shelter effect of a fir tree with different porosities

Journal of Mechanical Science and Technology 28 (2) (2014) 565~572

www.springerlink.com/content/1738-494x DOI 10.1007/s12206-013-1123-6

Shelter effect of a fir tree with different porosities†

Jin-Pyung Lee1, Eui-Jae Lee2 and Sang-Joon Lee2,* 1Equipment Development Team, Samsung Display, Yongin, 446-711, Korea

2Bio-fluid and Bio-mimic Research Center, Department of Mechanical Engineering, Pohang University of Science and Technology, Pohang, 790-784, Korea

(Manuscript Received February 14, 2013; Revised August 11, 2013; Accepted August 13, 2013)

----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Abstract Windbreak has been used for centuries as a natural wind-fence. The functional effects of windbreaks are directly related to the flow

structure. In the present study, the flow around a real fir tree placed in a wind tunnel was quantitatively visualized using PIV (particle image velocimetry) technique. The effects of tree porosity and leaves on the shelter effect were investigated as well. Tree leaves not only reduce the porosity, but also increase the wind-speed reduction and turbulence kinetic energy in the leeward region behind the tree can-opy. The leaves of tree canopy induce updraft toward the top region and downdraft toward the ground in the upstream region of the tree. The momentum difference of these two flows generates peculiar upwelling flow characteristics. The large reduction in mean velocities and turbulence intensities of flow around a tree is attributed to tree leaves providing a good shelter zone in the leeward region.

Keywords: Windbreak; Porosity; Sheltering effect; PIV; Fir tree ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 1. Introduction

Trees have been used as natural fences from sun, wind, sand, and snow for centuries. The primary purpose for establishing any windbreak system is to reduce wind speed. Wind-speed reduction brings about increase of turbulent transport proc-esses, and modifies the microclimate in the shelter zone. Windbreaks work as a flow resistance to the approaching wind, and force the air to reduce wind speed while accelerat-ing it over the top, providing shelter area near the ground up to some distance downstream of the windbreak (Wang et al., 2001). These functional effects are closely related to the flow characteristics of windbreaks.

The flow characteristics of windbreaks depend on the po-rosity, thickness, and shape of shelterbelt, as well as on envi-ronmental conditions such as wind speed and direction. In particular, the porosity of windbreaks is one of the most im-portant parameters with respect to the extent and magnitude of the shelter effect (Hagen et al., 1981; Wang and Takle, 1995; Guan et al., 2003).

Hagen et al. (1981) numerically investigated the effects of the width, height, porosity, and other parameters of two-dimensional (2-D) fences using a k-ε turbulence model. Gross (1987) studied the turbulent air flow around a single tree using

a three-dimensional (3-D) non-hydrostatic numerical model. Wang and Takle (1995) numerically analyzed the effects of porosity on the air flow and pressure distribution behind 2-D fences using a k-ε turbulence model. In these previous nu-merical simulations, however, the researchers simply assumed windbreaks as homogeneous porous fences. It is very difficult to evaluate the porosity of natural windbreaks, such as trees, because of their irregular shape and complicated inner struc-tures. Some problems may be encountered when simulating wind flow around a real tree.

Guan et al. (2003) performed wind tunnel tests on a natural windbreak model consisting of trees and shrubs. They esti-mated the aerodynamic porosity and drag coefficient of the windbreak from the flow speed information measured for surrounding wind. Torita and Satou (2007) measured wind speeds at several points around eight natural shelterbelts hav-ing various widths to investigate the fluid-mechanical features of wide shelterbelts. However, the spatial distributions of mean velocities and turbulence intensities, as well as their variations in the wake region of windbreaks were unaddressed in these studies.

Lee et al. (2012) carried out a wind tunnel test to study the shelter effect of a bank of real fir trees using a PIV system, and the results were compared with those for a single tree. Their experimental results explain the shelter mechanism of windbreak forests.

To understand the effectiveness of natural windbreaks, the porosity effect on the shelter effects of non-uniform real tree

*Corresponding author. Tel.: +82-54-279-2169, Fax.: +82-54-279-3199 E-mail address: [email protected]

† Recommended by Associate Editor Yang Na © KSME & Springer 2014

566 J.-P. Lee et al. / Journal of Mechanical Science and Technology 28 (2) (2014) 565~572

should be investigated systematically. This kind of demand has been mentioned for a long time (Wang et al. 2001; Zhou et al., 2004; Rosenfeld et al., 2010). Unfortunately, no attempt has thus far been made.

In the present study, the wind flow around a natural fir tree was experimentally investigated for two different optical po-rosities. The experimental results were also compared with those obtained for the leafless condition which was achieved by removing all the leaves from the same tree. Actually, the porosity of a real tree largely depends on the presence of leaves. On the basis of these comparisons, the porosity effect of tree leaves on the formation of shelter zones around a real tree was studied. The whole velocity fields of the flow around the real tree placed in a wind tunnel test section were meas-ured quantitatively using a particle image velocimetry (PIV) technique. The shelter effect was estimated by analyzing the information on the measured velocity field.

2. Experimental method

2.1 Experimental setup

The experimental setup and coordinate system used in the present study are shown in Fig. 1. The PIV velocity field mea-surement technique was employed to measure the velocity fields of wind flow around the test tree. The PIV system con-sists of a 200 mJ two-head Nd:YAG pulse laser, four sets of 2K X 2K CCD cameras, a delay generator, mirrors and optical lenses for illuminating a thin laser light sheet, and a personal computer. Olive oil droplets with a mean diameter of 1-3 μm were generated by Laskin nozzle and seeded into the wind tunnel test section as tracer particles. All experiments were performed in a closed-return type wind tunnel with test section dimensions of 0.72 m (W) × 0.65 m (H) × 8 m (L).

To extract velocity vectors by PIV, each interrogation win-dow of 32 × 32 was overlapped 50%. Four hundred instanta-neous velocity field vectors were consecutively obtained for each experimental condition. These velocity field vectors were ensemble-averaged to obtain the spatial distributions of mean velocity vectors and turbulence statistics.

The mean streamwise velocity and turbulence intensity pro-files of the atmospheric boundary layer simulated in the wind

tunnel were measured using a hot-wire anemometer (TSI IFA-100) at the location of the experimental model placed 4.5 m downstream from the leading edge of the test section. The mean streamwise velocity normalized by the reference veloc-ity (Uref) measured at the height of Yref = 19 cm has the flow-ing power-law profile :

( ) ( ) .n

ref ref

U y yU y

= (1)

The velocity profile is well fitted with a power-law expo-

nent (n) of 0.16. The turbulence intensity is about 20% near the ground surface (Fig. 2). In this study, the reference veloc-ity was maintained at 5m/s. The corresponding Reynolds number, based on the model height (h), was about 3.9 x 104. The velocity profile of ABL represents the offshore wind coming from sea, and the height is determined using the windbreak near the seashore.

2.2 Tree model

Evergreen trees have been used to make up windbreak for-ests. The stiff needles or soft foliage of evergreen trees effec-tively withstand strong winds. The year-round cover stays intact in the cold winter season. In this study, five-year-old white fir trees (Abides concolor) were used as the windbreak model. Fir trees have been commonly used as windbreaks because of their ability to withstand strong wind in high-temperature regions. The sample trees were grown according to the standard cultural practices in an open arboretum, receiv-ing full sunlight and water.

Optical porosity (β) and aerodynamic porosity (α) have been used to describe the porosity of a windbreak. Optical porosity is defined as the ratio of the open surface to the total surface of the windbreak. It can be determined by a digital image processing of black and white photographic silhouettes of windbreaks, as introduced by Kenney (1987). In the present study, the shelter effects for the control condition (tree with

Fig. 1. Schematic diagram of the experimental setup.

(a) (b) Fig. 2. Mean streamwise velocity and turbulence intensity profiles measured at the location of the tree model: (a) mean velocity; (b) tur-bulence intensity.

J.-P. Lee et al. / Journal of Mechanical Science and Technology 28 (2) (2014) 565~572 567

leaves), the rotated condition (the same tree was rotated 90 degree to get different optical porosity) and the leafless condi-tion (tree without leaves) were measured and the results were compared each other. The height (H) and width (W) of the model tree are 19 cm and 11 cm, respectively. The optical porosities of the control condition, the rotated condition and the leafless condition are estimated to be 0.06, 0.07 and 0.79, respectively. Velocity field information was measured at the center vertical plane passing through the tree sample.

Aerodynamic porosity is defined as the ratio of wind speed at the immediately leeward of windbreak under the windbreak height to the wind speed approaching the windbreak. Guan et al. (2003) proposed the following empirical relationship be-tween the porosity parameters α and β :

0.4 .a b= (2)

3. Results and discussion

3.1 Flow visualization

Fig. 3 shows the streamlines and the mean velocity fields for three different porosity conditions of the same tree. As expected, the axial velocity in the leeward of the canopy is reduced all cases. In the control and rotated conditions, the velocity in the upper part (0.5 < y/H <1.0) is lower than that in the lower part (0.0 < y/H <0.5). Similar results were reported by Gross (1987) and Rosenfeld et al. (2010) for a single tree with a solid stem. Wind-speed reduction in the leeward side of a tree is the primary goal of a windbreak. The presence of tree leaves (the control and rotated conditions with leaves) brings about better shelter effect, compared to the leafless condition. The control condition is shown large wind-speed reduction not only in the leeward region, but also in the upstream region. Gross (1987) also reported that the velocity profiles in the upstream flow were changed depending on the shape of test

trees. The porosity of the rotated condition is larger than that of

the control condition. In a sense, the lower porosity stands for high flow resistance that blocks some amounts of oncoming wind and deceases the bleed flow penetrating directly through the canopy. Actually, the wind-speed behind the rotated can-opy is larger than that of the control condition. Just in front of the canopy, the wind-speed reduction under the control condi-tion is also larger than that of the rotated condition.

A counter-clockwise recirculation zone is observed in the leeward of the canopy between x/H = 0 and x/H = 1.2 in cases of the control and rotated conditions. For a solid windbreak (Raine and Stevenson, 1977), dense shelterbelts with porosity less than 0.3 (Wang and Takle, 1995), a clockwise recircula-tion zone was formed behind the windbreak. A numerical simulation behind a single tree with zero porosity (Gross, 1987) also exhibited the presence of a similar recirculation zone. This different flow characteristic between previous stud-ies and present study was caused by the thickness of real tree. Most previous simulation studies simply assumed porous fences with zero thickness. In experimental studies, the thick-ness was constant and very thin. However, real trees have thickness because of the branches and leaves. This thickness of real tree is changed depending on the height of the tree. The variation of thickness influences the upstream flow character-istics of windbreak, especially the vertical velocity.

To analyze this phenomenon in more detail, the mean streamwise and vertical velocity profiles measured at the up-stream location of x/H = -0.5 are presented in Fig. 4. The streamwise wind velocity is reduced as the porosity deceases (Fig. 4(a)). This implies that the tree leaves block the oncom-ing wind as flow protectors, with reducing wind speed greatly.

Fig. 4(b) shows the vertical velocity profiles at the upstream region. The negative values of the vertical velocity component denote that the wind moves toward the ground, and the posi-

(a) Control condition

(b) Rotated condition

(c) Leafless condition Fig. 3. Mean velocity contours and streamlines in the vertical center plane of a single tree.

(a) (b) Fig. 4. Comparison of (a) mean streamwise; (b) vertical velocity pro-files measured at the upstream location of x/H = -0.5.


tive values demonstrate the presence of updrafts. The canopy with tree leaves induces updraft in the upper region (0.6 < y/H) and increase downdraft in the lower region (y/H <0.6) at the upstream of the tree for the cases of the leaf conditions. In the upper region, wind mostly moves toward the crest of the tree and goes over the tree. In the lower region, a dominant downdraft increases the wind speed in the lower region of the sample tree, because no exit is available to the wind because of the presence of the ground surface. Therefore, the mean streamwise velocity in the upper region is lower than that in the lower region, as shown in Figs. 3(a) and (b). In the middle part of the tree, the bleed flow passing through the canopy moves upward to the top part of the tree by the momentum difference between the two parts. In addition, there is a bottom gap through which most part of the oncoming wind passes through the trunk area under the tree canopy. The flow fun-neled underneath the canopy can acquire large momentum which is enough to generate an upwelling vortex immediately after the canopy. This upwelling movement makes a counter-clockwise recirculation zone. In addition, the low porosity of the control condition enhances the downdraft and updraft flows. These enhanced flows increases the momentum differ-ence between the two parts. Therefore, the lower porosity induces a larger counter-clockwise recirculation zone (Figs. 3(a) and (b)).

Fig. 5 shows the variations in vertical velocity profiles along the x-direction. The inlet flow condition is nearly the same for the three conditions tested in this study (data are not shown). The wind-speed for the leafless condition is much higher than the conditions with leaves in the near-wake region up to x/H = 4. Even though the velocity profiles are similar, the velocity magnitude for the rotated condition is larger than that for the control condition in the near-wake region up to x/H = 7. Because the bleed flow behind canopy is increased as the porosity increases. Beyond the downstream location of x/H = 7, the wind speed and velocity profile become nearly identical for the three cases. This phenomenon results from the fact that the flow perturbations induced by the windbreak are almost smoothed out. This stable region is called the “re-equilibration zone” (Rain and Stevenson, 1977). Using this velocity profile, aerodynamic porosity was calculated (Table 1). The theoretical aerodynamic porosity was calculated using Eq. (2). The aerodynamic porosity estimated from the experi-mental data matches relatively well with the theoretical results.

3.2 Leaves effect on the wind-speed reduction and turbu-lence characteristic

To analyze the effect of tree leaves on the flow characteris-tics around the tree, only the results of the control and leafless conditions are compared.

For quantitative comparison of the wind-speed reductions, a dimensionless wind-speed reduction coefficient was employed. Cornelis and Gabreils (2005) proposed this coefficient to ex-press the efficiency of a windbreak in reducing wind speed at a given height z and downstream distance x from the windbreak:

,

,0 ,

1 x zx z

x z

uRc

uD

DD

= - (3)

where RcΔx,z is the wind-speed reduction coefficient, Δx denotes the distance from the windbreak, z is the height above the ground surface, uΔx,z represents the time-averaged wind speed disturbed by the windbreak, and u0Δx,z is the time-averaged wind speed when no windbreak is present.

The overall wind-speed reduction at a given height z can be represented by the average overall reduction coefficient:

zTRc =1

,1

1 ( )Mx

x zxM

Rc xx x

D

DD

» D DD - D å (4)

where M is the number of observations. If RcΔx,z = 1 along a certain length considered, then TRcz = 1, which implies a 100% effective windbreak over the entire distance under con-sideration. Here, RcΔx,z and TRcz are dimensionless variables and their absolute values depend on the chosen integration limit, ΔxM.

Fig. 6 shows the variations in wind-speed reduction coeffi-cients along the downstream distance from the tree at five heights. For both control and leafless conditions, the general variation patterns at every height are similar, except for the leafless condition of z/H = 1.0. The patterns somewhat agree with the results obtained by Cornelis and Gabriels (2005) and Dong et al. (2011). For the control case with tree leaves, the variation patterns are similar, regardless of z/H. However, the peak positions at each height differ.

11

, ,1 1

1 1 ( ) .M

mxx

z x z x zxxM M

TRc Rc d x Rc xx x x x

DD

D DDD

= D » D DD - D D - D åò

(5)

Fig. 5. Variation of vertical velocity profiles along x-direction for the three different types of tree.

Table 1. Optical and aerodynamic porosities of three different types of tree tested in this study.

Aerodynamic porosity Tree type Optical porosity

Theory Experiment

Control 0.05981 0.3241 0.2985

Rotated 0.06869 0.3428 0.3440

Leafless 0.79 0.9100 0.8782


In the middle height range (z/H = 0.4 ~ 0.8), the wind-speed reduction coefficient is considerably larger than that in the other heights. At a lower height (z/H = 0.2), the streamwise velocity increases, because of the downdraft caused by the bottom gap effect. In the upper height (z/H = 1.0), the wind-speed reduction coefficient is substantially decreases because of the updraft induced by the tree and the sharp end canopy, which only provides a narrow shelter range. At the middle height, the wide canopy can provide a broad shelter range and the tree leaves effectively block oncoming wind. In addition, many branches and leaves cause the updraft and downdraft (Fig. 4).

For the leafless condition, the middle height range (z/H = 0.4 ~ 0.6) also provides good wind-speed reduction coefficient. In this condition, the branches serve only as windbreak against the oncoming wind. These branches of the middle height con-tribute to the formation of the shelter zone.

The overall wind-speed reduction coefficients (TRc) at the five heights are shown in Fig. 7. In the previous studies of Cornelis and Gabriels (2005) and Dong et al. (2011), the variations in coefficients in accordance with the vertical height are not so large. In the present study, however, the overall coefficient shows large variations with the vertical height, because the 3-D morphological structures of real tree (includ-ing the branches, leaves and trunk) generate updraft and

downdraft flow (Fig. 4). In the control condition, the overall wind-speed reduction

coefficients over the entire length (x/H = 0 ~ 9.5) are signifi-cantly smaller than those derived by the previous study of Dong et al. (2011) under similar porosity. This difference is caused by the 3-D configuration of the windbreak. Dong et al. used artificial fences of wide widths and the aspect ratio be-tween the width and height is higher than 9. This high aspect ratio can generate a wide shelter zone. In the present work, however, only one tree with a low aspect ratio was used. Therefore, the integration length (ΔxM) from the tree is limited to 3H. Within this reduced integration length, the overall wind-speed reduction coefficients for the control condition are slightly larger than that obtained by Dong et al. (2011) in the middle height range (z/H = 0.4~0.8). In the leafless condition, the overall coefficients are smaller than those of the control condition. However, the values are greater than that of the previous study, regardless of integration length. These results indicate that the real tree has a greater ability to induce wind-speed reduction than do artificial fences. Artificial fences are usually very thin, whereas a real tree has a thick canopy be-cause of its branches and leaves. The 3-D configuration of a tree canopy increases the contact length through which the tree interacts with oncoming wind. This condition enhances the reduction of wind speed.

For the comparison of turbulence structures in the wake re-gion behind the tree model, the turbulence kinetic energy (TKE) distributions are depicted in Fig. 8. TKE was normal-ized by the TKE value of the undisturbed upstream flow at the same height (TKE0) to minimize the effects of turbulent clo-sure schemes and windspeed variation measured for the undis-turbed flow without the tree model. In the control condition, the turbulence level is strong at the top height region and a weak in the near ground region, as shown in Fig. 8(a). Two strong turbulence zones are observed in the top height region. In the wake region, the center of the strong turbulence zone is located at x = 1.4 H and z = 1H. Another one occurs immedi-ately leeward the crest of the tree model, as shown in the sec-tion A of Fig. 8(a). These turbulence structures are consistent with those obtained in the previous experimental study (McNaughton, 1988) and numerical simulation (Wang and


(b) Leafless condition Fig. 6. Variations of wind-speed reduction coefficient at five different vertical heights.

Fig. 7. Variations of overall wind-speed reduction coefficient.


Takle, 1995). In these two previous studies, the turbulence structures also have two local peaks for a windbreak with a porosity of less than 0.4.

In the wake region behind the tree canopy, a quiet zone of reduced turbulence and smaller eddy size is observed immedi-ately behind the tree canopy. The quiet zone in the TKE distri-bution is well matched with the low wind-speed region shown in Fig. 3(a). Behind the artificial 2-D fences, the quiet zone has a triangular shape bounded by the fence, ground surface, and strong turbulence zone (Raine and Stevenson, 1977; McNaughton, 1988; Wang and Takle, 1995). In the present study, the tree model has a trunk between the bottom of the tree canopy and the ground surface. The wind speed is accelerated at the near-ground bottom region because the trunk of the tree forms a bottom gap for oncoming wind. This gap lifts the quiet zone in the downstream region behind the tree canopy. In the leafless condition, however, the turbulence level is not so strong and a weak turbulence zone is formed behind the tree trunk. The tree without leaves does not generate a strong turbu-lent flow, because the corresponding optical porosity (0.79) is significantly larger than that of the control condition (0.06).

Fig. 9 shows the contours of spanwise vorticity for the two different leaf conditions. For the control condition, the vortic-ity contours show negative vorticity values at the top height of the tree and positive vorticity values at the middle height. This vortical structure is closely related to the presence of the quiet zone. In the quiet zone, the mean velocity and turbulence ve-locity fluctuations have small values. To fill up this quiet zone, the surrounding wind is entrained into the leeward region of the tree canopy. This entrainment of surrounding air is ob-served in the spatial distribution of vorticity in the control case.

For the leafless condition, negative vorticity is also ob-served at the top height of the tree. The flow entrainment oc-curs in the range from the top to the middle height because the wind-speed reduction at the middle height is greater than that at the top height. Below the middle height, positive and nega-tive vorticity values are observed in turn. This result is attrib-uted to the bleed-flow directly passing through the open area

of the tree canopy without leaves.

3.3 Shelter effect

To express the shelter effect of a windbreak, Kim and Lee (2002) proposed a shelter parameter, which reflects the streamwise and vertical velocity. Although the vertical veloc-ity component is relatively smaller than the streamwise veloc-ity component, the former is included to accurately evaluate the shelter effect. This shelter parameter also considers changes in turbulence intensities. In the present study, the following shelter parameter was employed to evaluate the shelter effect of the fir tree.

2 2

( , ) ( , ) ( , ) ( , )( , ) 2

( ) ( )

( ).

( )x y x y x y x y

x y

ref y ref y

U u V v

U uy

¢ ¢+ + +=

¢+ (6)

Fig. 10 shows the contour plots of the shelter parameter ac-

cording to the porosity. As expected, the tree with leaves ex-hibits a much better shelter effect, compared to the leafless condition. For the control condition, a good shelter zone is formed behind the tree canopy, with a shelter parameter of less than 0.3. In the rotated condition, the contours of shelter parameter look similar in the upper shear layer, irrespective of the porosity. In the lower shear layer, however, the values of shelter parameter are increased, as the porosity increases. With increasing porosity, the permeability of a windbreak increases.

This strong bleed flow affects the shelter parameter in the near-wake region behind the tree. Therefore, the shelter pa-rameter of the rotated condition has larger values than that of control condition. In the leafless condition, the minimum shel-ter parameter is about 0.6. In the leafless condition, the bleed flow is much strong because there is only a few flow-blocking structures which prevent the entrainment of the oncoming wind into the leeward region behind the tree. This strong bleed


(b) Leafless condition Fig. 8. Contour plots of turbulent kinetic energy normalized by the undisturbed upstream TKE0 in the vertical center plane.


(b) Leafless condition Fig. 9. Contour plots of spanwise vorticity in the vertical center plane.


flow reduces the wind-speed of the oncoming wind (Figs. 5 and 6).

The best shelter zone is observed in the near wake region (x/H = 0-2) just behind the canopy of the control condition (Fig. 9(a)). This shelter zone well corresponds with the quiet zone region. In the quiet zone, the flow has not only the min-imum mean velocity but also has small turbulent velocity fluctuations. These large wind-speed reduction coefficient and low-turbulence characteristics create an excellent shelter effect in the leeward region.

4. Conclusions

Windbreaks provide shelter by reducing the speed of the approaching wind. This shelter effect of windbreaks is directly related to the modification of flow structure by the windbreak. However, few studies on the flow structure around a real tree have been conducted.

In the present study, the flow around a small white fir tree was quantitatively visualized. Based on the flow information obtained in this study, several fluid-mechanical distinct zones such as bleed flow, quiet zone and re-equilibration zone were analyzed and the effect of optical porosity on shelter effect was also investigated. Especially, a counter-clockwise recircu-lation zone is formed at the leeward of the canopy, which has not yet been reported. The tree with leaves brings about the updraft and downdraft flows in the upstream region. The mo-mentum difference of these two flows is the main cause of the formation of the upwelling recirculation zone.

The wind-speed reduction is increased, as the porosity of a tree decreases. In the leaf conditions (the control and rotated conditions), the general patterns of velocity profiles and the flow structures are similar, regardless of porosity. However, the flow characteristics such as the bleed flow are somewhat

different. These different flow structures bring about discerni-ble shelter effect in the low shear layer and in the re-equilibration zone. The contour plots of the shelter parameter have similar pattern in the upper shear layer, irrespective of tree porosity. However, the shelter effect increases as the po-rosity decreases in the lower region. The relationship between optical porosity and aerodynamic porosity was found to be well matched with the empirical formula of Guan et al. (2003).

Tree leaves significantly reduce the porosity of the canopy. This low porosity yields a high wind-speed reduction coeffi-cient in the leeward region behind the tree canopy. The leaves generate strong turbulent velocity fluctuations at the top height of the tree and create the quiet zone just behind the canopy. These turbulence structures are consistent with those observed in previous studies on artificial 2-D fences. However, the loca-tion of the quiet zone slightly shifts upward because of the presence of the bottom gap. The quiet zone and tree leaves considerably affect the formation of large-scale vortices in the region behind the tree. The vorticity distributions at the top height are similar, regardless of leaf condition. However, in the region below the middle height, the vorticity distributions are significantly different.

The shelter parameter exhibits different spatial distributions because of different porosity. The control condition with leaves provide much better shelter effect, compared to the leafless condition. A strong bleed flow passing through the leafless tree is observed. The quiet zone formed just behind the tree brings about the best shelter effect with greatly reduc-ing the oncoming wind speed and decreasing turbulence statis-tics.

Acknowledgment

This work was supported by the National Research Founda-tion of Korea(NRF) grant funded by the Korea government (MSIP) (No. 2008-0061991).

Nomenclature------------------------------------------------------------------------

U0 : Oncoming wind speed H : Height of the windbreak Uref : Reference velocity Yref : Reference height RcΔx,z : Wind-speed reduction coefficient Δx : Distance from the windbreak z : Height above the ground surface uΔx,z : Time-averaged wind speed with windbreak u0Δx,z : Time-averaged wind speed without windbreak TRcz : Overall wind-speed reduction M : Number of observations ΔxM : Integration length

References

[1] W. M. Cornelis and D. Gabriels, Optimal windbreak design


(b) Rotated condition

Fig. 10. Contours of shelter parameter in the vertical center plane.


for wind-erosion control, J. Arid. Environ., 61 (2005) 315-332.

[2] Z. Dong, W. Y. Lup, G. Q. Qian and H. T. Wang, Evaluat-ing the optimal porosity of fences for reducing wind erosion. Sci. in Cold Arid Regions, 3 (2011) 1-12.

[3] G. Gross, A numerical study of the air flow within and around a single tree, Boundary-Lay. Meteorol., 40 (1987) 311-327.

[4] D. Guan, Y. Zhang and T. Zhang, A wind-tunnel study of windbreak drag, Agric. For. Meteorol., 118 (2003) 75-84.

[5] J. L. Hagen, E. L. Skidmore, P. L. Miller and J. E. Kipp, Simulation of effect of wind barriers on airflow, Trans. ASAE., 24 (1981) 1002-1008.

[6] W. A. Kenney, A method for estimating windbreak porosity using digitized photographic silhouettes, Agric. For. Meteo-rol., 39 (1987) 91-94.

[7] H. B. Kim and S. J. Lee, The structure of turbulent shear flow around a two-dimensional porous fence having a bot-tom gap, J. Fluids. Struct., 16 (2002) 317-329.

[8] K. G. McNaughton, Effects of Windbreaks on turbulent transport and microclirnate, Agric. Ecosyst. Environ., 22 (1988) 17-39.

[9] J. K. Raine and D. C. Stevenson, Wind protection by model fences in a simulated atmospheric boundary layer, J. Wind Eng. Ind. Aerodyn., 2 (1977) 17-39.

[10] M. Rosendfield, G. Marom and A. Bitan, Numerical simu-lation of the airflow across trees in a windbreak, Bound-Lay. Meteorol., 135 (2010) 89-107.

[11] H. Torita and H. Satou, Relationship between shelterbelt structure and mean wind reduction, Agric. For. Meteorol., 145 (2007) 186-194.

[12] J. P. Lee and S. J. Lee, PIV analysis on the shelter effect of a bank of real fir trees, J. Wind Eng. Ind. Aerodyn., 110 (2012) 40-49.

[13] H. Wang and E. S. Takle, A numerical simulation of boundary-layer flows near shelterbelts, Bound-Lay. Meteo-rol., 75 (1995) 141-173.

[14] H. Wang, E. S. Takle and J. Shen, Shelterbelts and wind-breaks: mathematical modeling and computer simulations of turbulent flows, Annu. Rev. Fluid Mech., 33 (2001) 549-586.

[15] X. H. Zhou, J. R. Brandle, C. W. Mize and E. S. Takle, Three-dimensional aerodynamic structure of a tree shelter-belt: definition, characterization and working models, Agro-forest. Syst., 63 (2004) 133-147.

Sang Joon Lee He is currently a profes-sor in the department of Mechanical Engineering at POSTECH and a director of the Biofluid and Biomimic Research Center. His research interests are bio-fluid flows, microfluidics, quantitative flow visualization and experimental fluid mechanics.

Jin Pyung Lee He received his Ph.D. in School of Environmental Science and Engineering from POSTECH. Now He is in working in Samsung Display. His research interests are flow visualization, applications of PIV velocity field tech-niques, and wind tunnel experiments.

shelter effect of a fir tree with different porosities

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