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Verification of the shelter effect of a windbreak on coal piles in the POSCO open storage yards at the Kwang-Yang works
Atmospheric Environment 36 (2002) 2171–2185
Verification of the shelter effect of a windbreak on coal piles in the POSCO open storage yards at the Kwang-Yang works Cheol-Woo Park, Sang-Joon Lee* Department of Mechanical Engineering, Pohang University of Science and Technology, Pohang 790-784, South Korea Received 21 September 2001; received in revised form 28 January 2002; accepted 6 February 2002
Abstract The use of windbreaks to reduce wind-blown coal dust at the POSCO Kwang-Yang open storage yards was studied using wind simulations on a scale model of the yards. Based on these simulation results, a full-scale wind fence was constructed on two sides of the yard. Here, we present results on the wind behavior both for the real yard and for the simulation results that guided its construction. Wind-tunnel simulations were used to study the effect of a porous wind fence of porosity e ¼ 30% on the surface pressure and shear stress on coal piles using a 1/1200 model of the POSCO Kwang-Yang open storage yards. In addition, the shelter effects found in the model system were verified in field measurements on the full-scale system. The storage yard model was fully embedded in an atmospheric surface boundary layer over open terrain. The fence and coal pile model had the same height (12.2 mm) and Reynolds number (Re=1.6 104, based on the model height). The mean and fluctuating surface-pressure distributions on the coal piles, which are closely related to the dust emission from the surface, were measured for several directions of the oncoming wind. The wind directions pertinent to the study were determined by statistical analysis of seasonal wind data over the storage yard. A porous wind fence of porosity e ¼ 30% was found to be useful for reducing the wind speed without the formation of a recirculating bubble behind the fence. In addition, the fence caught the wind-borne particles when it was located behind the coal piles. The wind fence reduced the pressure fluctuations and surface shear stress on the coal piles to less than half of the levels observed in the no fence case. To verify the effectiveness of the porous wind fence installed around the Kwang-Yang open storage yard, the local wind speed and the concentration of suspended particles were measured directly. Full-scale porous fences installed around the Kwang-Yang open storage yard greatly decreased the turbulence intensity of the wind over the coal piles and reduced the total suspension particles by 70–80%. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Windbreak; Porosity; Wind fence; Surface pressure; Coal pile; Atmospheric boundary layer
1. Introduction The wind erosion of small particles such as snow and sand in regions of strong wind has been an important wind engineering problem over the past few decades. For example, atmospheric dispersion of wind-blown dust particles from open coal storage yards can cause serious air-pollution and environmental problems. Wind *Corresponding author. Tel.: +82-54-279-2169; fax: +8254-279-3199. E-mail address: [email protected] (S.-J. Lee).
erosion also leads to a loss of raw materials resulting in a needless waste of precious investment. From the standpoint of wind engineering it is valuable to investigate the wind erosion phenomenon in a systematic manner in order to develop methods for reducing undesirable dust emissions. The wind erosion of particles near the ground surface frequently occurs in the open storage yards of power generation plants and steel-making companies. This erosion causes environmental problems and the loss of raw materials. However, the mechanism of wind erosion is difficult to understand because the dispersion
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mechanism is very complicated and depends on many topographic and meteorological factors. For this reason only a few empirical correlations are available for predicting dust erosion, and adequate quantitative relationships between the fluctuating surface pressure and flow over a coal pile have yet to be established (Owen, 1964; Gillette, 1974; Iversen et al., 1987; Kind, 1990). Recently, the enhanced regulation for environmental protection has led to an increase in the importance of atmospheric wind tunnel simulations of wind-blown dusts in the field of wind engineering. Windbreaks have been widely used since early times, mainly to protect agricultural fields. Nowadays, they are also extensively used in a range of applications such as preventing snow drift, soil erosion, pollutant dispersion and spills of toxic materials. The wind fence used in the present study is one such windbreak. The wind fence withstands a drag force, causing a net loss of momentum of the oncoming airflow and thus has a sheltering effect. However, as the permeability of the wind fence is reduced, the bleed flow through the fence holes decreases and the drag force increases. From this point of view, use of solid fences as windbreaks can cause practical difficulties because of the increased drag force on such fences and increased construction cost. Many studies have endeavored to find effective wind fences that simultaneously reduce drag and enhance the shelter effect. Borges and Viegas (1988) investigated the shelter effect of windbreaks by measuring the mean velocity and shear stress of the near wake behind the windbreaks. Lee and Kim (1999) measured the velocity fields in the turbulent wake behind two-dimensional porous wind fences using a two-frame PTV technique. They found that as the fence porosity (e) increases the turbulence intensity and Reynolds shear stress decreases; however, the mean velocity deficit also decreases. In a study on porous windbreaks Perera (1981) investigated the effects of porosity and permeable-hole shape on the mean velocity deficit, reattachment length and Reynolds shear stress in the near wake. The separation bubble formed behind the fence disappeared when the fence porosity was >e ¼ 30%: Ranga et al. (1988) found that the drag force acting on wind fences is mainly affected by the fence porosity and fence height. Plate (1971) revealed that a windbreak can effectively reduce wind-blown dust when it is embedded in the atmospheric boundary layer over a plain terrain. Bofah and Alhinai (1986) investigated the encroachment of drifting sand in inhabited arid regions and studied the use of porous fences to control sand drift. A complete quantitative analysis of the effects of porous fences has not been performed, because wind erosion is a very complicated phenomenon that is greatly affected by climatic and topographical factors. Raine and Stevenson (1977) measured flow statistics such as the mean
velocity, Reynolds stress, turbulence intensity and power spectra in the lee of a solid fence and permeable shelter fences. In their study, a windbreak with low to medium permeability reduced the overall mean velocity more than a solid windbreak. A fence with even larger permeability gave better overall protection. Several previous studies have considered the wind erosion of pollutants in urban areas. Grant (1988) measured the pedestrian-level wind speed at several sites across an urban area and compared it with wind-tunnel simulation results using the plastic-pellet erosion technique. Chatzipanagiotidis and Olivari (1996) investigated the pollutant dispersion over a hill using a digital image analysis. They found that the boundary layer thickness of oncoming flow significantly influenced the wind erosion of pollutants. However, only limited attention has been given to the problem of wind erosion in large open storage yards. Lee and Park (1998, 1999) investigated the interaction between porous wind fences and the coal pile surface pressure. Using wind-tunnel tests they found that a wind fence of porosity e ¼ 40% is the most effective for reducing the pressure fluctuations on a 2-D prism surface. The shelter effect of the porous wind fence was found to extend up to the 5th–6th prism in consecutive triangular prism models. Wind erosion is closely related to the flow characteristics of the atmospheric surface layer such as the wind speed, pressure fluctuations and turbulent shear stress. In particular, the lift force that ejects dust particles from the coal pile surface is related to the magnitude of the oncoming wind speed and friction velocity near the surface. This implies that the surface-pressure fluctuations on the coal-pile surface are dependent on the near surface flow characteristics. Lee et al. (1997) proposed the installation of a porous wind fence of porosity e ¼ 30% around the coal pile yards at the POSCO Kwang-Yang Works, based on an optimization study on porous wind fences. At this factory, located on the southern seaside of the Korean peninsula, coal dust in the open storage yard is blown by wind and local gusts, especially in the summer stormy season. In order to support this proposal, the shelter effect of a porous wind fence of porosity e ¼ 30% on a scale-downed coal pile model of POSCO Kwang-Yang open storage yard was investigated experimentally through a wind-tunnel simulation. The proposed structure was accepted and a huge porous wind fence was installed along two sides of Yard 5 of the Kwang-Yang Works. After installing the wind fence, the wind environment around the storage yard was monitored annually to test its usefulness and effectiveness for abating the wind erosion of coal dust. From an engineering point of view, this construction represents a rare and very important opportunity to directly apply wind-tunnel simulation results to the
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full-scale situation and to check its usefulness through a field test. This paper summarizes the wind-tunnel simulation and the full-scale field tests. The results validate the shelter effect of porous wind fences for abating wind erosion of dust.
2. Experimental apparatus and methods The wind-tunnel simulation was carried out in a closed-return type subsonic wind-tunnel with a test section of 1.8 m wide 1.5 m high 11 m long. Spires and roughness elements were installed in front of the test section to generate a thermally neutral atmospheric boundary layer. The triangular spires of 71.2 cm height and 5.4 cm base-length were designed following Irwin (1981) and installed along the spanwise direction with a 35.6 cm gap. A schematic diagram of the wind-tunnel test section and measurement system is shown in Fig. 1. The right side of the first model coal pile encountered by the oncoming wind is selected as the positive X -coordinate, because the windward and leeward surfaces change according to the wind direction. The mean velocity and turbulence intensity profiles of the simulated atmospheric boundary layer were measured at the fence location, 5 m downstream from the leading edge of the test section, using a hot-wire
anemometer (TSI IFA-100). At each measurement point, 16,000 velocity data were acquired at a sampling rate of 2 kHz after low-pass filtering at 800 Hz. Since the measurement location is farther than six times of the spire height (Irwin, 1981), the simulated turbulent boundary layer would be an equilibrium state. In addition, the streamwise pressure gradient was nearly negligible due to corner fillets and small breathers located at the end of wind-tunnel test section. Fig. 2 shows the streamwise mean velocity and turbulence intensity profile measured at the fence location. The mean streamwise velocity normalized by the reference velocity (Uref ) at the height Yref ¼ 0:15 m has the power law profile: UðyÞ=Uref ¼ ðy=yref Þn :
Spires Y Uo
: Without Yard Model Fence H
Fence Hot-wire
h G
X or Z
α=40o
Pressure Scanning Selection Box FCO91 MkII
CTA TSI IFA-100
Micromanometer FCO 12
A/D Converter
PC
ð1Þ
The velocity profiles are well fitted with the power law exponent n ¼ 0:14; corresponding to the velocity profile over the open terrain. We assumed that the velocity profile is valid at the seaside. The turbulence intensity near the ground surface is about 20–25%. Ideally, all atmospheric parameters should be reproduced in the wind-tunnel simulation. Unfortunately, the atmospheric parameters scatter a lot. This is particularly true for the integral length scale (Lxu ) of the approach flow, although Geurts (1996) and some other researchers dispute this fact since the turbulence intensity is >10%. The integral
Wind Tunnel Test Section
Yards model
2173
Signal Conditioner
DT 2838
Fig. 1. Wind-tunnel test section and measurement system.
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40
40 U0.15 =5.8m/s
U0.15 =5.8m/s
U0.15 =9.5m/s
35
U0.15 =9.5m/s
35
U0.15=13.5m/s n=0.14
30
Y/ H
U0.15=13.5m/s
30
25
25
20
20
15
15
10
10
5
5
0
0 0.4
(a)
0.6
0.8
1.0
1.2
U/Uo
1.4
01 (b)
0
20
30
u'2 /U × 100(%)
Fig. 2. Mean streamwise velocity and turbulence intensity profiles measured at the model coal yard location (a) mean velocity, (b) turbulence intensity.
length scale (Lxu ) calculated from the wind-tunnel data is between 50 and 80 cm. The configuration and photograph of the open storage yards tested in this study are shown in Fig. 3. The wind-tunnel model includes various building structures and chimneys of the real coal plant. The locations in the 1/1200 scale yards where pressure measurements were made are specified by the row and column numbers. Each model coal pile had an inclination angle of 401 and 11 pressure taps were installed along the middle span of each model pile at intervals of 4 mm. The model coal pile was made of sawdust and a bonding agent. Fine sand particles were attached on the surface of model coal pile to simulate the surface roughness of real coal piles. There was no change in the morphology of the coal piles as a result of the wind erosion. The pressure taps were connected to the pressure scanning selection system (FCO91-MkII) with vinyl tubes of inner diameter 0.8 mm. Each pressure tap was scanned by a selection switch and the analog voltage output from the micromanometer (FCO-12) was digitized by a high-precision A/D converter (DT-2838). At each channel, 16,384 pressure data were acquired at a
sampling rate of 500 Hz and statistically averaged to obtain mean and rms pressure data. Based on previous wind-tunnel tests (Lee et al., 1997), a porous fence of porosity e ¼ 30% was selected to block the wind-blown dust from the coal piles. The porous fence was made of stainless-steel plate of thickness 0.5 mm, and the fence height H was made equal to the height of the coal pile crest, h ¼ 12:2 mm. The porous fence model was constructed of circular holes of 1 mm diameter with 0.6 mm spacing. The circular holes were precisely made using an etching technique. (Lee and Park, 1998; Bradley and Mulhearn, 1983) The distance between the fence and the front edge of the model coal pile was G ¼ 20 mm. The pressure difference between the surface pressure p and the reference static pressure p0 was non-dimensionalized by the dynamic pressure with velocity U0:15 at a height of Y ¼ 0:15 m, and air density ra : The pressure coefficient Cp is expressed as Cp ¼ 2ðp p0 Þ=ra ðU0:15 Þ2 :
ð2Þ
The reference pressure p0 was measured at Y ¼ 1:2 m, at which height the influence of the model was small and
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N E
W S
1, 2 COKE PLANT
500 (m)
0
Yard 1 & 2 1,2 SINTER PLANT
MCR
SINTER YARD
3,4 SINTER PLANT
3, 4 COKE PLANT
Yard 3 & 4
Row number for transverse measurement location 1
2
-Z Fence
3
Yard 5
Pressure tap (11 holes)
4
o oo o o oo o
5
X G=20mm
Wind Fence
4 3 2 1
Column number for each coal pile model
Fig. 3. Configuration and photograph of coal piles at Kwang-Yang open storage yards installed in the wind-tunnel test section and pressure measurement locations.
the flow was assumed to be inviscid. During the experiments the oncoming wind speed was fixed at UN ¼ 18 m s1 and its corresponding Reynolds number based on the height of the model coal pile (h) was approximately Re=1.6 104. The surface shear stress tw was measured along the centerline of the coal pile model with a Preston tube of small inner diameter. The shear stress was calculated from the pressure difference data between the Preston tube and the reference static tube, following the method of Patel (1965). The surface shear stress was nondimensionalized with the velocity U0:15 to give the friction coefficient Cf as follows: Cf ¼ 2tw =ra ðU0:15 Þ2 :
ð3Þ
Jovic and Driver (1995) mention that the friction coefficient Cf decreases in proportion to Re1/2, where Re is the Reynolds number. Following the wind-tunnel simulation results, a porous wind fence of porosity e ¼ 30% was installed along two sides of Yard 5 of the open storage yard of POSCO Kwang-Yang Works at a cost of about 3 million US dollars. The porous fence was installed along the eastern and northern boundaries of the yard. The wind fence consists of steel columns at intervals of 20 m and stretched net made of nylon wire. The mesh size was adjusted to create a fence porosity of e ¼ 30%: The height of the fence was chosen to be 17 m, slightly higher than the average height of the coal piles (15 m). The total length of the fences installed at Yard 5 is about 1800 m.
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W S
N E
Myo Island
Yard 3 & 4
The South Sea Line 3
Yard 1 & 2
Line 2 Line 1
350 m
Yard 5
Taein Island
Plant Area
1450 m
The South Sea
Wind Fence
.
: Dusts measurement location
Fig. 4. Measurement locations of wind-blown dust from the open storage yards of the Kwang-Yang Works.
A diagram of the wind fence installed at Yard 5 is shown in Fig. 4. Field measurements taken at the site measured the variation of local wind speed using a portable anemometer in conjunction with a direction finder. In addition, the monthly average wind speed along the centerline of Yard 5 was measured with an anemometer attached to a crane head at a height of 15 m above the ground. To confirm the shelter effect of the wind fence a sequential time series of instantaneous oncoming wind velocity measurements were made in front of and behind the installed wind fence at a height of 3.5 m above the ground. At each measurement point, 16,000 velocity data were measured with a Pitot-tube at a sampling rate of 100 Hz over a period of 160 s. The concentration of wind-blown dust was measured twice per month at a location near the wind fence, as shown in Fig. 4. Dry and wet depositions were gathered using dust collectors. The wind-blown dust concentration was defined as the total mass of suspended particles per unit volume (TSP, mg m3).
3. Wind-tunnel simulation 3.1. Wind direction rose Wind characteristics such as the wind speed and dominant wind direction were analyzed using the daily averaged wind data measured at the Kwang-Yang open storage yard over 1996 and 1997. A maximum wind speed of 13.4 m s1 was measured in March 1996, and the yearly mean wind speed was approximately 2.7 m s1. The instantaneous wind speed during storms
or gusts was greater than the daily average wind speed. Based on the wind data, the free stream velocity used in the model simulations was fixed at 18 m s1 to cover the extreme conditions. Fig. 5 shows the seasonally averaged wind direction rose obtained from the 1996 and 1997 wind data. The periphery of the wind rose is divided into sixteen segments indicating the wind direction. The annulus in the radial direction indicates the percentage of the wind rose probability. According to the wind data, northwesterly winds occur frequently in spring and winter, whereas north-easterly winds are dominant in summer and fall. In previous experimental simulations, we tested all coal piles and all wind directions that are likely to occur in the Kwang-Yang open storage yard (Lee et al., 1997). In the present paper, however, we consider only the three dominant wind directions that occur in Yard 5.
3.2. Surface pressure Fig. 6 shows the shelter effect of the wind fence installed around Yard 5 for an easterly wind incident on the model coal pile located at Row 3. For the coal pile of Column 1, the mean pressure and pressure fluctuations on the windward surface substantially decrease after installation of the porous fence. In addition, the magnitude of the mean pressure and pressure fluctuations in the crest region of the model coal pile is greatly reduced for the four columns tested. This reduction can be primarily attributed to the large decrease in the wind speed on passing through the fence. Without the fence, the pressure fluctuations are very high at the crest of the coal pile due to the flow separation near the crest. This seems to be closely related with the flow field variation
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Fig. 5. Wind direction rose based on 1996 and 1997 wind data.
around the coal pile model as explained in Lee and Kim (1998). The mean pressure on the windward surface of the second coal pile (Column 2) decreases due to the deceleration of the recirculating flow between the two adjacent coal piles. This recirculation flow reduces the mean pressure on the leeward side of the first coal pile and on the windward side of the second coal pile. The rms pressure fluctuations on the first coal pile surface also increase slightly due to the recirculation flow between the two consecutive coal piles (Ferreira and Viegas, 1995). For Columns 3 and 4 the mean pressure on the windward side increases due to the descending shear layer separated from the fence top and coal pile crest. The rms pressure fluctuations on the windward surface also increase slightly. Fig. 7 shows the surface-pressure distributions on the model coal piles at Row 2 of Yard 5 under a westerly wind. The mean pressure distributions show similar patterns to those observed for an easterly wind. However, the rms pressure distributions differ slightly from those of the eastern wind case. This difference can be attributed to the various high-rise structures such as
towers and cranes located in front of the wind fence, which are used at the port for unloading raw materials. The results indicate that nearby tall structures can disturb the flow structure behind the wind fence. The pressure fluctuations at the first column (Column 4) just behind the porous fence are even larger than those observed in the no fence case. For the next three columns the pressure distributions show that the installed wind fence provides a favorable shelter effect. Fig. 8 shows the mean and rms pressure variations on the model coal piles at Row 1 of Yard 5 under a northeasterly wind. For a north-easterly wind, the oncoming flow crosses Yard 5 obliquely and the shelter effect decreases slightly with moving toward Column 4 compared with the case of an easterly wind. The mean pressure distributions are nearly uniform on both sides of each coal pile, excluding the large negative pressure at the coal pile crest. This is attributed to the fact that the oncoming flow approaches obliquely and the consecutive coal piles are embedded in the shear layer separated from the pile crest and fence top edge. The mean pressure on the windward side of Column 4 is greatly increased and descends rapidly compared with the no
C.-W. Park, S.-J. Lee / Atmospheric Environment 36 (2002) 2171–2185
2178 0.8
0.2 No Fence Column 1
0.6
Wind direction
Wind direction
No Fence Column 4 Row 2, Column 4
0.4
Row 3, Column 2
0.2
Row 3, Column 3
0.0
Row 2, Column 3 Row 2, Column 2
Row 3, Column 4
0.0
Cp(mean)
Cp(mean)
Row 3, Column 1
-0.2
-0.2
Row 2, Column 1
-0.4
-0.4 -0.6
-0.6
-0.8 -1.0 -1.6
-1.2
-0.8
-0.4
(a)
0.0
0.4
0.8
1.2
-0.8
1.6
X/H
-1.6
-1.2
-0.8
-0.4
(a)
0.022
0.0
0.4
0.8
1.2
1.6
X/H 0.018
No Fence Column 1
No Fence Column 4
Row 3, Column 1
0.020
0.017
Row 2, Column 4
Row 3, Column 2
Row 2, Column 3 Row 3, Column 3
0.016
Row 2, Column 2
Row 3, Column 4
Cp(rms)
Cp(rms)
0.018
0.016
Row 2, Column 1
0.015 0.014
0.014
0.013 0.012 -1.6
-1.2
(b)
-0.8
-0.4
0.0
0.4
0.8
1.2
1.6
0.012 -1.6
X/H
Fig. 6. Mean and rms pressure distributions on coal piles (Row 3) of Yard 5 for a Eastern wind. (a) mean pressure, (b) rms pressure.
fence case, because the shear layer is separated from the fence top. The rms pressure around the crest of the fourth coal pile (Column 4) is slightly higher than for the other columns due to the momentum loss of the separated shear layer.
3.3. Power spectra The power spectral density (PSD) of the pressure signals measured at the coal pile surface was spectrally analyzed. Time series of pressure signals low-pass filtered at 50 Hz was acquired at 100 Hz to analyze its frequency characteristics. The PSD results for one model coal pile (Column 1) in Yard 5 under an eastern wind are shown in Fig. 9. In this figure, the effect of the wind fence on PSD attenuation in the low frequency range is obvious. The PSD has high values at frequencies up to 0.5 Hz, after which the power density of the pressure signal rapidly decays. Ogawa and Diosey (1980) observed several
(b)
-1.2
-0.8
-0.4
0.0
0.4
0.8
1.2
1.6
X/H
Fig. 7. Mean and rms pressure distributions on coal piles (Row 2) of Yard 5 for a Western wind. (a) mean pressure, (b) rms pressure.
frequency peaks in the wake of a 2-D fence, which they attributed to the high turbulence intensity of the oncoming flow near the ground surface and small vortex-induced eddies. Richardson (1989) found that a permeable windbreak substantially attenuated the PSD of the pressure signal at low frequency (0.14 Hz) in comparison to the drag force signal. This behavior is attributed to the characteristic length scale of the atmospheric flow, which exceeds the hole size of the windbreak at low frequencies. Geurts (1996) revealed that the attenuation of the pressure spectra was faster than that of the wind velocity spectra. The PSD attenuation on the windward surface is larger than that on the leeward surface in the low frequency band. On the leeward surface, the wind fence also decreases the PSD values to some extent in the low frequency band. This is primarily attributed to the fact that several consecutive coal piles are located below the shear layer separated from the fence top. Since the change of power spectra indicates the modification of the turbulent kinetic energy implied on the coal pile
C.-W. Park, S.-J. Lee / Atmospheric Environment 36 (2002) 2171–2185 0.2 No Fence Column 1
Wind direction
Row 1, Column 1
0.0
Row 1, Column 2 Row 1, Column 3 Row 1, Column 4
Cp(mean)
-0.2
-0.4
-0.6
-0.8 -1.6
-1.2
-0.8
-0.4
(a)
0.0
0.4
0.8
1.2
1.6
X/H
2179
windward surface, shown in Fig. 6. Beyond the location of X /H=0.66, however, the friction coefficients decrease slightly. This implies that the flow separation on the coal pile surface occurs earlier in this region compared with a smooth surface, which is reasonable because the model coal pile was roughened by adhering small sand particles to simulate the coal pile surface. Installation of the porous wind fence of e ¼ 30% in front of the coal pile reduced the friction coefficients down to half of the values observed for the no fence case. From this result we can conjecture that the wind fence should effectively reduce the wind-blown dust erosion, because the surface shear stress is one of the main causes of wind erosion.
0.018
4. Field test results
No Fence Column 1
0.017
Row 1, Column 1
4.1. Wind speed
Row 1, Column 2
Cp(rms)
0.016
Row 1, Column 3 Row 1, Column 4
0.015 0.014 0.013 0.012 -1.6
-1.2
-0.8
(b)
-0.4
0.0
0.4
0.8
1.2
1.6
X/H
Fig. 8. Mean and rms pressure distribution on coal piles (Row 1) of Yard 5 for a North-eastern wind. (a) mean pressure, (b) rms pressure.
surface, the wind erosion of coal dust particles would be changed. From this, we can see that the porous wind fence of e=30% installed in front of the coal pile can effectively attenuate the pressure power which may enhance the wind erosion.
3.4. Surface shear stress For the model systems with and without the wind fence of porosity 30%, the skin friction coefficients on the first coal pile model of Yard 5 under an eastern wind were measured. The results are compared in Fig. 10. Because the surface shear stress was measured using a Preston tube, which cannot be used in reverse flows, the surface shear stress was measured only on the windward surface of the model coal pile. Without the wind fence, the mean friction coefficient increases on the windward surface because the oncoming flow accelerates along this surface. This behavior correlates well with the mean pressure gradient on the
As mentioned in the previous section, the wind-tunnel simulation results, including the threshold velocity measurement of wind-blown sand particles stacked behind the wind fence, led to the installation of a wind fence of porosity e ¼ 30% on two sides of Yard 5 at the Kwang-Yang open storage yards. Fig. 11 shows the photograph of the porous wind fence installed around Yard 5. To ascertain the effectiveness of the porous wind fence installed around the yard, the local wind speeds were measured with a portable anemometer. The local wind measurement technique and measurement conditions are explained in detail in Lee et al. (2000). In addition, the monthly averaged wind speed at the centerline of Yard 5 and the quantity of total suspension particles (TSP) around the open storage yards were monitored. The wind speed was measured twice in the summer season along all measurement lines. In the present study, however, we consider only the three lines shown in Fig. 4. The main wind direction at the center of Yard 5 was south-eastern. Fig. 12 shows the monthly averaged wind speed calculated from daily wind data measured at the coal pile height (Y ¼ 15 m) after the installation of the porous wind fence at Yard 5. The measured wind speeds are compared with the 10 yr-averaged wind data for the corresponding month. The installation of the wind fence of e ¼ 30% around Yard 5 greatly decreased the mean wind speed inside the storage yard compared with the data obtained over 10 yr without the fence. The reduction rate of wind speed at the coal pile crest ranges from 28% to 71%. Even greater reductions of wind speed are expected in the region below the coal pile crest. On average, the wind speed is decreased to about half the value found in the absence of the fence. However, the reduction rate of the monthly averaged wind speed
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1.00 E-4 No Fence
Power Spectral Density, PSD
Porosity ε =30 %
1.00 E-5
1.00 E-6
1.00 E-7
1.00 E-8 2
4
0.1
6
8
2
4
1.0 Frequency (Hz)
6
8
2
10.0
4
0.1
6
8
2
1.0 Frequency (Hz)
4
6
8
10.0
Fig. 9. PSD distribution of pressure signal on the coal pile (Column 1, Row 3) of Yard 5 for an Eastern wind. (a) Windward side (X =H ¼ 1:0). (b) Leeward side (X =H ¼ 1:0).
4.0 E-3 No F ence
3.5 E-3
Porosity ε =30%
3.0 E-3 2.5 E-3 Cf
2.0 E-3 1.5 E-3 1.0 E-3 5.0 E-4 -0.4
0.0
0.4
0.8
1.2
1.6
2.0
X/H Fig. 10. Effect of the porous wind fence on the friction coefficient of the coal pile (Column 1, Row 3) of Yard 5 for an Eastern wind.
in summer is smaller than in other seasons. This is mainly due to the fact that the wind fence was not initially installed at the south boundary of Yard 5 because of economic considerations, even though the main wind in summer is a southern wind. Therefore, it was necessary to install an additional wind fence at the south boundary of Yard 5 to enhance the shelter effect.
Time series of oncoming wind speed, 3.5 m above the ground at locations in front of and behind the wind fence, were measured and the turbulence intensity of the flow around the fence was obtained. Measurements were performed at a distance of a quarter from the north-side boundary of Yard 5 along the east-side fence. As shown in Fig. 13, the velocity fluctuations ahead of the fence are very active and of large magnitude compared with
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Wind fence
Fig. 11. Photograph of the porous wind fence installed around POSCO Kwang-Yang Yard 5.
8
Monthly averaged (10 years)
7
With wind fence (ε =30%)
Wind speed (m/s)
6 5 4 3 2 1 (64%)
(57%)
(71%)
(58%)
(43%)
(51%)
‘99.10
‘99.11
‘99.12
‘00.1
‘00.2
‘00.3
(59%)
(32%)
(40%)
‘00.5
‘00.6
(28% reduction)
0
‘00.4
‘00.7
Month
Fig. 12. Comparison of monthly averaged wind speed and velocity reduction rate measured at POSCO Kwang-Yang Yard 5 at the elevation of 15 m above the ground.
those behind the fence. After passing the porous wind fence, however, the velocity fluctuations are remarkably reduced and a local wind speed of zero frequently occurs. From this velocity signal we can conjecture that the porous wind fence effectively decreases the oncoming wind speed and produces a good shelter effect in the region behind the fence. From the velocity signal shown in Fig. 13, the mean velocity and turbulence intensity at the location ahead of the fence are 1.52 m s1 and 10.5%, respectively. After passing the porous wind fence they were attenuated down to 0.26 m s1 and 5.3%, respectively. Consequently, with the installation of the porous wind fence of e ¼ 30% in front of coal piles, the turbulence intensity of oncoming wind are reduced down to a half in the region behind the fence. This reduction can be attributed to the substantial loss of momentum of the wind as it
penetrates the fence holes, leading to a bleed flow much lower in speed than the incident wind. Therefore, the wind fence can effectively reduce wind-blown dust erosion. However, the turbulence intensity of oncoming wind for the corresponding day was a little smaller, compared with that of wind-tunnel test. Therefore, there may be some Reynolds number dependency in the present results (Cermak, 1987). Fig. 14 shows the mean wind speed distribution measured along three lines perpendicular to the eastern boundary toward the western end of the yard (see Fig. 4) at a height of 3.5 m above the ground surface. The wind speed in front of the fence is quite fast because the eastern side of the storage yard is directly exposed to the sea. On passing through the porous wind fence, however, the wind speed just behind the fence is greatly decreased. With moving west along Line 1, the wind
C.-W. Park, S.-J. Lee / Atmospheric Environment 36 (2002) 2171–2185
2182
Wind Speed (m/s)
6
4
2
0 0
30
60
90
120
150
180
120
150
180
Time (sec.)
(a)
Wind Speed (m/s)
6
4
2
0 0
30
60
(b)
90
Time (sec.)
Fig. 13. Time series of blown-wind measured in front of and behind the wind fence located at Yard 5 at 3.5 m above the ground. (a) Before the installation of the wind fence, (b) After the installation of the wind fence.
speed increased slightly. This increase occurs because no wind fence is installed along the western boundary of Yard 5. However, on the whole the wind speed inside the storage yard is decreased due to installation of the wind fence along the eastern and western boundaries of the yard. In the region just behind the wind fence, the wind speeds measured in June and August of the year 2000 were almost the same. The wind speed distributions measured along Lines 2 and 3 show that the wind speed is greatly decreased in the region just behind the wind fence, where a conveyor belt system is located to distribute the raw coal inside the storage yard. Thus, the porous wind fence will abate the wind-blown emission of coal dust moving on the conveyor belt. In addition, the turbulence intensity of oncoming wind is reduced by about one-half in the region behind the porous wind fence. On moving further inside the storage yard, however, the wind speed increases again. This occurs because the shelter effect of the fence decreases as the wind moves away from the fence. The deterioration of the shelter effect will be particularly severe for southern and northern winds blowing along the longitudinal direction of the coal piles, because no wind fence is installed along the southern boundary and the coal piles are very long. One plan to further reduce the wind speed inside the storage yard as a whole would entail the installation of additional wind fences along the middle and southern boundary of the yard. Otherwise, adaptive distribution of the raw coal according to the local shelter effect information could provide an alternative solution. For
example, arrangement of large-size coal at the regions of weak shelter effect and placement of lighter coal of small size near the wind fence would help to prevent wind-blown dust emission. 4.2. Particle emission The variation of wind-blown dust concentration was measured twice per month at a location around Yard 5 of the Kwang-Yang Works. The dust concentration, defined as the total mass of suspended particles per unit volume (TSP, mg m3), was about 1300 mg m3 before the installation of wind fence, as shown in Fig. 15. This value is above the national environmental regulation limit of 1000 mg m3. However, the environmental regulation limit in the Kwang-Yang region was especially toughened to 500 mg m3 from the end of 1999. Installation of the porous wind fence reduced the wind-blown dust concentration to levels below the decreased environmental regulation limit. The TSP concentration has been reduced by 70–80% of the level before the construction of the fence. This implies that the porous wind fence installed along two sides of Yard 5 is very effective at abating wind-blown coal dust. The relatively high TSP values found just after the installation of the wind fence can be attributed to the dispersion of latent coal dust on trees and nearby structural elements. Further evidence of the efficacy of the fence is the reduction in civil appeals related to the wind erosion of coal dust. Before the installation of the wind fence many civil appeals were issued; however, in the
C.-W. Park, S.-J. Lee / Atmospheric Environment 36 (2002) 2171–2185
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Mean Velocity (m/s)
8
Wind Fence
2000.6.30
6
2000.8.8
4 2 0 -50
0
50
100
(a)
150
200
300 (m)
250
Measurement Location
Mean Velocity (m/s)
8
Wind Fence
2000.6.30
6
2000.8.8
4 2 0 -50
0
50
100
(b)
150
200
250
300
350
400 (m)
Measurement Location
Mean Velocity (m/s)
8
Wind Fence
2000.6.30
6
2000.8.8
4 2 0 -50
0
50
100
(c)
150
200
250
300
350
400 (m)
Measurement Location
Fig. 14. Mean velocity distributions in Yard 5 measured at the location of 3.5 m above the ground. (a) Line 1. (b) Line 2. (c) Line 3.
1500
1250
Before the wind fence installation
Middle of month
TSP (µg/m3)
End of month 1000
750
After the wind fence installation
500
250
0 ‘97.4
‘99.4
‘00.1
‘00.2
‘00.3
‘00.4
‘00.6
‘00.7
Date Fig. 15. Concentration of wind-blown dust measured around the coal yard.
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two years since the fence installation there have been no appeals.
5. Conclusions The shelter effect of a porous wind fence with porosity e ¼ 30% on the surface pressure and surface shear stress was investigated experimentally using a scaled-down model of the coal piles at the POSCO Kwang-Yang open storage yards. The mean and rms pressure fluctuations on the model coal piles were measured for several dominant wind directions. The presence of the fence decreased the surface shear stress on the windward surface to about half of that found in the no fence case. The wind fence of porosity e ¼ 30% was found to be effective for decreasing the surface pressure and surface shear stress on the coal piles in the open storage yard. Following the wind-tunnel simulation results, a fullsize wind fence of porosity e ¼ 30% was installed along two sides of Yard 5 of the Kwang-Yang open storage yards. The instantaneous wind speed was measured to test the effectiveness of the wind fence. A time series of the velocity signal measured behind the wind fence reveals a reduction of the turbulence intensity of the oncoming wind of about 50%. By installing the wind fence, about 70–80% of total suspension particle (TSP) was reduced and satisfied the environmental regulation safely. This field test has shown the installed wind fence to be very effective for reducing the wind speed and consequently for abating the wind-blown emission of coal dust. This implies that the shelter effect obtained from the wind-tunnel simulation is well matched with the full-scale measurement and it reflects a good practical application of windbreaks to the real field. Finally, the wind fence installation has saved the environment neighboring the POSCO Kwang-Yang Works from the wind erosion of coal dust.
Acknowledgements This work was supported by POSCO, and NRL (National Research Laboratory) Program of the Ministry of Science and Technology, Korea.
References Bofah, K.K., Alhinai, K.G., 1986. Field tests of porous fences in the regime of sand laden wind. Journal of Wind Engineering and Industrial Aerodynamics 23, 309–319. Borges, A.R., Viegas, D.X., 1988. Shelter effects on a row of coal piles to prevent wind erosion. Journal of Wind Engineering and Industrial Aerodynamics 29, 145–154.
Bradley, E.F., Mulhearn, P.J., 1983. Development of velocity and shear stress distributions in the wake of a porous shelter fence. Journal of Wind Engineering and Industrial Aerodynamics 15, 145–156. Cermak, J.E., 1987. Advances in physical modeling for wind engineering. Journal of Engineering Mechanics, ASCE 113 (5), 737–756. Chatzipanagiotidis, A., Olivari, D., 1996. Pollutant dispersal downstream of a hill in different wind conditions. Journal of Wind Engineering and Industrial Aerodynamics 64, 233–248. Ferreira, A.D., Viegas, D.X., 1995, Wind tunnel simulation of the adiabatic flow around a set of sinusoidal hills. Proceedings of the 9th International Conference on Wind Engineering, pp. 311–321. Geurts, C.P.W., 1996. Full-scale and wind-tunnel measurements of the wind and wind-induced pressures over suburban terrain. Journal of Wind Engineering and Industrial Aerodynamics 64, 89–100. Gillette, D.A., 1974. On the production of soil wind erosion aerosols having the potential for long range transport. Journal of Research Atmosphere 8 (3/4), 735–774. Grant, R.H., 1988. Full-scale comparison of a wind-tunnel simulation of windy locations in an urban area. Journal of Wind Engineering and Industrial Aerodynamics 31, 335–341. Irwin, H.P.A.H., 1981. The design of spires for wind simulation. Journal of Wind Engineering and Industrial Aerodynamics 7, 361–366. Iversen, J.D., Greeley, R.G., Marshall, J.R., Pollack, J.B., 1987. Aeolian saltation threshold: the effect of density ratio. Sedimentology 34, 699–706. Jovic, S., Driver, D., 1995. Reynolds number effect on the skin friction in separated flows behind a backward-facing step. Experiments in Fluids 18, 464–467. Kind, R.J., 1990. Mechanics of aeolian transport of snow and sand. Journal of Wind Engineering and Industrial Aerodynamics 36, 855–866. Lee, S.J., Kim, H.B., 1999. Laboratory measurements of velocity and turbulence field behind porous fences. Journal of Wind Engineering and Industrial Aerodynamics 80, 311–326. Lee, S.J., Kim, H.B., 1998. Velocity field measurements of flow around a triangular prism behind a porous fence. Journal of Wind Engineering and Industrial Aerodynamics 77, 521–530. Lee, S.J., Park, C.W., 1998. Surface-pressure variations on a triangular prism by porous fences in a simulated atmospheric boundary layer. Journal of Wind Engineering and Industrial Aerodynamics 73, 45–58. Lee, S.J., Park, C.W., 1999. Surface pressure characteristics on a triangular prism located behind a porous fence. Journal of Wind Engineering and Industrial Aerodynamics 80, 69–83. Lee, S.J., Park, C.W., Kim, H.B., 1997. Wind engineering study for Kwang-Yang open storage yard to abate wind-blown dusts. POSCO Technical Report-97G001. Lee, S.J., Park, C.W., Lim, H.C., Lee, S.I., 2000. Investigation on dustproof characteristics of wind fence for Kwang-Yang open storage yard. POSCO Technical Report-IPS99001. Ogawa, Y., Diosey, P.G., 1980. Surface roughness and thermal stratification effects on the flow behind a two-dimensional
C.-W. Park, S.-J. Lee / Atmospheric Environment 36 (2002) 2171–2185 fence—I. Field study. Atmospheric Environment 14, 1301–1308. Owen, P.R., 1964. Saltation of uniform grains in air. Journal of Fluid Mechanics 20, 225–242. Patel, V.C., 1965. Calibration of the preston tube and limitation on its use in pressure gradients. Journal of Fluid Mechanics 23, 185–208. Perera, M.A.E.S., 1981. Shelter behind two-dimensional solid and porous fences. Journal of Wind Engineering and Industrial Aerodynamics 8, 93–104. Plate, E.J., 1971. The aerodynamics of shelter belts. Agricultural Meteorology 8, 203–222.
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Raine, J.K., Stevenson, D.C., 1977. Wind protection by model fences in a simulated atmospheric boundary layer. Journal of Industrial Aerodynamics 2, 159–180. Ranga, R.K.G., Garde, R.J., Singh, S.K., Singh, N., 1988. Experimental study on characteristics of flow past porous fences. Journal of Wind Engineering and Industrial Aerodynamics 29, 155–163. Richardson, G.M., 1989. A permeable windbreak: its effect on the structure of the natural wind. Journal of Wind Engineering and Industrial Aerodynamics 32, 101–110.
Verification of the shelter effect of a windbreak on coal piles in the POSCO open storage yards at the Kwang-Yang works Cheol-Woo Park, Sang-Joon Lee* Department of Mechanical Engineering, Pohang University of Science and Technology, Pohang 790-784, South Korea Received 21 September 2001; received in revised form 28 January 2002; accepted 6 February 2002
Abstract The use of windbreaks to reduce wind-blown coal dust at the POSCO Kwang-Yang open storage yards was studied using wind simulations on a scale model of the yards. Based on these simulation results, a full-scale wind fence was constructed on two sides of the yard. Here, we present results on the wind behavior both for the real yard and for the simulation results that guided its construction. Wind-tunnel simulations were used to study the effect of a porous wind fence of porosity e ¼ 30% on the surface pressure and shear stress on coal piles using a 1/1200 model of the POSCO Kwang-Yang open storage yards. In addition, the shelter effects found in the model system were verified in field measurements on the full-scale system. The storage yard model was fully embedded in an atmospheric surface boundary layer over open terrain. The fence and coal pile model had the same height (12.2 mm) and Reynolds number (Re=1.6 104, based on the model height). The mean and fluctuating surface-pressure distributions on the coal piles, which are closely related to the dust emission from the surface, were measured for several directions of the oncoming wind. The wind directions pertinent to the study were determined by statistical analysis of seasonal wind data over the storage yard. A porous wind fence of porosity e ¼ 30% was found to be useful for reducing the wind speed without the formation of a recirculating bubble behind the fence. In addition, the fence caught the wind-borne particles when it was located behind the coal piles. The wind fence reduced the pressure fluctuations and surface shear stress on the coal piles to less than half of the levels observed in the no fence case. To verify the effectiveness of the porous wind fence installed around the Kwang-Yang open storage yard, the local wind speed and the concentration of suspended particles were measured directly. Full-scale porous fences installed around the Kwang-Yang open storage yard greatly decreased the turbulence intensity of the wind over the coal piles and reduced the total suspension particles by 70–80%. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Windbreak; Porosity; Wind fence; Surface pressure; Coal pile; Atmospheric boundary layer
1. Introduction The wind erosion of small particles such as snow and sand in regions of strong wind has been an important wind engineering problem over the past few decades. For example, atmospheric dispersion of wind-blown dust particles from open coal storage yards can cause serious air-pollution and environmental problems. Wind *Corresponding author. Tel.: +82-54-279-2169; fax: +8254-279-3199. E-mail address: [email protected] (S.-J. Lee).
erosion also leads to a loss of raw materials resulting in a needless waste of precious investment. From the standpoint of wind engineering it is valuable to investigate the wind erosion phenomenon in a systematic manner in order to develop methods for reducing undesirable dust emissions. The wind erosion of particles near the ground surface frequently occurs in the open storage yards of power generation plants and steel-making companies. This erosion causes environmental problems and the loss of raw materials. However, the mechanism of wind erosion is difficult to understand because the dispersion
1352-2310/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 0 2 ) 0 0 1 9 5 - 4
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mechanism is very complicated and depends on many topographic and meteorological factors. For this reason only a few empirical correlations are available for predicting dust erosion, and adequate quantitative relationships between the fluctuating surface pressure and flow over a coal pile have yet to be established (Owen, 1964; Gillette, 1974; Iversen et al., 1987; Kind, 1990). Recently, the enhanced regulation for environmental protection has led to an increase in the importance of atmospheric wind tunnel simulations of wind-blown dusts in the field of wind engineering. Windbreaks have been widely used since early times, mainly to protect agricultural fields. Nowadays, they are also extensively used in a range of applications such as preventing snow drift, soil erosion, pollutant dispersion and spills of toxic materials. The wind fence used in the present study is one such windbreak. The wind fence withstands a drag force, causing a net loss of momentum of the oncoming airflow and thus has a sheltering effect. However, as the permeability of the wind fence is reduced, the bleed flow through the fence holes decreases and the drag force increases. From this point of view, use of solid fences as windbreaks can cause practical difficulties because of the increased drag force on such fences and increased construction cost. Many studies have endeavored to find effective wind fences that simultaneously reduce drag and enhance the shelter effect. Borges and Viegas (1988) investigated the shelter effect of windbreaks by measuring the mean velocity and shear stress of the near wake behind the windbreaks. Lee and Kim (1999) measured the velocity fields in the turbulent wake behind two-dimensional porous wind fences using a two-frame PTV technique. They found that as the fence porosity (e) increases the turbulence intensity and Reynolds shear stress decreases; however, the mean velocity deficit also decreases. In a study on porous windbreaks Perera (1981) investigated the effects of porosity and permeable-hole shape on the mean velocity deficit, reattachment length and Reynolds shear stress in the near wake. The separation bubble formed behind the fence disappeared when the fence porosity was >e ¼ 30%: Ranga et al. (1988) found that the drag force acting on wind fences is mainly affected by the fence porosity and fence height. Plate (1971) revealed that a windbreak can effectively reduce wind-blown dust when it is embedded in the atmospheric boundary layer over a plain terrain. Bofah and Alhinai (1986) investigated the encroachment of drifting sand in inhabited arid regions and studied the use of porous fences to control sand drift. A complete quantitative analysis of the effects of porous fences has not been performed, because wind erosion is a very complicated phenomenon that is greatly affected by climatic and topographical factors. Raine and Stevenson (1977) measured flow statistics such as the mean
velocity, Reynolds stress, turbulence intensity and power spectra in the lee of a solid fence and permeable shelter fences. In their study, a windbreak with low to medium permeability reduced the overall mean velocity more than a solid windbreak. A fence with even larger permeability gave better overall protection. Several previous studies have considered the wind erosion of pollutants in urban areas. Grant (1988) measured the pedestrian-level wind speed at several sites across an urban area and compared it with wind-tunnel simulation results using the plastic-pellet erosion technique. Chatzipanagiotidis and Olivari (1996) investigated the pollutant dispersion over a hill using a digital image analysis. They found that the boundary layer thickness of oncoming flow significantly influenced the wind erosion of pollutants. However, only limited attention has been given to the problem of wind erosion in large open storage yards. Lee and Park (1998, 1999) investigated the interaction between porous wind fences and the coal pile surface pressure. Using wind-tunnel tests they found that a wind fence of porosity e ¼ 40% is the most effective for reducing the pressure fluctuations on a 2-D prism surface. The shelter effect of the porous wind fence was found to extend up to the 5th–6th prism in consecutive triangular prism models. Wind erosion is closely related to the flow characteristics of the atmospheric surface layer such as the wind speed, pressure fluctuations and turbulent shear stress. In particular, the lift force that ejects dust particles from the coal pile surface is related to the magnitude of the oncoming wind speed and friction velocity near the surface. This implies that the surface-pressure fluctuations on the coal-pile surface are dependent on the near surface flow characteristics. Lee et al. (1997) proposed the installation of a porous wind fence of porosity e ¼ 30% around the coal pile yards at the POSCO Kwang-Yang Works, based on an optimization study on porous wind fences. At this factory, located on the southern seaside of the Korean peninsula, coal dust in the open storage yard is blown by wind and local gusts, especially in the summer stormy season. In order to support this proposal, the shelter effect of a porous wind fence of porosity e ¼ 30% on a scale-downed coal pile model of POSCO Kwang-Yang open storage yard was investigated experimentally through a wind-tunnel simulation. The proposed structure was accepted and a huge porous wind fence was installed along two sides of Yard 5 of the Kwang-Yang Works. After installing the wind fence, the wind environment around the storage yard was monitored annually to test its usefulness and effectiveness for abating the wind erosion of coal dust. From an engineering point of view, this construction represents a rare and very important opportunity to directly apply wind-tunnel simulation results to the
C.-W. Park, S.-J. Lee / Atmospheric Environment 36 (2002) 2171–2185
full-scale situation and to check its usefulness through a field test. This paper summarizes the wind-tunnel simulation and the full-scale field tests. The results validate the shelter effect of porous wind fences for abating wind erosion of dust.
2. Experimental apparatus and methods The wind-tunnel simulation was carried out in a closed-return type subsonic wind-tunnel with a test section of 1.8 m wide 1.5 m high 11 m long. Spires and roughness elements were installed in front of the test section to generate a thermally neutral atmospheric boundary layer. The triangular spires of 71.2 cm height and 5.4 cm base-length were designed following Irwin (1981) and installed along the spanwise direction with a 35.6 cm gap. A schematic diagram of the wind-tunnel test section and measurement system is shown in Fig. 1. The right side of the first model coal pile encountered by the oncoming wind is selected as the positive X -coordinate, because the windward and leeward surfaces change according to the wind direction. The mean velocity and turbulence intensity profiles of the simulated atmospheric boundary layer were measured at the fence location, 5 m downstream from the leading edge of the test section, using a hot-wire
anemometer (TSI IFA-100). At each measurement point, 16,000 velocity data were acquired at a sampling rate of 2 kHz after low-pass filtering at 800 Hz. Since the measurement location is farther than six times of the spire height (Irwin, 1981), the simulated turbulent boundary layer would be an equilibrium state. In addition, the streamwise pressure gradient was nearly negligible due to corner fillets and small breathers located at the end of wind-tunnel test section. Fig. 2 shows the streamwise mean velocity and turbulence intensity profile measured at the fence location. The mean streamwise velocity normalized by the reference velocity (Uref ) at the height Yref ¼ 0:15 m has the power law profile: UðyÞ=Uref ¼ ðy=yref Þn :
Spires Y Uo
: Without Yard Model Fence H
Fence Hot-wire
h G
X or Z
α=40o
Pressure Scanning Selection Box FCO91 MkII
CTA TSI IFA-100
Micromanometer FCO 12
A/D Converter
PC
ð1Þ
The velocity profiles are well fitted with the power law exponent n ¼ 0:14; corresponding to the velocity profile over the open terrain. We assumed that the velocity profile is valid at the seaside. The turbulence intensity near the ground surface is about 20–25%. Ideally, all atmospheric parameters should be reproduced in the wind-tunnel simulation. Unfortunately, the atmospheric parameters scatter a lot. This is particularly true for the integral length scale (Lxu ) of the approach flow, although Geurts (1996) and some other researchers dispute this fact since the turbulence intensity is >10%. The integral
Wind Tunnel Test Section
Yards model
2173
Signal Conditioner
DT 2838
Fig. 1. Wind-tunnel test section and measurement system.
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40
40 U0.15 =5.8m/s
U0.15 =5.8m/s
U0.15 =9.5m/s
35
U0.15 =9.5m/s
35
U0.15=13.5m/s n=0.14
30
Y/ H
U0.15=13.5m/s
30
25
25
20
20
15
15
10
10
5
5
0
0 0.4
(a)
0.6
0.8
1.0
1.2
U/Uo
1.4
01 (b)
0
20
30
u'2 /U × 100(%)
Fig. 2. Mean streamwise velocity and turbulence intensity profiles measured at the model coal yard location (a) mean velocity, (b) turbulence intensity.
length scale (Lxu ) calculated from the wind-tunnel data is between 50 and 80 cm. The configuration and photograph of the open storage yards tested in this study are shown in Fig. 3. The wind-tunnel model includes various building structures and chimneys of the real coal plant. The locations in the 1/1200 scale yards where pressure measurements were made are specified by the row and column numbers. Each model coal pile had an inclination angle of 401 and 11 pressure taps were installed along the middle span of each model pile at intervals of 4 mm. The model coal pile was made of sawdust and a bonding agent. Fine sand particles were attached on the surface of model coal pile to simulate the surface roughness of real coal piles. There was no change in the morphology of the coal piles as a result of the wind erosion. The pressure taps were connected to the pressure scanning selection system (FCO91-MkII) with vinyl tubes of inner diameter 0.8 mm. Each pressure tap was scanned by a selection switch and the analog voltage output from the micromanometer (FCO-12) was digitized by a high-precision A/D converter (DT-2838). At each channel, 16,384 pressure data were acquired at a
sampling rate of 500 Hz and statistically averaged to obtain mean and rms pressure data. Based on previous wind-tunnel tests (Lee et al., 1997), a porous fence of porosity e ¼ 30% was selected to block the wind-blown dust from the coal piles. The porous fence was made of stainless-steel plate of thickness 0.5 mm, and the fence height H was made equal to the height of the coal pile crest, h ¼ 12:2 mm. The porous fence model was constructed of circular holes of 1 mm diameter with 0.6 mm spacing. The circular holes were precisely made using an etching technique. (Lee and Park, 1998; Bradley and Mulhearn, 1983) The distance between the fence and the front edge of the model coal pile was G ¼ 20 mm. The pressure difference between the surface pressure p and the reference static pressure p0 was non-dimensionalized by the dynamic pressure with velocity U0:15 at a height of Y ¼ 0:15 m, and air density ra : The pressure coefficient Cp is expressed as Cp ¼ 2ðp p0 Þ=ra ðU0:15 Þ2 :
ð2Þ
The reference pressure p0 was measured at Y ¼ 1:2 m, at which height the influence of the model was small and
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2175
N E
W S
1, 2 COKE PLANT
500 (m)
0
Yard 1 & 2 1,2 SINTER PLANT
MCR
SINTER YARD
3,4 SINTER PLANT
3, 4 COKE PLANT
Yard 3 & 4
Row number for transverse measurement location 1
2
-Z Fence
3
Yard 5
Pressure tap (11 holes)
4
o oo o o oo o
5
X G=20mm
Wind Fence
4 3 2 1
Column number for each coal pile model
Fig. 3. Configuration and photograph of coal piles at Kwang-Yang open storage yards installed in the wind-tunnel test section and pressure measurement locations.
the flow was assumed to be inviscid. During the experiments the oncoming wind speed was fixed at UN ¼ 18 m s1 and its corresponding Reynolds number based on the height of the model coal pile (h) was approximately Re=1.6 104. The surface shear stress tw was measured along the centerline of the coal pile model with a Preston tube of small inner diameter. The shear stress was calculated from the pressure difference data between the Preston tube and the reference static tube, following the method of Patel (1965). The surface shear stress was nondimensionalized with the velocity U0:15 to give the friction coefficient Cf as follows: Cf ¼ 2tw =ra ðU0:15 Þ2 :
ð3Þ
Jovic and Driver (1995) mention that the friction coefficient Cf decreases in proportion to Re1/2, where Re is the Reynolds number. Following the wind-tunnel simulation results, a porous wind fence of porosity e ¼ 30% was installed along two sides of Yard 5 of the open storage yard of POSCO Kwang-Yang Works at a cost of about 3 million US dollars. The porous fence was installed along the eastern and northern boundaries of the yard. The wind fence consists of steel columns at intervals of 20 m and stretched net made of nylon wire. The mesh size was adjusted to create a fence porosity of e ¼ 30%: The height of the fence was chosen to be 17 m, slightly higher than the average height of the coal piles (15 m). The total length of the fences installed at Yard 5 is about 1800 m.
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2176
W S
N E
Myo Island
Yard 3 & 4
The South Sea Line 3
Yard 1 & 2
Line 2 Line 1
350 m
Yard 5
Taein Island
Plant Area
1450 m
The South Sea
Wind Fence
.
: Dusts measurement location
Fig. 4. Measurement locations of wind-blown dust from the open storage yards of the Kwang-Yang Works.
A diagram of the wind fence installed at Yard 5 is shown in Fig. 4. Field measurements taken at the site measured the variation of local wind speed using a portable anemometer in conjunction with a direction finder. In addition, the monthly average wind speed along the centerline of Yard 5 was measured with an anemometer attached to a crane head at a height of 15 m above the ground. To confirm the shelter effect of the wind fence a sequential time series of instantaneous oncoming wind velocity measurements were made in front of and behind the installed wind fence at a height of 3.5 m above the ground. At each measurement point, 16,000 velocity data were measured with a Pitot-tube at a sampling rate of 100 Hz over a period of 160 s. The concentration of wind-blown dust was measured twice per month at a location near the wind fence, as shown in Fig. 4. Dry and wet depositions were gathered using dust collectors. The wind-blown dust concentration was defined as the total mass of suspended particles per unit volume (TSP, mg m3).
3. Wind-tunnel simulation 3.1. Wind direction rose Wind characteristics such as the wind speed and dominant wind direction were analyzed using the daily averaged wind data measured at the Kwang-Yang open storage yard over 1996 and 1997. A maximum wind speed of 13.4 m s1 was measured in March 1996, and the yearly mean wind speed was approximately 2.7 m s1. The instantaneous wind speed during storms
or gusts was greater than the daily average wind speed. Based on the wind data, the free stream velocity used in the model simulations was fixed at 18 m s1 to cover the extreme conditions. Fig. 5 shows the seasonally averaged wind direction rose obtained from the 1996 and 1997 wind data. The periphery of the wind rose is divided into sixteen segments indicating the wind direction. The annulus in the radial direction indicates the percentage of the wind rose probability. According to the wind data, northwesterly winds occur frequently in spring and winter, whereas north-easterly winds are dominant in summer and fall. In previous experimental simulations, we tested all coal piles and all wind directions that are likely to occur in the Kwang-Yang open storage yard (Lee et al., 1997). In the present paper, however, we consider only the three dominant wind directions that occur in Yard 5.
3.2. Surface pressure Fig. 6 shows the shelter effect of the wind fence installed around Yard 5 for an easterly wind incident on the model coal pile located at Row 3. For the coal pile of Column 1, the mean pressure and pressure fluctuations on the windward surface substantially decrease after installation of the porous fence. In addition, the magnitude of the mean pressure and pressure fluctuations in the crest region of the model coal pile is greatly reduced for the four columns tested. This reduction can be primarily attributed to the large decrease in the wind speed on passing through the fence. Without the fence, the pressure fluctuations are very high at the crest of the coal pile due to the flow separation near the crest. This seems to be closely related with the flow field variation
C.-W. Park, S.-J. Lee / Atmospheric Environment 36 (2002) 2171–2185
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Fig. 5. Wind direction rose based on 1996 and 1997 wind data.
around the coal pile model as explained in Lee and Kim (1998). The mean pressure on the windward surface of the second coal pile (Column 2) decreases due to the deceleration of the recirculating flow between the two adjacent coal piles. This recirculation flow reduces the mean pressure on the leeward side of the first coal pile and on the windward side of the second coal pile. The rms pressure fluctuations on the first coal pile surface also increase slightly due to the recirculation flow between the two consecutive coal piles (Ferreira and Viegas, 1995). For Columns 3 and 4 the mean pressure on the windward side increases due to the descending shear layer separated from the fence top and coal pile crest. The rms pressure fluctuations on the windward surface also increase slightly. Fig. 7 shows the surface-pressure distributions on the model coal piles at Row 2 of Yard 5 under a westerly wind. The mean pressure distributions show similar patterns to those observed for an easterly wind. However, the rms pressure distributions differ slightly from those of the eastern wind case. This difference can be attributed to the various high-rise structures such as
towers and cranes located in front of the wind fence, which are used at the port for unloading raw materials. The results indicate that nearby tall structures can disturb the flow structure behind the wind fence. The pressure fluctuations at the first column (Column 4) just behind the porous fence are even larger than those observed in the no fence case. For the next three columns the pressure distributions show that the installed wind fence provides a favorable shelter effect. Fig. 8 shows the mean and rms pressure variations on the model coal piles at Row 1 of Yard 5 under a northeasterly wind. For a north-easterly wind, the oncoming flow crosses Yard 5 obliquely and the shelter effect decreases slightly with moving toward Column 4 compared with the case of an easterly wind. The mean pressure distributions are nearly uniform on both sides of each coal pile, excluding the large negative pressure at the coal pile crest. This is attributed to the fact that the oncoming flow approaches obliquely and the consecutive coal piles are embedded in the shear layer separated from the pile crest and fence top edge. The mean pressure on the windward side of Column 4 is greatly increased and descends rapidly compared with the no
C.-W. Park, S.-J. Lee / Atmospheric Environment 36 (2002) 2171–2185
2178 0.8
0.2 No Fence Column 1
0.6
Wind direction
Wind direction
No Fence Column 4 Row 2, Column 4
0.4
Row 3, Column 2
0.2
Row 3, Column 3
0.0
Row 2, Column 3 Row 2, Column 2
Row 3, Column 4
0.0
Cp(mean)
Cp(mean)
Row 3, Column 1
-0.2
-0.2
Row 2, Column 1
-0.4
-0.4 -0.6
-0.6
-0.8 -1.0 -1.6
-1.2
-0.8
-0.4
(a)
0.0
0.4
0.8
1.2
-0.8
1.6
X/H
-1.6
-1.2
-0.8
-0.4
(a)
0.022
0.0
0.4
0.8
1.2
1.6
X/H 0.018
No Fence Column 1
No Fence Column 4
Row 3, Column 1
0.020
0.017
Row 2, Column 4
Row 3, Column 2
Row 2, Column 3 Row 3, Column 3
0.016
Row 2, Column 2
Row 3, Column 4
Cp(rms)
Cp(rms)
0.018
0.016
Row 2, Column 1
0.015 0.014
0.014
0.013 0.012 -1.6
-1.2
(b)
-0.8
-0.4
0.0
0.4
0.8
1.2
1.6
0.012 -1.6
X/H
Fig. 6. Mean and rms pressure distributions on coal piles (Row 3) of Yard 5 for a Eastern wind. (a) mean pressure, (b) rms pressure.
fence case, because the shear layer is separated from the fence top. The rms pressure around the crest of the fourth coal pile (Column 4) is slightly higher than for the other columns due to the momentum loss of the separated shear layer.
3.3. Power spectra The power spectral density (PSD) of the pressure signals measured at the coal pile surface was spectrally analyzed. Time series of pressure signals low-pass filtered at 50 Hz was acquired at 100 Hz to analyze its frequency characteristics. The PSD results for one model coal pile (Column 1) in Yard 5 under an eastern wind are shown in Fig. 9. In this figure, the effect of the wind fence on PSD attenuation in the low frequency range is obvious. The PSD has high values at frequencies up to 0.5 Hz, after which the power density of the pressure signal rapidly decays. Ogawa and Diosey (1980) observed several
(b)
-1.2
-0.8
-0.4
0.0
0.4
0.8
1.2
1.6
X/H
Fig. 7. Mean and rms pressure distributions on coal piles (Row 2) of Yard 5 for a Western wind. (a) mean pressure, (b) rms pressure.
frequency peaks in the wake of a 2-D fence, which they attributed to the high turbulence intensity of the oncoming flow near the ground surface and small vortex-induced eddies. Richardson (1989) found that a permeable windbreak substantially attenuated the PSD of the pressure signal at low frequency (0.14 Hz) in comparison to the drag force signal. This behavior is attributed to the characteristic length scale of the atmospheric flow, which exceeds the hole size of the windbreak at low frequencies. Geurts (1996) revealed that the attenuation of the pressure spectra was faster than that of the wind velocity spectra. The PSD attenuation on the windward surface is larger than that on the leeward surface in the low frequency band. On the leeward surface, the wind fence also decreases the PSD values to some extent in the low frequency band. This is primarily attributed to the fact that several consecutive coal piles are located below the shear layer separated from the fence top. Since the change of power spectra indicates the modification of the turbulent kinetic energy implied on the coal pile
C.-W. Park, S.-J. Lee / Atmospheric Environment 36 (2002) 2171–2185 0.2 No Fence Column 1
Wind direction
Row 1, Column 1
0.0
Row 1, Column 2 Row 1, Column 3 Row 1, Column 4
Cp(mean)
-0.2
-0.4
-0.6
-0.8 -1.6
-1.2
-0.8
-0.4
(a)
0.0
0.4
0.8
1.2
1.6
X/H
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windward surface, shown in Fig. 6. Beyond the location of X /H=0.66, however, the friction coefficients decrease slightly. This implies that the flow separation on the coal pile surface occurs earlier in this region compared with a smooth surface, which is reasonable because the model coal pile was roughened by adhering small sand particles to simulate the coal pile surface. Installation of the porous wind fence of e ¼ 30% in front of the coal pile reduced the friction coefficients down to half of the values observed for the no fence case. From this result we can conjecture that the wind fence should effectively reduce the wind-blown dust erosion, because the surface shear stress is one of the main causes of wind erosion.
0.018
4. Field test results
No Fence Column 1
0.017
Row 1, Column 1
4.1. Wind speed
Row 1, Column 2
Cp(rms)
0.016
Row 1, Column 3 Row 1, Column 4
0.015 0.014 0.013 0.012 -1.6
-1.2
-0.8
(b)
-0.4
0.0
0.4
0.8
1.2
1.6
X/H
Fig. 8. Mean and rms pressure distribution on coal piles (Row 1) of Yard 5 for a North-eastern wind. (a) mean pressure, (b) rms pressure.
surface, the wind erosion of coal dust particles would be changed. From this, we can see that the porous wind fence of e=30% installed in front of the coal pile can effectively attenuate the pressure power which may enhance the wind erosion.
3.4. Surface shear stress For the model systems with and without the wind fence of porosity 30%, the skin friction coefficients on the first coal pile model of Yard 5 under an eastern wind were measured. The results are compared in Fig. 10. Because the surface shear stress was measured using a Preston tube, which cannot be used in reverse flows, the surface shear stress was measured only on the windward surface of the model coal pile. Without the wind fence, the mean friction coefficient increases on the windward surface because the oncoming flow accelerates along this surface. This behavior correlates well with the mean pressure gradient on the
As mentioned in the previous section, the wind-tunnel simulation results, including the threshold velocity measurement of wind-blown sand particles stacked behind the wind fence, led to the installation of a wind fence of porosity e ¼ 30% on two sides of Yard 5 at the Kwang-Yang open storage yards. Fig. 11 shows the photograph of the porous wind fence installed around Yard 5. To ascertain the effectiveness of the porous wind fence installed around the yard, the local wind speeds were measured with a portable anemometer. The local wind measurement technique and measurement conditions are explained in detail in Lee et al. (2000). In addition, the monthly averaged wind speed at the centerline of Yard 5 and the quantity of total suspension particles (TSP) around the open storage yards were monitored. The wind speed was measured twice in the summer season along all measurement lines. In the present study, however, we consider only the three lines shown in Fig. 4. The main wind direction at the center of Yard 5 was south-eastern. Fig. 12 shows the monthly averaged wind speed calculated from daily wind data measured at the coal pile height (Y ¼ 15 m) after the installation of the porous wind fence at Yard 5. The measured wind speeds are compared with the 10 yr-averaged wind data for the corresponding month. The installation of the wind fence of e ¼ 30% around Yard 5 greatly decreased the mean wind speed inside the storage yard compared with the data obtained over 10 yr without the fence. The reduction rate of wind speed at the coal pile crest ranges from 28% to 71%. Even greater reductions of wind speed are expected in the region below the coal pile crest. On average, the wind speed is decreased to about half the value found in the absence of the fence. However, the reduction rate of the monthly averaged wind speed
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1.00 E-4 No Fence
Power Spectral Density, PSD
Porosity ε =30 %
1.00 E-5
1.00 E-6
1.00 E-7
1.00 E-8 2
4
0.1
6
8
2
4
1.0 Frequency (Hz)
6
8
2
10.0
4
0.1
6
8
2
1.0 Frequency (Hz)
4
6
8
10.0
Fig. 9. PSD distribution of pressure signal on the coal pile (Column 1, Row 3) of Yard 5 for an Eastern wind. (a) Windward side (X =H ¼ 1:0). (b) Leeward side (X =H ¼ 1:0).
4.0 E-3 No F ence
3.5 E-3
Porosity ε =30%
3.0 E-3 2.5 E-3 Cf
2.0 E-3 1.5 E-3 1.0 E-3 5.0 E-4 -0.4
0.0
0.4
0.8
1.2
1.6
2.0
X/H Fig. 10. Effect of the porous wind fence on the friction coefficient of the coal pile (Column 1, Row 3) of Yard 5 for an Eastern wind.
in summer is smaller than in other seasons. This is mainly due to the fact that the wind fence was not initially installed at the south boundary of Yard 5 because of economic considerations, even though the main wind in summer is a southern wind. Therefore, it was necessary to install an additional wind fence at the south boundary of Yard 5 to enhance the shelter effect.
Time series of oncoming wind speed, 3.5 m above the ground at locations in front of and behind the wind fence, were measured and the turbulence intensity of the flow around the fence was obtained. Measurements were performed at a distance of a quarter from the north-side boundary of Yard 5 along the east-side fence. As shown in Fig. 13, the velocity fluctuations ahead of the fence are very active and of large magnitude compared with
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Wind fence
Fig. 11. Photograph of the porous wind fence installed around POSCO Kwang-Yang Yard 5.
8
Monthly averaged (10 years)
7
With wind fence (ε =30%)
Wind speed (m/s)
6 5 4 3 2 1 (64%)
(57%)
(71%)
(58%)
(43%)
(51%)
‘99.10
‘99.11
‘99.12
‘00.1
‘00.2
‘00.3
(59%)
(32%)
(40%)
‘00.5
‘00.6
(28% reduction)
0
‘00.4
‘00.7
Month
Fig. 12. Comparison of monthly averaged wind speed and velocity reduction rate measured at POSCO Kwang-Yang Yard 5 at the elevation of 15 m above the ground.
those behind the fence. After passing the porous wind fence, however, the velocity fluctuations are remarkably reduced and a local wind speed of zero frequently occurs. From this velocity signal we can conjecture that the porous wind fence effectively decreases the oncoming wind speed and produces a good shelter effect in the region behind the fence. From the velocity signal shown in Fig. 13, the mean velocity and turbulence intensity at the location ahead of the fence are 1.52 m s1 and 10.5%, respectively. After passing the porous wind fence they were attenuated down to 0.26 m s1 and 5.3%, respectively. Consequently, with the installation of the porous wind fence of e ¼ 30% in front of coal piles, the turbulence intensity of oncoming wind are reduced down to a half in the region behind the fence. This reduction can be attributed to the substantial loss of momentum of the wind as it
penetrates the fence holes, leading to a bleed flow much lower in speed than the incident wind. Therefore, the wind fence can effectively reduce wind-blown dust erosion. However, the turbulence intensity of oncoming wind for the corresponding day was a little smaller, compared with that of wind-tunnel test. Therefore, there may be some Reynolds number dependency in the present results (Cermak, 1987). Fig. 14 shows the mean wind speed distribution measured along three lines perpendicular to the eastern boundary toward the western end of the yard (see Fig. 4) at a height of 3.5 m above the ground surface. The wind speed in front of the fence is quite fast because the eastern side of the storage yard is directly exposed to the sea. On passing through the porous wind fence, however, the wind speed just behind the fence is greatly decreased. With moving west along Line 1, the wind
C.-W. Park, S.-J. Lee / Atmospheric Environment 36 (2002) 2171–2185
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Wind Speed (m/s)
6
4
2
0 0
30
60
90
120
150
180
120
150
180
Time (sec.)
(a)
Wind Speed (m/s)
6
4
2
0 0
30
60
(b)
90
Time (sec.)
Fig. 13. Time series of blown-wind measured in front of and behind the wind fence located at Yard 5 at 3.5 m above the ground. (a) Before the installation of the wind fence, (b) After the installation of the wind fence.
speed increased slightly. This increase occurs because no wind fence is installed along the western boundary of Yard 5. However, on the whole the wind speed inside the storage yard is decreased due to installation of the wind fence along the eastern and western boundaries of the yard. In the region just behind the wind fence, the wind speeds measured in June and August of the year 2000 were almost the same. The wind speed distributions measured along Lines 2 and 3 show that the wind speed is greatly decreased in the region just behind the wind fence, where a conveyor belt system is located to distribute the raw coal inside the storage yard. Thus, the porous wind fence will abate the wind-blown emission of coal dust moving on the conveyor belt. In addition, the turbulence intensity of oncoming wind is reduced by about one-half in the region behind the porous wind fence. On moving further inside the storage yard, however, the wind speed increases again. This occurs because the shelter effect of the fence decreases as the wind moves away from the fence. The deterioration of the shelter effect will be particularly severe for southern and northern winds blowing along the longitudinal direction of the coal piles, because no wind fence is installed along the southern boundary and the coal piles are very long. One plan to further reduce the wind speed inside the storage yard as a whole would entail the installation of additional wind fences along the middle and southern boundary of the yard. Otherwise, adaptive distribution of the raw coal according to the local shelter effect information could provide an alternative solution. For
example, arrangement of large-size coal at the regions of weak shelter effect and placement of lighter coal of small size near the wind fence would help to prevent wind-blown dust emission. 4.2. Particle emission The variation of wind-blown dust concentration was measured twice per month at a location around Yard 5 of the Kwang-Yang Works. The dust concentration, defined as the total mass of suspended particles per unit volume (TSP, mg m3), was about 1300 mg m3 before the installation of wind fence, as shown in Fig. 15. This value is above the national environmental regulation limit of 1000 mg m3. However, the environmental regulation limit in the Kwang-Yang region was especially toughened to 500 mg m3 from the end of 1999. Installation of the porous wind fence reduced the wind-blown dust concentration to levels below the decreased environmental regulation limit. The TSP concentration has been reduced by 70–80% of the level before the construction of the fence. This implies that the porous wind fence installed along two sides of Yard 5 is very effective at abating wind-blown coal dust. The relatively high TSP values found just after the installation of the wind fence can be attributed to the dispersion of latent coal dust on trees and nearby structural elements. Further evidence of the efficacy of the fence is the reduction in civil appeals related to the wind erosion of coal dust. Before the installation of the wind fence many civil appeals were issued; however, in the
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Mean Velocity (m/s)
8
Wind Fence
2000.6.30
6
2000.8.8
4 2 0 -50
0
50
100
(a)
150
200
300 (m)
250
Measurement Location
Mean Velocity (m/s)
8
Wind Fence
2000.6.30
6
2000.8.8
4 2 0 -50
0
50
100
(b)
150
200
250
300
350
400 (m)
Measurement Location
Mean Velocity (m/s)
8
Wind Fence
2000.6.30
6
2000.8.8
4 2 0 -50
0
50
100
(c)
150
200
250
300
350
400 (m)
Measurement Location
Fig. 14. Mean velocity distributions in Yard 5 measured at the location of 3.5 m above the ground. (a) Line 1. (b) Line 2. (c) Line 3.
1500
1250
Before the wind fence installation
Middle of month
TSP (µg/m3)
End of month 1000
750
After the wind fence installation
500
250
0 ‘97.4
‘99.4
‘00.1
‘00.2
‘00.3
‘00.4
‘00.6
‘00.7
Date Fig. 15. Concentration of wind-blown dust measured around the coal yard.
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two years since the fence installation there have been no appeals.
5. Conclusions The shelter effect of a porous wind fence with porosity e ¼ 30% on the surface pressure and surface shear stress was investigated experimentally using a scaled-down model of the coal piles at the POSCO Kwang-Yang open storage yards. The mean and rms pressure fluctuations on the model coal piles were measured for several dominant wind directions. The presence of the fence decreased the surface shear stress on the windward surface to about half of that found in the no fence case. The wind fence of porosity e ¼ 30% was found to be effective for decreasing the surface pressure and surface shear stress on the coal piles in the open storage yard. Following the wind-tunnel simulation results, a fullsize wind fence of porosity e ¼ 30% was installed along two sides of Yard 5 of the Kwang-Yang open storage yards. The instantaneous wind speed was measured to test the effectiveness of the wind fence. A time series of the velocity signal measured behind the wind fence reveals a reduction of the turbulence intensity of the oncoming wind of about 50%. By installing the wind fence, about 70–80% of total suspension particle (TSP) was reduced and satisfied the environmental regulation safely. This field test has shown the installed wind fence to be very effective for reducing the wind speed and consequently for abating the wind-blown emission of coal dust. This implies that the shelter effect obtained from the wind-tunnel simulation is well matched with the full-scale measurement and it reflects a good practical application of windbreaks to the real field. Finally, the wind fence installation has saved the environment neighboring the POSCO Kwang-Yang Works from the wind erosion of coal dust.
Acknowledgements This work was supported by POSCO, and NRL (National Research Laboratory) Program of the Ministry of Science and Technology, Korea.
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