
0.58
0.45
0.48
0.45
1.5B
0.90
0.52
0.49
0
0
0
0
.6B
.93
.78
.66
l.OB
0.87
0.66
0.56
0.36
0.28
72
-------
between windbreaks of height l.OH and 1.5H, for the same porosity,
length, and position, were not nearly as great as for those between
0.5H and l.OH. In general, for a given windbreak height, length, and
position, the 50% porous windbreak caused lower umax/ur than did the
65% porous windbreak. Placing a given windbreak a distance 3H from the
pile base was more effective than placing it 1H from the conical pile
for nearly all the cases tested here. For the larger pile, the 1H
position tended to be more effective except for the higher, longer 50%
porous windbreaks and differences in umax/ur between the two positions
for the 65% porous windbreak were not significant. Windbreaks shorter
than the length of the pile base were clearly less effective.
Given a windbreak height, length, porosity and location, the
direction of the incident wind will also affect the wind speed
reductions about a pile. Wind direction effect was studied with the
50% porous windbreak of height l.OH, length l.OD, placed 1H from the
conical pile base. Since the wind direction is constant in a wind
tunnel, a windbreak oriented at an angle to the tunnel center-line
simulates a full-scale case of a fixed windbreak with the air flow at
an angle to the windbreak different from normal. Figure 6.10 shows
reduction factors for wind directions normal to the windbreak, and at
20°, and 40° to the normal. The windbreak positions are also shown in
the figures. The maximum u/ur was 0.31, 0.69, and 1.12 for the 0°,
20°, and 40° cases, respectively. For the 20° case, a region of much
lower reductions was observed on the side of the pile opposite the
windbreak, indicating that the windbreak length and position are
important. For the 40° case, the region of reductions greater than 40%
73
-------
direction
flow
direction
flow
direction
60
(a)
(c)
Figure 6.10 Wind speed reduction factor for the 50% porous windbreak of
height l.OH and and length l.OD placed 1H from the conical pile base
oriented (a) normal* (b) 20°, and (c) 40° to the flow direction.
74
-------
was quite small; indeed wind speed increases were observed. Clearly,
windbreak effectiveness decreases with increasing angle of flow from
the normal.
6.2 WINDBREAKS ON THE PILE TOP
In addition to placing windbreaks upstream of both piles, they were
placed on the top of the oval, flat-topped pile in the positions shown
in Figure 6.11. As stated earlier, the 65% porous windbreaks were of
two heights and two lengths and were placed either near the leading
edge or centerline of the pile. For a given windbreak in both
positions, the pile was rotated 20° and 40° from the original position
to assess wind direction effects.
Large reduction factors (up to 65%) on the pile top were observed
for all the cases. The location and extent of the area with significant
wind speed reduction depended upon windbreak size, location, and angle
of the incident flow. Higher, longer windbreaks caused larger sheltered
regions as shown in Figure 6.12. The width of the sheltered region
(parallel to the windbreak) is larger than the windbreak and areas of
wind speed increase (negative reductions) were generally observed to
the sides of the windbreaks. The greatest reductions were observed
between about 3h and 6h downstream of the windbreak. On the sloping
surfaces of the pile, reduction factors of 5 to 10% were observed
upstream of the three windbreaks placed near the leading edge and
negative reduction factors in the lee of the pile downstream of the
windbreaks. A windbreak placed near the centerline caused wind speed
reductions upstream, as well as downstream, of the windbreak on the
V
pile top (Figure 6.13(a)). The area of coverage and magnitude of the
75
-------
Figure 6.11 Sketch of windbreak positions with respect to thermistor
positions on the top of the oval, flat-topped pile.
76
-------
A
U
0)
O)
in
t.
cn
0)
-C
O
t_
O
Lft
-
t- en
O c
O T3
•)-> C
O nt
f-. •
C Cvf I—
o . vo
•r- O •— •
O 4-> O
3 J=
-O cr>-C
O) -r- *J
C_ O) 0>
J= C
-o o>
TO O>
c em
•r- TD f^
3 r— -^ ^
•r- «r- O
U. Q-- --
77
-------
CsJ
O
+-> •(->
o c
«o o>
«*- o
4J CX
o
13
XJ TJ
OJ 1)
O) C
CX
U1 T3
O)
•o o
c
78
-------
reductions were greater for a higher windbreak. Reduction factors for
the 0.14H high, 0.5T long windbreak placed near the centerline with the
incident flow 40° to the windbreak normal clearly show that the area of
coverage shifted with the wind direction (Figure 6.13(b}). Smaller
reductions were observed on the slope downstream of the windbreak.
Here the reduction factor was defined with respect to the no windbreak
case with the pile at 40° to the flow.
To summarize, windbreaks on the pile top reduce wind speeds on the
top both upstream and downstream of the windbreak. Greater reductions
were observed a distance 3h to 6h downstream of the windbreak than
immediatedly in its lee. Small reductions occurred along the slope
upstream of the windbreaks placed near the leading edge, but, in
general, the areas of high wind speed without any windbreak were not
affected. Hence windbreaks located on the top of the pile may not
provide much protection against high winds, but may be used to locally
protect regions on the top of the pile.
79
-------
7. FURTHER ANALYSES AND DISCUSSION OF RESULTS
In this section, windbreak efficiencies based upon the
relationships between particle uptake and wind speed are presented.
i
Then the results of both parts of the experiment (flows about a
windbreak and windbreak-protected piles) are compared to results of
previous studies.
7.1 RELATION TO PARTICLE UPTAKE
Relative windbreak effectiveness may be assessed by several
methods. Wind speed reduction patterns and the normalized maximum wind
speed have been described. Further analyses based on particle uptake
at wind speeds exceeding a threshold value will be described. Results
for windbreaks placed upwind of the pile will be discussed first.
If emissions had been measured directly, the windbreak efficiency E
would have been defined as
E = 1 - (Q/Qo) (7.1)
where Q and Q0 are the storage-pile fugitive-dust emission rates with
and without the windbreak, respectively. Since wind speeds have been
measured here, assumed relationships between wind speed and emissions
are used to calculate efficiencies. Depending on the reference wind
speed, the wind speed on the lee side of the piles and on part of the
top of the oval, flat-topped pile in the absence of any windbreak may
be less than the threshold. In general, with certain windbreaks, the
speeds in these regions could become greater than the threshold.
Furthermore, a windbreak may reduce wind speeds to values less than the
threshold over partner all of the pile. Hence a better definition of
efficiency would include a threshol-d value. However, threshold speeds •
80
-------
have been determined only for a few cases as discussed in section 2.1.
The relationship between threshold speed and particle size and moisture
content is not understood. If a given threshold speed were applied to
the data here, a full-scale reference wind speed would have to be
assumed. To a first approximation, it is assumed here that the
reference wind speed is sufficiently high that wind speeds everywhere,
with and without a windbreak, exceed the threshold. Percentage
efficiencies Ej and £3 are defined based upon linear and cubic relations
between wind speed and particle uptake, respectively. Hence
= [1 - ICuiAO/lKjAi)] x 100 (7.2)
i i
E3 = [1 - RuO/lK i-)] x 100 (7.3)
i i
where the summation is over the entire pile. In effect, these
efficiencies are l-(un/u0n), where n is either 1 or 3 and un and uQn are
the area-averaged values over the pile surface with and without a
windbreak, respectively. Later in this subsection, as an example,
threshold and reference wind speeds are assumed and the areal extent of
erosion is noted for the various windbreaks.
Previous analyses by the present author of the data for the conical
pile utilized an overall or area-averaged reduction factor,
R = 1-u.j/u.j Q, and a windbreak effectiveness factor, E = I-U^/UQ ^6t
(Billman, 1984). However, if the objective was to define a parameter
which is the fractional amount by which fugitive emissions have been
reduced, then Ej and £3 would be better parameters than R^ and E. In the
previous analyses, if the wind speed at some points in the no windbreak
case were zero, theh R" or E would be infinite. In addition, the regions
81
-------
of low wind speed in the absence of the windbreak make relatively large
contributions to R and E. This limitation has been removed by using the
terms E^ and £3 in the present analysis.
EI for the windbreaks placed upstream of the conical and larger
piles with normal incident flow are given in Tables 7.1 and 7.2,
respectively. In general, a windbreak is more effective (higher E^)
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 Ej with changes in height, length, location and
porosity.of the windbreak are similar for both piles. In general,
increasing windbreak height is desirable. Efficiencies range from
28-45% and 13-21% for 0.5H high windbreaks placed upstream of the
conical and larger piles, respectively, and for higher windbreaks,
44-77% and 27-62%, again for the two piles. The 1.5H height was more
effective than the l.OH height with the oval, flat-topped pile,
reflecting increased wind speed reductions on the pile top for the
highest windbreak. For most cases with the conical pile, windbreaks of
heights l.OH and 1.5H were equally effective, within experimental
scatter. Efficiencies were higher with the less porous windbreak
material. Except for the windbreaks of height one half the pile height
(0.5H), efficiency was lower when the windbreak was not as long as the
pile base length. Length was not significant for the 0.5H high
windbreaks because only the lower part of the pile sides were
significantly affected with the longer windbreak, not the higher part,
the location of high wind speeds in the absence of any windbreak (refer
to Figure 5.3). However, with a higher windbreak, increased length
82
-------
TABLE 7.1 EFFICIENCY (Ej) FOR THE VARIOUS WINDBREAKS PLACED UPSTREAM
OF THE CONICAL PILE
position
length:
height
0.5H
l.OH
1.5H
65%
J
l.OD
34
48
47
porous
1H
1.5D
32
45
44
TABLE 7.2 EFFICIENCY (
position
length:
height
0.5 H
n 7RH
1.0 H
1 ?RH
1.5 H
65%
0.6B
15
27
33
OF
porous
1H
l.OB
16
34
39
windbreak
3H
l.OD
28
53
55
50% porous
1H
1.5D l.OD 1.5D
30 46 45
52 66 67
54 64 65
El) FOR THE VARIOUS WINDBREAKS
THE OVAL
windbreak
3H
0.6B 1
13
28
38
, FLAT-TOPPED PILE
50% porous
1H
.OB 0.6B l.OB
18 20
41
37 34 53
56
44 58
windbreak
3H
l.OD 1.5D
36 36
65 71
71 77
PLACED UPSTREAM
windbreak
3H
1.5B 0.6B l.OB
21 15 17
34
51 31 49
57
59 43 62
83
-------
caused wind speed reductions in the high wind speed area on the sides.
For both piles, the windbreak locations of 1H and 3H upwind of the pile
were better for the windbreaks of height 0.5H and 1.5H, respectively.
Tables 7.3 and 7.4 list £2,'the efficiency based upon the u3
relation to dust uptake for the various windbreak cases for the conical
and larger piles, respectively. Again, the efficiencies tend to be
higher with the conical pile. Many of the same qualitative results as
with EI are apparent. The trends are clearly seen in a plot of
efficiency (£3) vs. windbreak height as functions of windbreak length
and porosity for both piles (Figure 7.1). Windbreaks of the lowest
height were least effective; windbreaks higher than the pile were
significantly more effective only with the oval, flat-topped pile.
Windbreaks shorter than the pile base length were less effective than
those at least as long as the pile. The less porous material was
clearly more effective. These trends were observed for windbreaks at
either the 1H or 3H position.
Although the highest efficiencies of 99% and 96% correspond to the
50% porous material of height 1.5H, 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
efficiencies of the more economical windbreak of the same porosity,
height equal to the pile height and length equal to the pile base
length are only slightly lower (97% and 89%, respectively). Clearly,
the latter size would be preferable on the basis of cost effectiveness.
Any location between 1H and 3H from the base of the pile could be
chosen depending onvthe convenience.
84
-------
TABLE 7.3 EFFICIENCY (E3) FOR THE VARIOUS WINDBREAKS PLACED UPSTREAM
OF THE CONICAL PILE
65%
position:
length: l.OD
height
0.5H
l.OH
1.5H
74
88
86
porous
1H
1.5D
72
85
82
windbreak
3H
l.OD
66
91
92
1.5D
67
90
91
50%
l.OD
82
97
95
porous
1H
1.5D
80
97
95
windbreak
3H
l.OD
76
97
98
1.5D
76
98
99
TABLE 7.4 EFFICIENCY (E3) FOR THE VARIOUS WINDBREAKS PLACED UPSTREAM
OF THE OVAL, FLAT-TOPPED PILE
65% porous
position: 1H
length: 0.6B
height
0.5 H
0.75H
1.0 H
1.25H
1.5 H
42
63
67
l.OB
45
74
73
windbreak
3H
0.6B l.OB
39
66 79
75
50% porous windbreak
1H 3H
0.6B
48
73
80
l.OB
49
84
92
91
92
1.5B
51
89
92
0.6B
45
70
81
l.OB
50
74
89
95
96
85
-------
o
CO
XJ •
Ol O)
O r—
.*: CL
4-> O
O)
x:
O)
>> c
o o
c u
O)
LU 01
in
-------
As discussed earlier in this section, windbreak effectiveness may
also be assessed by assuming threshold (u^) and full-scale reference
(ur) speeds. An example calculation was made using the u/ur wind
tunnel data for windbreaks placed upwind, assuming a reference wind
speed of 10 m/s and a threshold velocity of 2.8 m/s to determine the
percentage surface area over which no emissions would occur (i.e. where
u < u^). (This threshold speed was also used in an example calculation
by Martin and Drehmel (1980), and may be applicable to some fine
material). With no windbreak, 22% (25%) of the conical (oval) pile
surface area had wind speeds less than the threshold. With all the 65%
porous windbreaks and the 0.5H high 50% porous windbreaks, 40-65%
(24-49%) of the pile surface area would have no dust emissions. The
area increased to 77-100% (40-99%) for the other 50% porous windbreaks.
Note that the percentage surface areas calculated here would increase
with a decrease in reference speed and/or increase in threshold
velocity.
For windbreaks placed on top of the larger pile, efficiencies (Ej
and £3) were quite low, less than 12%. As discussed in section 6.2,
wind speeds on the pile top were reduced significantly, but the
windbreaks had very little effect in the high wind speed region on the
windward face. This suggests that fugitive-dust emissions on the top
of the pile may be controlled locally through the use of a windbreak.
Since the windbreaks were not very high, (0.14H and 0.27H), in practice,
they could be portable, providing protection to specific areas on the
top of the pile, depending upon the location of activity and wind
direction. v
87
-------
7.2 COMPARISON TO PREVIOUS STUDIES
Many results compare well with previous studies discussed in
section 2, but several windbreak design guidelines are introduced here.
The flow structure about a porous windbreak will be discussed first,
then that about windbreak-protected piles.
The flow structure downwind of a porous windbreak was qualitatively
similar to that observed by Raine and Stevenson (1977). The area of
minimum wind speed and high turbulence intensity occurred not
immediately, but farther downwind of the windbreak and turbulence was
generated in the shear layer at the top of the windbreak. As observed
by Perera (1981), the hot-wire anemometer (HWA) overpredicted wind
speeds in the highly turbulent region. Also, for heights greater than
the windbreak height, the longitudinal turbulence intensity, measured
with the pulsed-wire anemometer (PWA), was greater than that measured
with the HWA. However, unlike that observed by Perera, the intensity
was less when measured with the PWA for heights less than In and the
mean wind speed measured by the two instruments was approximately the
same for heights above about 2h. Perera used a solid barrier (0%
porous); here it was 50% porous.
Results with the windbreak-protected piles may be compared with
two previous laboratory studies in which windbreaks were located upwind
of a pile. Davies (1980) recommended a windbreak height of 1.4H, which,
based on our results, should be very good. We did not examine the case
of a windbreak of varying porosity as Davies did. However, our results
cannot adequately be compared with those by Davies since only the final
result was reported*, not the details of the various cases tested. The
88
-------
present work extends that reported by Soo et al. (1981) and Cai et al
(1983). . In both studies, optimal windbreak location was found to be
related to the windbreak height, and lower wind speeds were observed
with less porous windbreaks. In the present study, with three-
dimensional piles, effects of windbreak length, additional windbreak
heights and porosities, and various incident flow directions were
examined.
Windbreak efficiencies presented here are generally much higher
than those estimated by Bohn et al. (1978) and Jutze et al. (1977),
indicating that windbreaks may be a highly effective fugitive-dust
control method.
89
-------
8. CONCLUSIONS
This wind tunnel study has shown that windbreaks normal to the wind
direction placed upwind of a conical and a larger, oval, flat-topped
storage pile reduce wind speeds hear the surface of the pile and hence
suggest reductions in fugitive-dust emissions. Of the windbreaks
tested for each pile, the largest (height 1.5 times the pile height
(1.5H) and length equal to the larger pile base length (l.OB) or 1.5
times the conical pile base diameter (1.5D)) 50% porous windbreak
placed 3H from the pile appears to be best in terms of greatest wind
speed reduction. However, all the 50* porous windbreaks 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 windbreak 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. A farther
location could be more effective, but the pile could then be beyond the
sheltered region, hence, a less effective windbreak position.
Windbreaks of height 1.5H caused greater wind speed reductions on the
top of the oval, flat-topped pile than those of height equal to or less
than the pile height, but the difference in the effectiveness of l.OH
and 1.5H high windbreaks with the conical pile was not significant.
Windbreak length and position are even more important in
determining effectiveness when the air flow is not normal to a
windbreak. With a windbreak of height and length equal to the pile
dimensions, fairly nigh wind speed reductions resulted when the
90
-------
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 (0.14H and 0.27H'high) placed on the top of the oval,
flat-topped pile caused large areas of high wind speed reductions on
the pile top both downwind and upwind of the windbreak, but very small
reductions to the high wind speeds on the windward face occurring in
the absence of any windbreak. The area of greatest reduction was not
immediately downwind of the windbreak, but displaced farther downstream.
Changes in wind direction shifted the location of the sheltered 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.
Design guidelines developed by Soo et al. (1981) and Cai et al.
(1983) have been extended since more windbreak configurations were
examined and three-dimensional piles were used. Windbreak efficiencies
were generally much higher than expected (Bohn et al., 1978 and Jutze
et al., 1977). With the design guidelines presented here, the use of
windbreaks for fugitive-dust control appears promising.
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 speed also affect emissions. A clearer
understanding of the relationship of wind speed and threshold speed to
fugitive-dust emissions would allow for better analysis of the data
presented. Additional field measurements of fugitive dust from storage
91
-------
piles with and without windbreaks would be helpful for comparison to
the efficiencies and design guidelines presented here.
92
-------
9. REFERENCES
Axetell, Jr., K., 1978: Survey of fugitive dust from coal mines.
EPA-908/1-78-003, EPA Region VIII, Office of Energy Activities,
Denver, CO 80295.
i
Bagnold, R.A., 1941: The Physics of Blown Sand and Desert Dunes.
Methuen, London, 265 pp.
Billman, B.J., 1984: Windbreak effectiveness for the control of
fugitive-dust emissions from storage piles — A wind tunnel study.
Presented at the Fifth Symposium on the Transfer and Utilization
of Particulate Control Technology, Kansas City, MO, August 27-30,
1984. Proceedings to be published by the Environmental Protection
Agency and Electric Power Research Institute.
Blackwood, T.R. and Wachter, R.A, 1978: Source assessment: Coal storage
piles. EPA-600/2-78-004k, U.S. Environmental Protection Agency,
Cincinnati", OH.
Bohn, R., T. Cuscino, and C. Cowherd, Jr., 1978: Fugitive emissions
from integrated iron and steel plants. EPA-600/2-78-050. U.S.
Environmental Protection Agency.
Bradbury, L.J.S. and I.P. Castro, 1971: A pulsed-wire technique for
velocity measurements in highly turbulent flows. J. Fluid Mech. 49:
657.
The Bureau of National Affairs, Inc., 1983: Environment Reporter.
336:0553-0554, Rule 302(f).
The Bureau of National Affairs, Inc., 1982: Environment Reporter.
411:0516-0517, R336.1371, R336.1372.
Caborn, J.M., 1957: Shelter-belts and microclimate. Forestry Commun.
Bull. 29:1.
Cai, S., Chen, F.F., and Soo, S.L., 1983: Wind penetration into a
porous storage pile and use of barriers. Environ. Sci. Technol.
H_: 298.
Caput, C., Belot, Y., Guyot, G., Sarnie, C., and Seguin, B., 1973:
Transport of a diffusing material over a thin wind-break. Atmos.
Environ. ]_: 75.
Castro, I.P., and W.H. Snyder, 1982: A wind tunnel study of dispersion
from' sources downwind of three-dimensional hills. Atmos. Environ.
Jj>: 1869.
Counihan, J, 1975: ^Adiabatic atmospheric boundary layers: A review and
analysis of data from the period 1880-1972. Atmos. Environ, j): 871.
93
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Cowherd, Jr., C., K. Axetell, Jr., C.M. Guenther, and G. Jutze, 1974:
Development of emission factors for fugitive dust sources. EPA-450/
3-74-037, U.S. Environmental Protection Agency, Office of Air
Quality Planning and Standards.
Cowherd, Jr., C., R. Bohn, and T.A. Cuscino, 1979: Iron and Steel
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