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The shelter effect of porous wind fences on coal piles in POSCO open storage yard
Journal of Wind Engineering and Industrial Aerodynamics 84 (2000) 101}118
The shelter e!ect of porous wind fences on coal piles in POSCO open storage yard Sang-Joon Lee*, Cheol-Woo Park Department of Mechanical Engineering, Advanced Fluids Engineering Research Center, Pohang University of Science and Technology, Pohang 790-784, South Korea Received 12 October 1998; received in revised form 13 April 1999; accepted 9 June 1999
Abstract The shelter e!ect of porous wind fences on surface pressure and wall shear stress acting on the consecutive coal piles of 1/800 scale POSCO open storage yard model was investigated experimentally. The storage yard model was fully embedded in a neutral atmospheric surface boundary layer over open terrain. Reynolds number based on the coal pile height was Re"18,000. The mean and #uctuating surface-pressure distributions on the coal piles, which were directly related to the dust emission from the surface, were measured for several oncoming wind directions. The daily and monthly wind data over the storage yard during two years were statistically analyzed. As a result, a fence of porosity e"40% was found to be e!ective for decreasing the mean pressure and pressure #uctuations on the coal piles. In addition, the wall shear stress on the windward surface decreased more than half of that for the no fence case. In order to get a good shelter e!ect for a large-scale open storage yard, the porous fence should be installed along all peripheral sides of the storage yard, and an additional middle fence is needed for every "ve consecutive piles to prevent the decreasing shelter e!ect from descending shear #ow separated from the wind fence. ( 2000 Elsevier Science Ltd. All rights reserved. Keywords: Shelter e!ect; Porous wind fences; POSCO; Coal piles; Open storage yard
1. Introduction The wind erosion of small particles such as snow and sand in the region of strong wind has been one of the important wind engineering problems over the past several decades. The wind-erosion phenomena occur frequently at open storage yards of * Corresponding author. 0167-6105/00/$ - see front matter ( 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 1 6 7 - 6 1 0 5 ( 9 9 ) 0 0 0 4 6 - X
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a power generation plant or steel company, and it causes environmental problems in addition to the loss of raw material itself. At POSCO (Pohang Steel Co.) located at the south-eastern seaside of Korea Peninsula, coal dusts of open storage yard are blown by strong wind and local gusts, especially in the summer stormy season. However, the wind-erosion mechanism is di$cult to understand since the dispersion phenomena are very complicated and depend on many topographic and meteorological factors. Therefore, only a few empirical correlations are available for predicting dust erosion [1]. Recently the atmospheric wind tunnel simulation of wind-blown dusts is of increasing importance in the wind engineering "eld with enhanced regulation for the protection of the environment. There are several previous studies on the wind erosion of air pollutants over urban areas. Grant et al. [2] measured the pedestrian-level wind speed at several sites across an urban area and compared it with the wind tunnel simulation result using the plastic-pellet erosion technique. Sierputowski et al. [3] studied the mean and #uctuating velocity of ground-level wind over a hill-valley con"guration to determine the di!usion characteristics. Chatzipanagiotidis and Olivari [4] investigated the pollutant dispersion over a hill and obtained statistical properties of pollutant concentration using digital image analysis. They found that the boundary layer thickness of oncoming #ow in#uenced the wind-erosion of pollutants signi"cantly. Borges et al. [5] investigated the shelter e!ect of windbreaks by measuring the mean velocity and shear stress of near wake #ow behind the windbreaks. Lee et al. [6] measured velocity "elds of the turbulent wake behind two-dimensional porous wind fences using a two-frame PTV technique. They found that as the fence porosity increases, the turbulence intensity and Reynolds shear stress decrease, and the fence e!ect on mean velocity reduction was decreased. The force which ejects dust particles from the coal pile is closely related to the instantaneous pressure #uctuations on the coal surface. Generally, the pressure #uctuations are high in the region of strong vortex #ow or near the separation point. Ogawa et al. [7] carried out a power spectral analysis for the pressure signals measured from a semi-circular prism surface. They found that the turbulence intensity of oncoming #ow largely a!ected the surface pressure #uctuations. However, there were limited studies on the wind-erosion problem which can be applied to a large open storage yard. The interaction between the porous fence and the coal pile surface pressure was not known in detail. Lee et al. [8,9] found that a fence of porosity e"40% was most e!ective for reducing the pressure #uctuations and shear stress on the prism surface from two-dimensional wind tunnel tests. The time-averaged velocity "elds around a porous fence of e"38.5% and a triangular prism were measured using PTV technique to understand the shelter e!ect on #ow characteristics around the prism model [10]. The shelter e!ect of porous wind fences was found to extend up to the "fth}sixth prism of consecutive triangular prism models. The main objective of this study is to investigate the shelter e!ect of the porous wind fence of porosity e"40% on surface-pressure characteristics on coal piles located in the POSCO open storage yard.
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2. Experimental apparatus and methods The experiments were carried out in a closed-return-type subsonic wind tunnel having 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 make a thermally neutral atmospheric boundary layer. The schematic diagram of the wind tunnel test section and measurement system is shown in Fig. 1. The right-hand side of the "rst-in-line coal pile model is selected as the positive X-coordinate since the windward and leeward surfaces are changed according to the wind direction. The velocity pro"les of the simulated atmospheric boundary layer were measured by a Pitot tube and a hot-wire anemometer (TSI IFA-100). At each measurement point, 16,000 velocity data were acquired at a 2 KHz sampling rate after low-pass "ltering at 800 Hz. The mean velocity and turbulence intensity pro"les measured at the coal yard location, 5 meters downstream from the leading edge of the test section, are shown in Fig. 2. The turbulence intensity at reference height (> "0.15 m) is about 8%. The mean 3%& streamwise velocity has the following power law pro"le:
A B
;(y) y n . " ; y 3%& 3%&
(1)
The velocity pro"le is well "tted with n"0.14, which corresponds to the velocity pro"le over open terrain.
Fig. 1. Wind tunnel test section and measurement system.
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Fig. 2. Mean streamwise velocity and turbulence intensity pro"les measured at X"0. (a) mean velocity; (b) turbulent intensity.
The con"guration of the open storage yard and the wind directions tested in this study are shown in Fig. 3. The pressure measuring locations in the 1 scaled-down 800 yard are speci"ed with the row and the column numbers. Each coal pile model has an inclination angle of 403 and 13 pressure taps were installed along the middle span of the coal model with 5 mm intervals. Fig. 4 shows the POSCO open storage yard model installed in the wind tunnel test section. The pressure taps were connected to the Scanivalve system (48J9-1) with vinyl tubes of 0.8 mm inner diameter. Each pressure tap was selected by a solenoid controller (CTLR/S2) and the analog voltage output from the pressure transducer (PDCR22-1psid) was digitized by a high-precision A/D converter (DT2838). The frequency response of the pressure transducer used was about 330 ls. At each channel, 16,384 pressure data were acquired at sampling rate of 500 samples/s and statistically averaged. In this study, a fence of porosity e"40% was selected to shelter the wind-blown dusts from the coal piles. This fence proved to be one of the most e!ective fences to abate the wind-erosion dusts from our previous experiment with simpli"ed twodimensional triangular prism models [8]. In addition, the e!ects of back fence and the side (L: left or R: right) fences of the same porosity e"40% were also investigated.
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Fig. 3. Yards model con"guration and coordinate system.
The porous fence was made of aluminum plate 0.5 mm thick, and the fence height H was set to have the same height as the coal pile crest, h"18.8 mm. The distance between the fence and leading edge of the coal pile model was 2.1H. The pressure di!erence between the surface pressure p and the reference static pressure p was non-dimensionalized by the dynamic pressure with velocity 0 ; at the reference height and air density o . The pressure coe$cient C is 0.15 ! p expressed as: p!p 0 . C" p (1/2)o ;2 ! 0.15
(2)
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Fig. 4. 1 scale POSCO open storage yard model installed in POSTECH wind tunnel. 800
The reference pressure p was measured at >"1.2 m where the in#uence of the 0 experimental model was small and assumed to be inviscid. During experiments, the oncoming wind velocity was "xed at ; "13.5 m/s and its corresponding 0.15 Reynolds number based on the coal pile model height h was Re"18,000. The shear stress was calculated using the pressure di!erence between the Preston tube and the reference static tube following the Patel method [11]. The Preston tube can be applied to the inner layer of the log law to allow the pressures to relate to shear. Therefore, the technique cannot be used for three-dimensional reverse #ows. The time-averaged velocity "eld results [10] measured around a triangular prism located behind a porous fence revealed that the #ow near the windward surface was nearly homogeneous and there was no reverse #ow. The wall shear stress q was 8 measured along the centerline of the coal pile model with a Preston tube having small inner diameter. The wall shear stress was non-dimensionalized with the velocity ; at the reference height and the air density o to give the friction coe$cient C 0.15 ! & as follows: q 8 C" . (3) & (1/2)o ;2 ! 0.15 In general, the friction coe$cient C decreases in proportion to the Reynolds & number in a form of Re~1@2 [12].
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3. Results and discussions 3.1. Wind rose The wind characteristics such as wind speed and dominant wind direction were analyzed based on the daily averaged wind data measured at POSCO open storage yard during two years 1993 and 1994. The maximum wind speed of 13.3 m/s was obtained in March 1993, and the yearly mean wind speed was about 2.5 m/s. The instantaneous wind speed, however, in storm or gust was greater than the daily average wind speed. From the wind data, the free stream velocity was "xed at 18 m/s to cover the extreme conditions. Fig. 5 shows the wind rose obtained from the 1993 and 1994 wind data at POSCO open storage yard. The periphery of the wind rose was subdivided into sixteen segments for indicating the wind directions. The annulus in the radial direction indicates the percentage of the wind rose probability. The two dominant oncoming winds are the western wind and the north-eastern wind. According to the seasonly averaged wind data, the western wind rose occurred frequently in spring and fall, and the north-eastern wind rose in summer. We chose four wind directions (northern, western, north-eastern and north-western wind) which are most likely to occur over the open storage yard. 3.2. Surface pressure The shelter e!ect of the porous fence on the coal pile models was tested for 4 blown-wind directions. For Yard 1 (COREX), the surface pressures were measured
Fig. 5. Annually averaged wind rose obtained from 1993 and 1994 wind data.
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at a "xed coal pile located at the left front corner of the yard to see the shelter e!ect of the side and back fences. In the case of Yard 2, the surface pressure distributions over all the coal piles marked by row and column numbers were measured. Figs. 6 and 7 show the e!ect of a partly installed wind fence of porosity e"40% on the surface pressure distributions on the coal pile model located at Pohang Yard 1 for the northern and the north-western winds. The shelter e!ect of the front fence was dominant at the windward side of the coal pile model. This is attributed to the fact that the windward surface faced directly toward the northern and north-western wind. But, the e!ect of the side and back fence on the surface pressures acting on the coal pile was not so signi"cant compared with that of the front fence. From this, we can see that the front fence reduces the surface pressure predominantly, especially the pressure #uctuations on the windward surface and crest region. This is attributed to the large decrease of the oncoming wind speed after passing through the fence. Without the fence, the pressure #uctuations are very high at the crest of the coal pile due to the impingement of the turbulent shear layer separated from the top of the fence. The mean and rms pressure distributions for the western wind at Yard 1 are shown in Fig. 8. The mean pressures at the windward side are decreased largely compared
Fig. 6. The e!ect of porous wind fences on the mean and rms pressure distributions on coal pile of Yard 1 for northern wind.
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Fig. 7. The e!ect of porous wind fences on the mean and rms pressure distributions on coal pile of Yard 1 for north-western wind.
with the no fence case. But, on the leeward surface, the mean pressures for the case of a front and a back fence combination are higher than those for the no fence case. This results from the fact that the western wind is nearly aligned with the longitudinal axis of the coal pile, i.e., blows along the leeward surface. The pressure #uctuations on the lower part of the windward side are decreased largely because the coal pile tested for Yard 1 is located close to the front and left side fences, decreasing the oncoming wind speed. But, the existence of the side and back fences without the front fence does not improve the surface pressure distributions from those of the no fence case; in fact the pressure distributions are even worse at the crest region of the coal pile. For the western wind, the combination of front and back fence shows the best shelter e!ect on the coal pile surface. Fig. 9 shows the mean and rms pressure variations on the coal pile of Yard 1 for the north-eastern wind. The mean pressure distributions show more or less similar patterns on both the windward and leeward sides because the north-eastern wind is parallel to the longitudinal direction of the coal pile model. For the north-eastern wind, the oncoming #ow separated from the right fence is reattached and accelerated
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Fig. 8. Mean and rms pressure distributions on coal pile models in POSCO Yard 1 for western wind.
again along the coal piles because Yard 1 is very long in the blown-wind direction. When the left side fence is installed, the magnitude of mean pressures on the coal pile located far away from the right-hand side fence increased, and the pressure #uctuations are also decreased. This indicates that the separated shear #ow along the coal piles is blocked by the back (left) fence and forms a recirculating #ow between the fence and the last coal pile. However, the shelter e!ect of the right wind fence for the north-eastern wind on the last coal pile is di$cult to identify and the pressure #uctuations are even worse compared to the no-fence case. Therefore, in order to abate the wind-blown dusts for the main wind directions, it is necessary to install the porous fence along all peripheral sides of Yard 1 and an additional fence is also recommended at the middle of the yard. The open storage Yard 2 is very large in scale and its coal pile arrangement is di!erent from that of Yard 1. The coordinate system used for Yard 2 is slightly di!erent from Yard 1. The >-axis for coal piles is the same, but, the X- and Z-axis are rotated 903 in the counter-clockwise direction. Fig. 10 shows the mean and rms pressure distributions measured along the Row 1 of POSCO Yard 2 for the north-eastern wind approaching from the East Sea. The
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Fig. 9. Mean and rms pressure distributions on coal pile models in POSCO Yard 1 for north-eastern wind.
windward surface is the right-side of the coal piles for the north-eastern wind, i.e. the positive X-axis. For the coal piles in the "rst column (Column 1), the wind fence of porosity 40% decreased the mean pressure to have a nearly uniform distribution on the coal pile surface. Other coal piles located consecutively behind the "rst column show negative pressure distributions since they are embedded in a shear layer separated from the pile crest and wide fence. Further downstream, the rms pressure #uctuations also have high values due to the re-circulation bubbles between the coal piles. The mean and rms pressures on the windward surface of the "fth coal pile (Column 5) were increased signi"cantly, even higher than that of the no fence case at the lower side. This is attributed to the fact that the shear #ow separated from the fence crest interacts with the re-circulation #ow ascending along the leeward surface of each pile and "nally descends in the vicinity of the "fth coal pile due to momentum loss. The mean and rms pressure distributions on the coal piles at Column 1 of Yard 2 for the north-eastern wind are shown in Fig. 11. The pressure distributions on both the windward and leeward surfaces are nearly uniform since all coal piles in the "rst column are fully embedded in the turbulent shear #ow separated from the porous
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Fig. 10. Mean and rms pressure distributions on coal piles at Row 1 of POSCO Yard 2 for north-eastern wind.
wind fence. The mean pressure decreases downward along the column because some of the blown-wind accelerates along the coal pile surface and separates near the coal pile crest. The rms pressure #uctuations have nearly similar distributions irrespective of the row number. The pressure distributions along the row and column array for the western wind are shown in Figs. 12 and 13. For the western wind, a di!erent kind of boundary layer is formed because there are many factory plants and chimneys located in front of the fence as shown in Fig. 4. The fence and coal piles are located in the wake of those roughness elements. Therefore, the surface pressure distributions are much di!erent from those for the north-eastern wind due to the highly turbulent wake separated from the blu! bodies and wind fence. Since the tenth column (Column 10) is faced with the wind at "rst, the mean and rms pressures on the coal pile are largely decreased by installing the fence. The mean pressure and rms pressure #uctuations are largely increased on the windward surface further downstream at Columns 5 and 1 compared with the no fence case. Especially, the rms pressure at the crest of Column 5 pile is increased signi"cantly. This indicates that the separated shear #ow descends here and the shelter
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Fig. 11. Mean and rms pressure distributions on coal piles at Column 1 of POSCO Yard 2 for north-eastern wind.
e!ect of the front fence nearly disappears. Therefore, an additional wind fence is needed at this location to compensate for the e!ect of descending shear #ow and to prevent wind-blown dust emissions from the consecutive piles. Fig. 13 shows the mean and rms pressure distributions on the coal piles at Column 10 for the western wind. For the no wind fence case, the mean pressure has very high values at the lower part of the windward surface and rapidly decreases at the upper part and maintains low values nearly uniformly along the leeward surface. The rms pressure results show a wavy distribution on the coal pile surface, and it has a peak value at the pile crest. When a fence of porosity e"40% is installed in front of Column 10, the mean and rms pressure #uctuations on the coal piles are largely decreased, especially on the windward surface. They have nearly similar shape irrespective of row number though the roughness conditions in front of each coal pile are di!erent. 3.3. Power spectra The PSD (power spectral density) of pressure signals measured from the coal pile surface was analyzed. The sequential time series of pressure signal which was low-pass
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Fig. 12. Mean amd rms pressure distributions on coal piles at Row 1 of POSCO Yard 2 for western wind.
"ltered at 50 Hz was acquired at 100 samples/s to analyze the frequency characteristics of the pressure signal. The PSD results of the coal pile models (Column 1, Row 2) in Yard 2 for the north-eastern wind are shown in Fig. 14. This "gure shows the wind fence e!ect on the PSD reduction at lower frequency clearly. The PSD has high values at frequency less than 0.5 Hz, but beyond this the power density of the pressure signal decays rapidly. Ogawa et al. [13] observed several frequency peaks in their two-dimensional fence wake. They explained that the peaks were attributed to the high turbulence intensity of the oncoming #ow near the surface and the small vorticity-induced eddies. Richardson [14] carried out a PSD analysis on pressure signals measured from the wake behind a permeable windbreak (e"50%) and found that the fence attenuated the pressure power largely at lower frequency (0.14 Hz), compared to the drag force signal. This is attributed to the characteristic length scale of the #ow which exceeds the windbreak size at low frequency. Guerts [15] revealed that the pressure spectra attenuation was faster than the wind velocity spectra. The fence e!ect on the PSD attenuation on the windward surface is larger than that on the leeward surface at low frequency band. But, on the leeward surface, the wind
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Fig. 13. Mean and rms pressure distributions on coal piles at Column 10 of POSCO Yard 2 for western wind.
fence increases more or less the PSD values at a higher-frequency band. This is basically attributed to the re-circulation #ow formed between the "rst coal pile and the adjacent coal pile. 3.4. Wall shear stress The mean friction coe$cients on a coal pile surface of Yard 2 for the north-eastern wind are compared for the cases with and without the fence of porosity 40% and the result is shown in Fig. 15. The wall shear stress was measured using a Preston tube having a small inner diameter. But, since the Preston tube method cannot be used for the region of reverse #ow, we measured the wall shear stress only on the windward surface. Even though the windward surface has nearly uniform pressure gradient, the skin friction coe$cient near the crest region maybe contain some errors, since the shear #ow is separated at that region. Bradley et al. [16] found reduction of shear stress near the wall for the wake behind a e"50% permeable windbreak.
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Fig. 14. Power spectral density distributions on the coal pile model at POSCO Yard 2 (Column 1, Row 2) for north-eastern wind. (a) windward side (X"20 mm); (b) leeward side (X"!20 mm).
Fig. 15. Comparison of skin friction coe$cients on a coal pile in POSCO Yard 2 for north-eastern wind.
For the no fence case, the mean friction coe$cient increases on the windward surface because the oncoming #ow is accelerated along the windward surface. By installing a porous wind fence of e"40% in front of the coal pile, the friction coe$cients are decreased up to one-half of those for the no fence case. This is attributed to the decrease of the bleed #ow penetrating through the fence holes. From this, we can see that the wind fence can reduce the wind-blown dust erosion e!ectively, because the wall shear stress is one of the main causes of wind erosion.
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4. Conclusion The shelter e!ect of a wind fence with porosity e"40% on the surface pressure and wall shear stress on consecutive coal piles in the POSCO open storage yard was investigated experimentally. The yard model was fully embedded in a neutral atmospheric surface boundary layer over open-terrain. The mean and rms #uctuations of surface pressure on the coal pile models were measured by varying the wind directions, which were selected based on the wind rose analysis over the open storage yards during the past two years. The wall shear stress on the windward surface was decreased to half of that for the no fence case. The wind fence of porosity e"40% was found to be e!ective for decreasing the surface pressure and wall shear stress on the coal piles of the POSCO open storage yard, the same as for the previous twodimensional experiments. In order to get good shelter e!ect for all possible wind directions, the fence should be installed along all peripheral sides of the storage yard, and also an additional middle fence should be installed for large-scale open storage yard having more than "ve consecutive coal piles to prevent the e!ect of descending shear layer separated from the wind fence.
Acknowledgements This work was "nancially supported in part by POSCO and AFERC (Advanced Fluids Engineering Research Center).
References [1] D.A. Gillette, On the production of soil wind erosion aerosols having the potential for long range transport, J. Rech. Atmos. 8 (3/4) (1974) 735}774. [2] R.H. Grant, Full-scale comparison of a wind-tunnel simulation of windy locations in an urban area, J. Wind Eng. Ind. Aerodyn. 31 (1988) 335}341. [3] P. Sierputowski, J. Ostrowski, A. Cenedese, Experimental study of wind #ow over the model of a valley, J. Wind Eng. Ind. Aerodyn. 57 (1995) 127}136. [4] A. Chatzipanagiotidis, D. Olivari, Pollutant dispersal downstream of a hill in di!erent wind conditions, J. Wind Eng. Ind. Aerodyn. 64 (1996) 233}248. [5] A.R. Borges, D.X. Viegas, Shelter e!ects on a row of coal piles to prevent wind erosion, J. Wind Eng. Ind. Aerodyn. 29 (1988) 145}154. [6] S.J. Lee, H.B. Kim, Laboratory measurements of velocity and turbulence "eld behind porous fences, J. Wind Eng. Ind. Aerodyn. 80 (1999) 311}326. [7] T. Ogawa, M. Nakayama, S. Murayama, Y. Sasaki, Characteristics of wind pressure on basic structures with curved surfaces and their response in turbulent #ow, J. Fluid Mech. 38 (1991) 427}438. [8] S.J. Lee, C.W. Park, Surface-pressure variations on a triangular prism by porous fences in a simulated atmospheric boundary layer, J. Wind Eng. Ind. Aerodyn. 73 (1998) 45}58. [9] S.J. Lee, C.W. Park, Surface-pressure characteristics on a triangular prism located behind a porous fence, J. Wind Eng. Ind. Aerodyn. 80 (1999) 69}83. [10] S.J. Lee, H.B. Kim, Velocity "eld measurements of #ow around a triangular prism behind a porous fence, J. Wind Eng. Ind. Aerodyn. 77}78 (1998) 521}530.
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[11] V.C. Patel, Calibration of the Preston tube and limitation on it's use in pressure gradients, J. Fluid Mech. 23 (1965) 185}208. [12] S. Jovic, D. Driver, Reynolds number e!ect on the skin friction in separated #ows behind a backward-facing step, Exp. Fluids 18 (1995) 464}467. [13] Y. Ogawa, P.G. Diosey, Surface roughness and thermal strati"cation e!ects on the #ow behind a two-dimensional fence } I. Field study, Atmos. Environ. 14 (1980) 1301}1308. [14] G.M. Richardson, A permeable windbreak: it's e!ect on the structure of the natural wind, J. Wind Eng. Ind. Aerodyn. 32 (1989) 101}110. [15] C.P.W. Geurts, Full-scale and wind-tunnel measurements of the wind and wind-induced pressures over suburban terrain, J. Wind Eng. Ind. Aerodyn. 64 (1996) 89}100. [16] E.F. Bradley, P.J. Mulhearn, Development of velocity and shear stress distributions in the wake of porous shelter fence, J. Wind Eng. Ind. Aerodyn. 15 (1983) 145}156.
The shelter e!ect of porous wind fences on coal piles in POSCO open storage yard Sang-Joon Lee*, Cheol-Woo Park Department of Mechanical Engineering, Advanced Fluids Engineering Research Center, Pohang University of Science and Technology, Pohang 790-784, South Korea Received 12 October 1998; received in revised form 13 April 1999; accepted 9 June 1999
Abstract The shelter e!ect of porous wind fences on surface pressure and wall shear stress acting on the consecutive coal piles of 1/800 scale POSCO open storage yard model was investigated experimentally. The storage yard model was fully embedded in a neutral atmospheric surface boundary layer over open terrain. Reynolds number based on the coal pile height was Re"18,000. The mean and #uctuating surface-pressure distributions on the coal piles, which were directly related to the dust emission from the surface, were measured for several oncoming wind directions. The daily and monthly wind data over the storage yard during two years were statistically analyzed. As a result, a fence of porosity e"40% was found to be e!ective for decreasing the mean pressure and pressure #uctuations on the coal piles. In addition, the wall shear stress on the windward surface decreased more than half of that for the no fence case. In order to get a good shelter e!ect for a large-scale open storage yard, the porous fence should be installed along all peripheral sides of the storage yard, and an additional middle fence is needed for every "ve consecutive piles to prevent the decreasing shelter e!ect from descending shear #ow separated from the wind fence. ( 2000 Elsevier Science Ltd. All rights reserved. Keywords: Shelter e!ect; Porous wind fences; POSCO; Coal piles; Open storage yard
1. Introduction The wind erosion of small particles such as snow and sand in the region of strong wind has been one of the important wind engineering problems over the past several decades. The wind-erosion phenomena occur frequently at open storage yards of * Corresponding author. 0167-6105/00/$ - see front matter ( 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 1 6 7 - 6 1 0 5 ( 9 9 ) 0 0 0 4 6 - X
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a power generation plant or steel company, and it causes environmental problems in addition to the loss of raw material itself. At POSCO (Pohang Steel Co.) located at the south-eastern seaside of Korea Peninsula, coal dusts of open storage yard are blown by strong wind and local gusts, especially in the summer stormy season. However, the wind-erosion mechanism is di$cult to understand since the dispersion phenomena are very complicated and depend on many topographic and meteorological factors. Therefore, only a few empirical correlations are available for predicting dust erosion [1]. Recently the atmospheric wind tunnel simulation of wind-blown dusts is of increasing importance in the wind engineering "eld with enhanced regulation for the protection of the environment. There are several previous studies on the wind erosion of air pollutants over urban areas. Grant et al. [2] measured the pedestrian-level wind speed at several sites across an urban area and compared it with the wind tunnel simulation result using the plastic-pellet erosion technique. Sierputowski et al. [3] studied the mean and #uctuating velocity of ground-level wind over a hill-valley con"guration to determine the di!usion characteristics. Chatzipanagiotidis and Olivari [4] investigated the pollutant dispersion over a hill and obtained statistical properties of pollutant concentration using digital image analysis. They found that the boundary layer thickness of oncoming #ow in#uenced the wind-erosion of pollutants signi"cantly. Borges et al. [5] investigated the shelter e!ect of windbreaks by measuring the mean velocity and shear stress of near wake #ow behind the windbreaks. Lee et al. [6] measured velocity "elds of the turbulent wake behind two-dimensional porous wind fences using a two-frame PTV technique. They found that as the fence porosity increases, the turbulence intensity and Reynolds shear stress decrease, and the fence e!ect on mean velocity reduction was decreased. The force which ejects dust particles from the coal pile is closely related to the instantaneous pressure #uctuations on the coal surface. Generally, the pressure #uctuations are high in the region of strong vortex #ow or near the separation point. Ogawa et al. [7] carried out a power spectral analysis for the pressure signals measured from a semi-circular prism surface. They found that the turbulence intensity of oncoming #ow largely a!ected the surface pressure #uctuations. However, there were limited studies on the wind-erosion problem which can be applied to a large open storage yard. The interaction between the porous fence and the coal pile surface pressure was not known in detail. Lee et al. [8,9] found that a fence of porosity e"40% was most e!ective for reducing the pressure #uctuations and shear stress on the prism surface from two-dimensional wind tunnel tests. The time-averaged velocity "elds around a porous fence of e"38.5% and a triangular prism were measured using PTV technique to understand the shelter e!ect on #ow characteristics around the prism model [10]. The shelter e!ect of porous wind fences was found to extend up to the "fth}sixth prism of consecutive triangular prism models. The main objective of this study is to investigate the shelter e!ect of the porous wind fence of porosity e"40% on surface-pressure characteristics on coal piles located in the POSCO open storage yard.
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2. Experimental apparatus and methods The experiments were carried out in a closed-return-type subsonic wind tunnel having 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 make a thermally neutral atmospheric boundary layer. The schematic diagram of the wind tunnel test section and measurement system is shown in Fig. 1. The right-hand side of the "rst-in-line coal pile model is selected as the positive X-coordinate since the windward and leeward surfaces are changed according to the wind direction. The velocity pro"les of the simulated atmospheric boundary layer were measured by a Pitot tube and a hot-wire anemometer (TSI IFA-100). At each measurement point, 16,000 velocity data were acquired at a 2 KHz sampling rate after low-pass "ltering at 800 Hz. The mean velocity and turbulence intensity pro"les measured at the coal yard location, 5 meters downstream from the leading edge of the test section, are shown in Fig. 2. The turbulence intensity at reference height (> "0.15 m) is about 8%. The mean 3%& streamwise velocity has the following power law pro"le:
A B
;(y) y n . " ; y 3%& 3%&
(1)
The velocity pro"le is well "tted with n"0.14, which corresponds to the velocity pro"le over open terrain.
Fig. 1. Wind tunnel test section and measurement system.
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Fig. 2. Mean streamwise velocity and turbulence intensity pro"les measured at X"0. (a) mean velocity; (b) turbulent intensity.
The con"guration of the open storage yard and the wind directions tested in this study are shown in Fig. 3. The pressure measuring locations in the 1 scaled-down 800 yard are speci"ed with the row and the column numbers. Each coal pile model has an inclination angle of 403 and 13 pressure taps were installed along the middle span of the coal model with 5 mm intervals. Fig. 4 shows the POSCO open storage yard model installed in the wind tunnel test section. The pressure taps were connected to the Scanivalve system (48J9-1) with vinyl tubes of 0.8 mm inner diameter. Each pressure tap was selected by a solenoid controller (CTLR/S2) and the analog voltage output from the pressure transducer (PDCR22-1psid) was digitized by a high-precision A/D converter (DT2838). The frequency response of the pressure transducer used was about 330 ls. At each channel, 16,384 pressure data were acquired at sampling rate of 500 samples/s and statistically averaged. In this study, a fence of porosity e"40% was selected to shelter the wind-blown dusts from the coal piles. This fence proved to be one of the most e!ective fences to abate the wind-erosion dusts from our previous experiment with simpli"ed twodimensional triangular prism models [8]. In addition, the e!ects of back fence and the side (L: left or R: right) fences of the same porosity e"40% were also investigated.
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Fig. 3. Yards model con"guration and coordinate system.
The porous fence was made of aluminum plate 0.5 mm thick, and the fence height H was set to have the same height as the coal pile crest, h"18.8 mm. The distance between the fence and leading edge of the coal pile model was 2.1H. The pressure di!erence between the surface pressure p and the reference static pressure p was non-dimensionalized by the dynamic pressure with velocity 0 ; at the reference height and air density o . The pressure coe$cient C is 0.15 ! p expressed as: p!p 0 . C" p (1/2)o ;2 ! 0.15
(2)
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Fig. 4. 1 scale POSCO open storage yard model installed in POSTECH wind tunnel. 800
The reference pressure p was measured at >"1.2 m where the in#uence of the 0 experimental model was small and assumed to be inviscid. During experiments, the oncoming wind velocity was "xed at ; "13.5 m/s and its corresponding 0.15 Reynolds number based on the coal pile model height h was Re"18,000. The shear stress was calculated using the pressure di!erence between the Preston tube and the reference static tube following the Patel method [11]. The Preston tube can be applied to the inner layer of the log law to allow the pressures to relate to shear. Therefore, the technique cannot be used for three-dimensional reverse #ows. The time-averaged velocity "eld results [10] measured around a triangular prism located behind a porous fence revealed that the #ow near the windward surface was nearly homogeneous and there was no reverse #ow. The wall shear stress q was 8 measured along the centerline of the coal pile model with a Preston tube having small inner diameter. The wall shear stress was non-dimensionalized with the velocity ; at the reference height and the air density o to give the friction coe$cient C 0.15 ! & as follows: q 8 C" . (3) & (1/2)o ;2 ! 0.15 In general, the friction coe$cient C decreases in proportion to the Reynolds & number in a form of Re~1@2 [12].
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3. Results and discussions 3.1. Wind rose The wind characteristics such as wind speed and dominant wind direction were analyzed based on the daily averaged wind data measured at POSCO open storage yard during two years 1993 and 1994. The maximum wind speed of 13.3 m/s was obtained in March 1993, and the yearly mean wind speed was about 2.5 m/s. The instantaneous wind speed, however, in storm or gust was greater than the daily average wind speed. From the wind data, the free stream velocity was "xed at 18 m/s to cover the extreme conditions. Fig. 5 shows the wind rose obtained from the 1993 and 1994 wind data at POSCO open storage yard. The periphery of the wind rose was subdivided into sixteen segments for indicating the wind directions. The annulus in the radial direction indicates the percentage of the wind rose probability. The two dominant oncoming winds are the western wind and the north-eastern wind. According to the seasonly averaged wind data, the western wind rose occurred frequently in spring and fall, and the north-eastern wind rose in summer. We chose four wind directions (northern, western, north-eastern and north-western wind) which are most likely to occur over the open storage yard. 3.2. Surface pressure The shelter e!ect of the porous fence on the coal pile models was tested for 4 blown-wind directions. For Yard 1 (COREX), the surface pressures were measured
Fig. 5. Annually averaged wind rose obtained from 1993 and 1994 wind data.
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at a "xed coal pile located at the left front corner of the yard to see the shelter e!ect of the side and back fences. In the case of Yard 2, the surface pressure distributions over all the coal piles marked by row and column numbers were measured. Figs. 6 and 7 show the e!ect of a partly installed wind fence of porosity e"40% on the surface pressure distributions on the coal pile model located at Pohang Yard 1 for the northern and the north-western winds. The shelter e!ect of the front fence was dominant at the windward side of the coal pile model. This is attributed to the fact that the windward surface faced directly toward the northern and north-western wind. But, the e!ect of the side and back fence on the surface pressures acting on the coal pile was not so signi"cant compared with that of the front fence. From this, we can see that the front fence reduces the surface pressure predominantly, especially the pressure #uctuations on the windward surface and crest region. This is attributed to the large decrease of the oncoming wind speed after passing through the fence. Without the fence, the pressure #uctuations are very high at the crest of the coal pile due to the impingement of the turbulent shear layer separated from the top of the fence. The mean and rms pressure distributions for the western wind at Yard 1 are shown in Fig. 8. The mean pressures at the windward side are decreased largely compared
Fig. 6. The e!ect of porous wind fences on the mean and rms pressure distributions on coal pile of Yard 1 for northern wind.
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Fig. 7. The e!ect of porous wind fences on the mean and rms pressure distributions on coal pile of Yard 1 for north-western wind.
with the no fence case. But, on the leeward surface, the mean pressures for the case of a front and a back fence combination are higher than those for the no fence case. This results from the fact that the western wind is nearly aligned with the longitudinal axis of the coal pile, i.e., blows along the leeward surface. The pressure #uctuations on the lower part of the windward side are decreased largely because the coal pile tested for Yard 1 is located close to the front and left side fences, decreasing the oncoming wind speed. But, the existence of the side and back fences without the front fence does not improve the surface pressure distributions from those of the no fence case; in fact the pressure distributions are even worse at the crest region of the coal pile. For the western wind, the combination of front and back fence shows the best shelter e!ect on the coal pile surface. Fig. 9 shows the mean and rms pressure variations on the coal pile of Yard 1 for the north-eastern wind. The mean pressure distributions show more or less similar patterns on both the windward and leeward sides because the north-eastern wind is parallel to the longitudinal direction of the coal pile model. For the north-eastern wind, the oncoming #ow separated from the right fence is reattached and accelerated
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Fig. 8. Mean and rms pressure distributions on coal pile models in POSCO Yard 1 for western wind.
again along the coal piles because Yard 1 is very long in the blown-wind direction. When the left side fence is installed, the magnitude of mean pressures on the coal pile located far away from the right-hand side fence increased, and the pressure #uctuations are also decreased. This indicates that the separated shear #ow along the coal piles is blocked by the back (left) fence and forms a recirculating #ow between the fence and the last coal pile. However, the shelter e!ect of the right wind fence for the north-eastern wind on the last coal pile is di$cult to identify and the pressure #uctuations are even worse compared to the no-fence case. Therefore, in order to abate the wind-blown dusts for the main wind directions, it is necessary to install the porous fence along all peripheral sides of Yard 1 and an additional fence is also recommended at the middle of the yard. The open storage Yard 2 is very large in scale and its coal pile arrangement is di!erent from that of Yard 1. The coordinate system used for Yard 2 is slightly di!erent from Yard 1. The >-axis for coal piles is the same, but, the X- and Z-axis are rotated 903 in the counter-clockwise direction. Fig. 10 shows the mean and rms pressure distributions measured along the Row 1 of POSCO Yard 2 for the north-eastern wind approaching from the East Sea. The
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Fig. 9. Mean and rms pressure distributions on coal pile models in POSCO Yard 1 for north-eastern wind.
windward surface is the right-side of the coal piles for the north-eastern wind, i.e. the positive X-axis. For the coal piles in the "rst column (Column 1), the wind fence of porosity 40% decreased the mean pressure to have a nearly uniform distribution on the coal pile surface. Other coal piles located consecutively behind the "rst column show negative pressure distributions since they are embedded in a shear layer separated from the pile crest and wide fence. Further downstream, the rms pressure #uctuations also have high values due to the re-circulation bubbles between the coal piles. The mean and rms pressures on the windward surface of the "fth coal pile (Column 5) were increased signi"cantly, even higher than that of the no fence case at the lower side. This is attributed to the fact that the shear #ow separated from the fence crest interacts with the re-circulation #ow ascending along the leeward surface of each pile and "nally descends in the vicinity of the "fth coal pile due to momentum loss. The mean and rms pressure distributions on the coal piles at Column 1 of Yard 2 for the north-eastern wind are shown in Fig. 11. The pressure distributions on both the windward and leeward surfaces are nearly uniform since all coal piles in the "rst column are fully embedded in the turbulent shear #ow separated from the porous
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Fig. 10. Mean and rms pressure distributions on coal piles at Row 1 of POSCO Yard 2 for north-eastern wind.
wind fence. The mean pressure decreases downward along the column because some of the blown-wind accelerates along the coal pile surface and separates near the coal pile crest. The rms pressure #uctuations have nearly similar distributions irrespective of the row number. The pressure distributions along the row and column array for the western wind are shown in Figs. 12 and 13. For the western wind, a di!erent kind of boundary layer is formed because there are many factory plants and chimneys located in front of the fence as shown in Fig. 4. The fence and coal piles are located in the wake of those roughness elements. Therefore, the surface pressure distributions are much di!erent from those for the north-eastern wind due to the highly turbulent wake separated from the blu! bodies and wind fence. Since the tenth column (Column 10) is faced with the wind at "rst, the mean and rms pressures on the coal pile are largely decreased by installing the fence. The mean pressure and rms pressure #uctuations are largely increased on the windward surface further downstream at Columns 5 and 1 compared with the no fence case. Especially, the rms pressure at the crest of Column 5 pile is increased signi"cantly. This indicates that the separated shear #ow descends here and the shelter
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Fig. 11. Mean and rms pressure distributions on coal piles at Column 1 of POSCO Yard 2 for north-eastern wind.
e!ect of the front fence nearly disappears. Therefore, an additional wind fence is needed at this location to compensate for the e!ect of descending shear #ow and to prevent wind-blown dust emissions from the consecutive piles. Fig. 13 shows the mean and rms pressure distributions on the coal piles at Column 10 for the western wind. For the no wind fence case, the mean pressure has very high values at the lower part of the windward surface and rapidly decreases at the upper part and maintains low values nearly uniformly along the leeward surface. The rms pressure results show a wavy distribution on the coal pile surface, and it has a peak value at the pile crest. When a fence of porosity e"40% is installed in front of Column 10, the mean and rms pressure #uctuations on the coal piles are largely decreased, especially on the windward surface. They have nearly similar shape irrespective of row number though the roughness conditions in front of each coal pile are di!erent. 3.3. Power spectra The PSD (power spectral density) of pressure signals measured from the coal pile surface was analyzed. The sequential time series of pressure signal which was low-pass
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Fig. 12. Mean amd rms pressure distributions on coal piles at Row 1 of POSCO Yard 2 for western wind.
"ltered at 50 Hz was acquired at 100 samples/s to analyze the frequency characteristics of the pressure signal. The PSD results of the coal pile models (Column 1, Row 2) in Yard 2 for the north-eastern wind are shown in Fig. 14. This "gure shows the wind fence e!ect on the PSD reduction at lower frequency clearly. The PSD has high values at frequency less than 0.5 Hz, but beyond this the power density of the pressure signal decays rapidly. Ogawa et al. [13] observed several frequency peaks in their two-dimensional fence wake. They explained that the peaks were attributed to the high turbulence intensity of the oncoming #ow near the surface and the small vorticity-induced eddies. Richardson [14] carried out a PSD analysis on pressure signals measured from the wake behind a permeable windbreak (e"50%) and found that the fence attenuated the pressure power largely at lower frequency (0.14 Hz), compared to the drag force signal. This is attributed to the characteristic length scale of the #ow which exceeds the windbreak size at low frequency. Guerts [15] revealed that the pressure spectra attenuation was faster than the wind velocity spectra. The fence e!ect on the PSD attenuation on the windward surface is larger than that on the leeward surface at low frequency band. But, on the leeward surface, the wind
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Fig. 13. Mean and rms pressure distributions on coal piles at Column 10 of POSCO Yard 2 for western wind.
fence increases more or less the PSD values at a higher-frequency band. This is basically attributed to the re-circulation #ow formed between the "rst coal pile and the adjacent coal pile. 3.4. Wall shear stress The mean friction coe$cients on a coal pile surface of Yard 2 for the north-eastern wind are compared for the cases with and without the fence of porosity 40% and the result is shown in Fig. 15. The wall shear stress was measured using a Preston tube having a small inner diameter. But, since the Preston tube method cannot be used for the region of reverse #ow, we measured the wall shear stress only on the windward surface. Even though the windward surface has nearly uniform pressure gradient, the skin friction coe$cient near the crest region maybe contain some errors, since the shear #ow is separated at that region. Bradley et al. [16] found reduction of shear stress near the wall for the wake behind a e"50% permeable windbreak.
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Fig. 14. Power spectral density distributions on the coal pile model at POSCO Yard 2 (Column 1, Row 2) for north-eastern wind. (a) windward side (X"20 mm); (b) leeward side (X"!20 mm).
Fig. 15. Comparison of skin friction coe$cients on a coal pile in POSCO Yard 2 for north-eastern wind.
For the no fence case, the mean friction coe$cient increases on the windward surface because the oncoming #ow is accelerated along the windward surface. By installing a porous wind fence of e"40% in front of the coal pile, the friction coe$cients are decreased up to one-half of those for the no fence case. This is attributed to the decrease of the bleed #ow penetrating through the fence holes. From this, we can see that the wind fence can reduce the wind-blown dust erosion e!ectively, because the wall shear stress is one of the main causes of wind erosion.
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4. Conclusion The shelter e!ect of a wind fence with porosity e"40% on the surface pressure and wall shear stress on consecutive coal piles in the POSCO open storage yard was investigated experimentally. The yard model was fully embedded in a neutral atmospheric surface boundary layer over open-terrain. The mean and rms #uctuations of surface pressure on the coal pile models were measured by varying the wind directions, which were selected based on the wind rose analysis over the open storage yards during the past two years. The wall shear stress on the windward surface was decreased to half of that for the no fence case. The wind fence of porosity e"40% was found to be e!ective for decreasing the surface pressure and wall shear stress on the coal piles of the POSCO open storage yard, the same as for the previous twodimensional experiments. In order to get good shelter e!ect for all possible wind directions, the fence should be installed along all peripheral sides of the storage yard, and also an additional middle fence should be installed for large-scale open storage yard having more than "ve consecutive coal piles to prevent the e!ect of descending shear layer separated from the wind fence.
Acknowledgements This work was "nancially supported in part by POSCO and AFERC (Advanced Fluids Engineering Research Center).
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[11] V.C. Patel, Calibration of the Preston tube and limitation on it's use in pressure gradients, J. Fluid Mech. 23 (1965) 185}208. [12] S. Jovic, D. Driver, Reynolds number e!ect on the skin friction in separated #ows behind a backward-facing step, Exp. Fluids 18 (1995) 464}467. [13] Y. Ogawa, P.G. Diosey, Surface roughness and thermal strati"cation e!ects on the #ow behind a two-dimensional fence } I. Field study, Atmos. Environ. 14 (1980) 1301}1308. [14] G.M. Richardson, A permeable windbreak: it's e!ect on the structure of the natural wind, J. Wind Eng. Ind. Aerodyn. 32 (1989) 101}110. [15] C.P.W. Geurts, Full-scale and wind-tunnel measurements of the wind and wind-induced pressures over suburban terrain, J. Wind Eng. Ind. Aerodyn. 64 (1996) 89}100. [16] E.F. Bradley, P.J. Mulhearn, Development of velocity and shear stress distributions in the wake of porous shelter fence, J. Wind Eng. Ind. Aerodyn. 15 (1983) 145}156.