- Email: [email protected]
Wind loads and flow regimes associated with sheltered lightweight greenhouses
Journal o[ Wind Engineering and Industrial Aerodynamics, 38 (1991) 419-426
419
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
Wind loads and flow regimes associated with sheltered lightweight greenhouses B. Tahouri, N. Toy and J. Booker Department of Civil Engineering, University of Surrey, Guildford, Surrey GU2 5XH, UK
Summary A wind tunnel investigation of the effects of fences on the surface pressure distribution of a greenhouse structure is reported. The investigation includes a set of parameter studies based on the windbreak characteristics to obtain optimum shelter for the structure. The influence of the fence parameters on the mean surface pressures in two different approaching boundary layers is investigated. Mean velocity profiles in the wake of fences of different porosities are presented together with the effect of fence porosity on the loading of the model.
Notation
C~ C~ Cd h H L p Po Re U u X Z
pressure coefficient, ( p - p o ) / 1 / 2 p O 2 lift coefficient drag coefficient height of fence height of model length of model pressure on model surface static pressure Reynolds number, pUH//~ mean freestream velocity longitudinal mean velocity (X direction) velocity fluctuation in the x direction streamwise Cartesian coordinate vertical Cartesian coordinate
Greek letters 0 elevation angle dynamic viscosity of air # density of air 0167-6105/91/$03.50 © 1991 Elsevier Science Publishers B.V. All rights reserved.
420
1. Introduction
The use of shelter screens or fences to reduce the wind loads on lightweight, film-plastic clad greenhouses has been frequent during the past two decades. With no quantitative information on the degree of protection afforded to a greenhouse structure by a fence, wind shelters have been constructed using an empirical approach. A series of full-scale tests on one type of sheltered greenhouse, carried out by AFRC U.K., provided some initial guidelines in terms of a shelter factor for the design of that particular type of building [ 1 ]. The precise mechanism of flow past a fence, studied by numerous workers, has shown that the performance of such fences is defined in terms of the size of the shelter area and the nature of the flow within it. These are both affected by many parameters such as porosity, and size and geometry of the fence. The most important factor is the porosity (ratio of open to total area) of the fence. It has been shown that the recirculating bubble attached to a fence of zero porosity, detaches from the fence and moves downstream as the porosity increases up to a critical value of about 30% when no more reverse flow occurs [2,3]. Within this range, as the porosity increases the wind speed reduction decreases, but the sheltered zone increases. In fact there are several optimum fence porosities, depending upon the application of the fence [3-6]. Other parameters, such as the fence dimensions, geometry and orientation with respect to the object to be protected, may also have different optimal values. The extent to which these parameters influence the results and the complex nature of the flow field initiated the present wind tunnel investigation, which is a continuation of an earlier work on greenhouses [ 7 ]. This paper presents experimental data regarding the influence of two-dimensional fences on the mean surface pressure distributions on a wind tunnel model of a typical semi-cylindrical greenhouse used in the agricultural industry. The effects of fence height and length, fence/building spacing, porosity of the fence and the approach flow conditions on the surface pressures and the loading on the model are discussed. In identifying the size of the sheltered region and the reductions in the flow velocities, detailed information is given regarding the mean longitudinal velocities of the flows past two-dimensional fences with three different porosities. 2. E x p e r i m e n t a l details
The experiments were carried out in the low-speed, blow-down, open circuit wind tunnel with working section dimensions of 1.68 m height x1.37 m width X 9.0 m length in the Department of Civil Engineering. The measurements were undertaken in two different boundary layers, denoted "thin" and "rough2", with thicknesses of 135 mm and 600 ram, respectively. Additional details are included in ref. 8. The greenhouse model was a solid, closed-ended
421
semi-cylinder with a height of 95 m m and a length of 380 ram. Pressure tappings of 1 m m diameter were provided over the mid-length at 5 ° intervals. To simulate high Re flow conditions [9], the surface of the model was roughened with 0.7-1.0 m m diameter spherical beads, giving an approximate roughness ratio of 0.01. The fence models were constructed from 3 m m thick aluminium alloy plate with 45 ° bevelled edges. Three heights of fences were used, 60 mm, 95 mm and 120 ram, giving fence height to model height ratios (h/H) of 0.63, 1.00 and 1.26, respectively• In addition, two 60 m m high two-dimensional fences of 25% and 50% porosity were used with horizontal open slats of 3 m m and 6 m m width. Microcomputer-controlled systems for data acquisition and analysis were used in both the surface pressure and the near-wake measurements. The mean pressure data were normalised against the freestream dynamic pressure, with results presented in coefficient form. The mean velocity and turbulence intensity profiles were obtained using a standard Pela Instruments pulsed-wire anemometer system, attached to a computer-controlled traversing mechanism, measuring the streamwise mean and fluctuating velocity component. A freestream velocity of 10 m s- 1 was used throughout, giving a Reynolds number of 6.6 X 104 based on the model height. 3. Discussion
The mean surface pressure distributions for the unsheltered model in the two boundary layers were initially measured. Generally, the magnitude of the stagnation, base and peak negative pressures were higher in the case of the "thin" boundary layer due to the greater m o m e n t u m of the flow. The influence on these pressure distributions of the three two-dimensional solid fences was studied at different fence-model spacings. Figure 1 shows the pressure distribution of the model in the "thin" boundary layer with the 60 m m two-dimensional fence at three upstream positions. It 0.4
UNSHELTERED ""-'"
Cp 0.2 x
xx
~xJ x
/J
h/H=
0.63
xxxx~
0 30 -0.2
~ x60 °~ °~ ~xo
90
x°
ooo°
-0.8
\
~
18N ~ - X/h= 1 810,
o XxSX..---'.
•
1 ..•.•...•
I0
~
x°o
-0.4 -0.6
120
z /•
.
.....
.
-
.
~
•
J
Fig. I. M e a n surface pressure distribution of the model sheltered by the 60 m m 2-D fence in the "thin" boundary layer.
422
can be noted that the magnitude of the pressures are generally reduced except at small spacings (e.g. X/h = 1), where windward surface and base pressures reach greater values than those of the unsheltered model. The effect of increasing the fence height to model height ratio (h/H) may be studied by comparing Figs. 1, 2 and 3. At small fence/model spacings, there is a critical fence height (h/H= 0.63-1 ) at which the flow is deflected over the top of the model, exposing the entire surface to an almost uniform negative pressure. Similar patterns were observed during the tests carried out in the "rough2" boundary layer, with peak suctions of smaller magnitude and base pressures that were generally of a greater value. The flow was found to attach onto the model at a smaller fencemodel spacing than in the "thin" boundary layer, with the flow separating from the model at closer spacing (Figs. 4, 5 and 6). Lift and drag coefficients were determined by integrating the centre-line pressure data in each case. Although not strictly the overall lift and drag coefficients, they provide a basis for the comparison of all the presented data. The variations of these coefficients, with the model sheltered by the two-dimensional fences in the two boundary layers, are shown in Figs. 7 and 8. In each 0,4 Cp 0 . 2 0
x\l~ p
'
'
3'0
,
,
-0.2
\~ 1.26
°°
,
I
i
120××
i
i
150
I
J
Soo°"
/
,
0
, X2h- 21-47
~ ~, x/,, \ o°°,°
. . . . . . . . .
i
x x x x x o ~ ~ ~ x ~ x ox ×o °~.~_...~ ° ~ 1 1 . 3 7
o°~
-0.4
,
90
~o.too x °°°o
11.37
-0-6
~
',×,, 6 0
'
/.,
1-26 ......
•
. . • ......
\./"
-0.8
Fig. 2. Mean surface pressure distribution of the model sheltered by the 95 mm 2-D fence in the "thin" boundary layer. 0.4 -
\
1
CpO.2
~ xxx
h/H=
126
xx\
0
30
9° x
-0.2
x
X×x×X -0-4 -0.6
o o°/ ooo
oo~ooo
o~,°
9. 1.
/
1
-0'8
°°.o
. . . . . . . . . .
~,..°°°°°°°°°.
Fig. 3. Mean surface pressure distribution of the model sheltered by the 120 mm 2-D fence in the "thin" boundary layer.
423
UNSHELTERED CpO.2
//
~-"-
o
h/H= 0-63
o o t',,~
0
[
,
30
- 0.2 •
°
.
.
.
.
.
,
-"
.
.
90
o
,~ ~
• •
-
? O '
'7.
',.:
×
.
.
120 ~%-..
•
.
.
.
150
o1~0-- X/h= 10.
-.-.--r~-~-~--; • . - . _ _ _ _ ~
o
17.7 z
Fig. 4. Mean surface pressure distribution of the model sheltered b y t h e 60 m m 2 - D fence in t h e " r o u g h 2 " b o u n d m ' y layer.
h/H= 1.
Cp 0.2 °
°o:.
0 30 . 2,.° ~6.0
"'...-°
-0.2
\
6-3
90
o°..
",, "
-0-4
" •.
"-,
120
150
oooollle=e.°'~
,:'~'"'~ . . . . . . . . . .
- ~18--O--y/h-lZ. q ,,- -
~
~ . ~
6-3
,"
Fig. 5. Mean surface pressure distribution of the model sheltered by the 95 m m 2-D fence in the "rough2" boundary layer.
Cp 0.2 0
22.5
h / H : 1"26
x,.~ - ' x - Y*"~ . b ~
\
~x
11 30
-0-2 -0.4
"~ .60 •, × ° o
"~××°° \ × 4
90 Oo
150 xxXXX~;~;~,~ ox,,-#~ . . . . . . ., . . . ,. ., ., , ,
. .X,." "x~ = ',"/"
......
,
z, ~180 ~ ~,~----_X/h= 22-5 , , " ~ 1 1 ... ~
4
Fig. 6. Mean surface pressure distribution of the model sheltered by the 120 m m 2-D fence in the "rough2" boundary layer.
case, values of the coefficients for the unsheltered model are plotted as dotted lines. The variations in the lift and drag coefficients with the fence-model spacing are generally greater in the "thin" boundary layer flow. The pressure recovery associated with the "rough2" flow provides a more effective shelter at closer spacings, whereas the fences in the "thin" flow are only effective at some distances away from the model, depending on their heights. The drag coefficients are well below the values of the unprotected model, with negative values at close spacings (up to X/h= 10-13) in the "thin" boundary layer. From the range of results taken in this study, the drag coefficient was not found to be a good indicator of the shelter effect of the solid fences, because at every fence position the drag coefficients of the protected model were less than those of the unprotected model. The lift coefficient, however, provided a good guide to the effectiveness of each solid windbreak. In the "thin" boundary layer,
424
(-
[
0Bi "-,
0.6~
0.4
.~
. ..
0.2
oJ o
'
~
'
d
I
,
i
'
~
'
1
'
,
~
~"
-
~
' l'o'
1
,
1'2'
1;
'1'6'
1;'
io'
i/h
0,4 0.2 C d 1
4
'
6
5.z15
, 4"
I..~'YI
,
,4
1
,6
l
I
,8
,
1
20
,
1
22xJh
[email protected] -0.4 Fig. 7. Lift and drag coefficients of model sheltered by the 2-D fences in the "thin" boundary layer. O, Large fence (120 ram) h/H=l.26; 0, medium fence (95 ram) h/H=l.O0; X, small fence (60 mm) h/H= 0.63; .... unsheltered.
for instance, the best shelter was provided by adopting fence-model spacing (X/h) ratiosorS, 9, and 10 for fence height ratios (h/H) of 0.63, 1.00 and 1.26, respectively.In the rougher boundary layer,the spacing ratios were reduced to 3.5, 5.0 and 7.0, respectively. Figure 9 illustratesthe longitudinal mean velocity profiles in the wake of three 60 m m high, two-dimensional fences of 0%, 25% and 50% porosity in the "thin" boundary layer. It can be noted that as the porosity is increased, the reduction in wind speed decreases with the recirculationzone becoming smaller. There is no reverse flow taking place behind the 50% porous fence and the shelter provided by this fence is minimal. The influence of these fences on the pressure distribution of the greenhouse model is shown in Fig. 10. The magnitude of the peak negative and base pressures together with the pressure on the windward face were found to increase with increasing porosity, and it was concluded that, in that configuration (h/H=0.63 and X/h= 10), the best shelter would be provided by a fence with about 25% porosity.
425 0.5
04 Ct 0.2 -
o
o--------
x 0
I
I
I
I
I
I
I
i
I
I
I
2
4
6
6
10
12
1~,
16
18
20
22
o1
0°2
........................................................................................................................................................... x..~.
~,/"
6
~.
4(._.
6
~
. . . . .
10
12
x---
1~-
.
.
.
16
.
.
.
.
18
o---
-, X /i ~ . ,
i
20
22
h
- 0.7
-0.4 Fig. 8. Lift and drag coefficients of model sheltered by the 2-D fences in the "rough2" boundary layer. O, Large fence (120 ram) h/H= 1.26; O, medium fence (95 ram) h/H= 1.00; X, small fence (60 ram) h/H=0.63; .... u n s h e l t e r e d .
,u/F4
0
1'0
Z/h
•
e,
3 j*
q o×
Z, oo ~
~x
oo
,/
~
4
6
10
12
i , I'8 14
16
X/h
Fig. 9. Mean longitudinal velocity profiles in t h e w a k e of the 60 mm porous two-dimensional fences in the "thin" boundary layer (O, solid; O, 25%; X, 50% ).
Further work is being undertaken on the study of the flow streamlines behind porous fences with various geometries of perforation. The results of this work together with the influence of three-dimensional windbreaks on the loading of the greenhouse model will be reported in the near future.
426 UNSHELTERED //
0.4
\
0.2 X
O.
h/H= 0 . 6 3
×
ZX
\o x , x , o~ ~, o o 002?0° °4~, .t.o
-0.2
e
8'0 ~ 160' 1201 1401 160 I 180
i
-0.4 ×
-0.6 \
/
X
/
Fig. 10. Mean surface pressure distribution of greenhouse model sheltered by 2-D fences in the thin boundary layer ( 0 , solid; O, 25%; × , 50% ).
Acknowledgement
This work was supported by the Agricultural and Food Research Council, U.K. References 1 G.M. Richardson, Wind loads on a full-scale f'flm-plastic clad greenhouses: With and without shelter from a windbreak, Proc. 6th Colloquium on Ind. Aerodyn., Aachen, 1985. 2 I.P. Castro, Wake characteristics of two-dimensional perforated plates normal to an air-stream, J. Fluid Mech., 46 (1971) 599-609. 3 M.D.A.E.S. Perera, Shelter behind two-dimensional solid and porous fences, J. Wind Eng. Ind. Aerodyn., 8 (1981) 93-104. 4 G. Guyot, Les Effets Aerodynamiques des Brise Vent, Promocline, Etudes Thermiques et Aeroliques, 8E (3) (1977). 5 L.J. Hagen and E.L. Skidmore, Turbulent velocity fluctuations and vertical flow as affected by windbreak porosity, Trans. ASAE, {1971 ) 634-637. 6 J.K. Raine and D.C. Stevenson, Wind protection by model fences in a simulated atmospheric boundary layer, J. Ind. Aerodyn., 2 (1977) 159-180. 7 N. Toy and B. Tahouri, Pressure distributions on semi-cylindrical structures of different geometrical cross-sections, J. Wind Eng. Ind. Aerodyn., 29 (1988) 263-272. 8 E. Savory, N. Toy, S. Dalley and J. Trussler, Wind loading on a portal frame agricultural building, Proc. 8th Colloquium on Industrial Aerodynamics, Aachen, September, 1989. 9 E. Savory and N.Toy, Hemispheres and hemisphere-cylinders in turbulent boundary layers, J. Wind Eng. Ind. Aerodyn., 23 (1986) 345-364.
419
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
Wind loads and flow regimes associated with sheltered lightweight greenhouses B. Tahouri, N. Toy and J. Booker Department of Civil Engineering, University of Surrey, Guildford, Surrey GU2 5XH, UK
Summary A wind tunnel investigation of the effects of fences on the surface pressure distribution of a greenhouse structure is reported. The investigation includes a set of parameter studies based on the windbreak characteristics to obtain optimum shelter for the structure. The influence of the fence parameters on the mean surface pressures in two different approaching boundary layers is investigated. Mean velocity profiles in the wake of fences of different porosities are presented together with the effect of fence porosity on the loading of the model.
Notation
C~ C~ Cd h H L p Po Re U u X Z
pressure coefficient, ( p - p o ) / 1 / 2 p O 2 lift coefficient drag coefficient height of fence height of model length of model pressure on model surface static pressure Reynolds number, pUH//~ mean freestream velocity longitudinal mean velocity (X direction) velocity fluctuation in the x direction streamwise Cartesian coordinate vertical Cartesian coordinate
Greek letters 0 elevation angle dynamic viscosity of air # density of air 0167-6105/91/$03.50 © 1991 Elsevier Science Publishers B.V. All rights reserved.
420
1. Introduction
The use of shelter screens or fences to reduce the wind loads on lightweight, film-plastic clad greenhouses has been frequent during the past two decades. With no quantitative information on the degree of protection afforded to a greenhouse structure by a fence, wind shelters have been constructed using an empirical approach. A series of full-scale tests on one type of sheltered greenhouse, carried out by AFRC U.K., provided some initial guidelines in terms of a shelter factor for the design of that particular type of building [ 1 ]. The precise mechanism of flow past a fence, studied by numerous workers, has shown that the performance of such fences is defined in terms of the size of the shelter area and the nature of the flow within it. These are both affected by many parameters such as porosity, and size and geometry of the fence. The most important factor is the porosity (ratio of open to total area) of the fence. It has been shown that the recirculating bubble attached to a fence of zero porosity, detaches from the fence and moves downstream as the porosity increases up to a critical value of about 30% when no more reverse flow occurs [2,3]. Within this range, as the porosity increases the wind speed reduction decreases, but the sheltered zone increases. In fact there are several optimum fence porosities, depending upon the application of the fence [3-6]. Other parameters, such as the fence dimensions, geometry and orientation with respect to the object to be protected, may also have different optimal values. The extent to which these parameters influence the results and the complex nature of the flow field initiated the present wind tunnel investigation, which is a continuation of an earlier work on greenhouses [ 7 ]. This paper presents experimental data regarding the influence of two-dimensional fences on the mean surface pressure distributions on a wind tunnel model of a typical semi-cylindrical greenhouse used in the agricultural industry. The effects of fence height and length, fence/building spacing, porosity of the fence and the approach flow conditions on the surface pressures and the loading on the model are discussed. In identifying the size of the sheltered region and the reductions in the flow velocities, detailed information is given regarding the mean longitudinal velocities of the flows past two-dimensional fences with three different porosities. 2. E x p e r i m e n t a l details
The experiments were carried out in the low-speed, blow-down, open circuit wind tunnel with working section dimensions of 1.68 m height x1.37 m width X 9.0 m length in the Department of Civil Engineering. The measurements were undertaken in two different boundary layers, denoted "thin" and "rough2", with thicknesses of 135 mm and 600 ram, respectively. Additional details are included in ref. 8. The greenhouse model was a solid, closed-ended
421
semi-cylinder with a height of 95 m m and a length of 380 ram. Pressure tappings of 1 m m diameter were provided over the mid-length at 5 ° intervals. To simulate high Re flow conditions [9], the surface of the model was roughened with 0.7-1.0 m m diameter spherical beads, giving an approximate roughness ratio of 0.01. The fence models were constructed from 3 m m thick aluminium alloy plate with 45 ° bevelled edges. Three heights of fences were used, 60 mm, 95 mm and 120 ram, giving fence height to model height ratios (h/H) of 0.63, 1.00 and 1.26, respectively• In addition, two 60 m m high two-dimensional fences of 25% and 50% porosity were used with horizontal open slats of 3 m m and 6 m m width. Microcomputer-controlled systems for data acquisition and analysis were used in both the surface pressure and the near-wake measurements. The mean pressure data were normalised against the freestream dynamic pressure, with results presented in coefficient form. The mean velocity and turbulence intensity profiles were obtained using a standard Pela Instruments pulsed-wire anemometer system, attached to a computer-controlled traversing mechanism, measuring the streamwise mean and fluctuating velocity component. A freestream velocity of 10 m s- 1 was used throughout, giving a Reynolds number of 6.6 X 104 based on the model height. 3. Discussion
The mean surface pressure distributions for the unsheltered model in the two boundary layers were initially measured. Generally, the magnitude of the stagnation, base and peak negative pressures were higher in the case of the "thin" boundary layer due to the greater m o m e n t u m of the flow. The influence on these pressure distributions of the three two-dimensional solid fences was studied at different fence-model spacings. Figure 1 shows the pressure distribution of the model in the "thin" boundary layer with the 60 m m two-dimensional fence at three upstream positions. It 0.4
UNSHELTERED ""-'"
Cp 0.2 x
xx
~xJ x
/J
h/H=
0.63
xxxx~
0 30 -0.2
~ x60 °~ °~ ~xo
90
x°
ooo°
-0.8
\
~
18N ~ - X/h= 1 810,
o XxSX..---'.
•
1 ..•.•...•
I0
~
x°o
-0.4 -0.6
120
z /•
.
.....
.
-
.
~
•
J
Fig. I. M e a n surface pressure distribution of the model sheltered by the 60 m m 2-D fence in the "thin" boundary layer.
422
can be noted that the magnitude of the pressures are generally reduced except at small spacings (e.g. X/h = 1), where windward surface and base pressures reach greater values than those of the unsheltered model. The effect of increasing the fence height to model height ratio (h/H) may be studied by comparing Figs. 1, 2 and 3. At small fence/model spacings, there is a critical fence height (h/H= 0.63-1 ) at which the flow is deflected over the top of the model, exposing the entire surface to an almost uniform negative pressure. Similar patterns were observed during the tests carried out in the "rough2" boundary layer, with peak suctions of smaller magnitude and base pressures that were generally of a greater value. The flow was found to attach onto the model at a smaller fencemodel spacing than in the "thin" boundary layer, with the flow separating from the model at closer spacing (Figs. 4, 5 and 6). Lift and drag coefficients were determined by integrating the centre-line pressure data in each case. Although not strictly the overall lift and drag coefficients, they provide a basis for the comparison of all the presented data. The variations of these coefficients, with the model sheltered by the two-dimensional fences in the two boundary layers, are shown in Figs. 7 and 8. In each 0,4 Cp 0 . 2 0
x\l~ p
'
'
3'0
,
,
-0.2
\~ 1.26
°°
,
I
i
120××
i
i
150
I
J
Soo°"
/
,
0
, X2h- 21-47
~ ~, x/,, \ o°°,°
. . . . . . . . .
i
x x x x x o ~ ~ ~ x ~ x ox ×o °~.~_...~ ° ~ 1 1 . 3 7
o°~
-0.4
,
90
~o.too x °°°o
11.37
-0-6
~
',×,, 6 0
'
/.,
1-26 ......
•
. . • ......
\./"
-0.8
Fig. 2. Mean surface pressure distribution of the model sheltered by the 95 mm 2-D fence in the "thin" boundary layer. 0.4 -
\
1
CpO.2
~ xxx
h/H=
126
xx\
0
30
9° x
-0.2
x
X×x×X -0-4 -0.6
o o°/ ooo
oo~ooo
o~,°
9. 1.
/
1
-0'8
°°.o
. . . . . . . . . .
~,..°°°°°°°°°.
Fig. 3. Mean surface pressure distribution of the model sheltered by the 120 mm 2-D fence in the "thin" boundary layer.
423
UNSHELTERED CpO.2
//
~-"-
o
h/H= 0-63
o o t',,~
0
[
,
30
- 0.2 •
°
.
.
.
.
.
,
-"
.
.
90
o
,~ ~
• •
-
? O '
'7.
',.:
×
.
.
120 ~%-..
•
.
.
.
150
o1~0-- X/h= 10.
-.-.--r~-~-~--; • . - . _ _ _ _ ~
o
17.7 z
Fig. 4. Mean surface pressure distribution of the model sheltered b y t h e 60 m m 2 - D fence in t h e " r o u g h 2 " b o u n d m ' y layer.
h/H= 1.
Cp 0.2 °
°o:.
0 30 . 2,.° ~6.0
"'...-°
-0.2
\
6-3
90
o°..
",, "
-0-4
" •.
"-,
120
150
oooollle=e.°'~
,:'~'"'~ . . . . . . . . . .
- ~18--O--y/h-lZ. q ,,- -
~
~ . ~
6-3
,"
Fig. 5. Mean surface pressure distribution of the model sheltered by the 95 m m 2-D fence in the "rough2" boundary layer.
Cp 0.2 0
22.5
h / H : 1"26
x,.~ - ' x - Y*"~ . b ~
\
~x
11 30
-0-2 -0.4
"~ .60 •, × ° o
"~××°° \ × 4
90 Oo
150 xxXXX~;~;~,~ ox,,-#~ . . . . . . ., . . . ,. ., ., , ,
. .X,." "x~ = ',"/"
......
,
z, ~180 ~ ~,~----_X/h= 22-5 , , " ~ 1 1 ... ~
4
Fig. 6. Mean surface pressure distribution of the model sheltered by the 120 m m 2-D fence in the "rough2" boundary layer.
case, values of the coefficients for the unsheltered model are plotted as dotted lines. The variations in the lift and drag coefficients with the fence-model spacing are generally greater in the "thin" boundary layer flow. The pressure recovery associated with the "rough2" flow provides a more effective shelter at closer spacings, whereas the fences in the "thin" flow are only effective at some distances away from the model, depending on their heights. The drag coefficients are well below the values of the unprotected model, with negative values at close spacings (up to X/h= 10-13) in the "thin" boundary layer. From the range of results taken in this study, the drag coefficient was not found to be a good indicator of the shelter effect of the solid fences, because at every fence position the drag coefficients of the protected model were less than those of the unprotected model. The lift coefficient, however, provided a good guide to the effectiveness of each solid windbreak. In the "thin" boundary layer,
424
(-
[
0Bi "-,
0.6~
0.4
.~
. ..
0.2
oJ o
'
~
'
d
I
,
i
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~
'
1
'
,
~
~"
-
~
' l'o'
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,
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1;'
io'
i/h
0,4 0.2 C d 1
4
'
6
5.z15
, 4"
I..~'YI
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,4
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,8
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[email protected] -0.4 Fig. 7. Lift and drag coefficients of model sheltered by the 2-D fences in the "thin" boundary layer. O, Large fence (120 ram) h/H=l.26; 0, medium fence (95 ram) h/H=l.O0; X, small fence (60 mm) h/H= 0.63; .... unsheltered.
for instance, the best shelter was provided by adopting fence-model spacing (X/h) ratiosorS, 9, and 10 for fence height ratios (h/H) of 0.63, 1.00 and 1.26, respectively.In the rougher boundary layer,the spacing ratios were reduced to 3.5, 5.0 and 7.0, respectively. Figure 9 illustratesthe longitudinal mean velocity profiles in the wake of three 60 m m high, two-dimensional fences of 0%, 25% and 50% porosity in the "thin" boundary layer. It can be noted that as the porosity is increased, the reduction in wind speed decreases with the recirculationzone becoming smaller. There is no reverse flow taking place behind the 50% porous fence and the shelter provided by this fence is minimal. The influence of these fences on the pressure distribution of the greenhouse model is shown in Fig. 10. The magnitude of the peak negative and base pressures together with the pressure on the windward face were found to increase with increasing porosity, and it was concluded that, in that configuration (h/H=0.63 and X/h= 10), the best shelter would be provided by a fence with about 25% porosity.
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-0.4 Fig. 8. Lift and drag coefficients of model sheltered by the 2-D fences in the "rough2" boundary layer. O, Large fence (120 ram) h/H= 1.26; O, medium fence (95 ram) h/H= 1.00; X, small fence (60 ram) h/H=0.63; .... u n s h e l t e r e d .
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Further work is being undertaken on the study of the flow streamlines behind porous fences with various geometries of perforation. The results of this work together with the influence of three-dimensional windbreaks on the loading of the greenhouse model will be reported in the near future.
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Acknowledgement
This work was supported by the Agricultural and Food Research Council, U.K. References 1 G.M. Richardson, Wind loads on a full-scale f'flm-plastic clad greenhouses: With and without shelter from a windbreak, Proc. 6th Colloquium on Ind. Aerodyn., Aachen, 1985. 2 I.P. Castro, Wake characteristics of two-dimensional perforated plates normal to an air-stream, J. Fluid Mech., 46 (1971) 599-609. 3 M.D.A.E.S. Perera, Shelter behind two-dimensional solid and porous fences, J. Wind Eng. Ind. Aerodyn., 8 (1981) 93-104. 4 G. Guyot, Les Effets Aerodynamiques des Brise Vent, Promocline, Etudes Thermiques et Aeroliques, 8E (3) (1977). 5 L.J. Hagen and E.L. Skidmore, Turbulent velocity fluctuations and vertical flow as affected by windbreak porosity, Trans. ASAE, {1971 ) 634-637. 6 J.K. Raine and D.C. Stevenson, Wind protection by model fences in a simulated atmospheric boundary layer, J. Ind. Aerodyn., 2 (1977) 159-180. 7 N. Toy and B. Tahouri, Pressure distributions on semi-cylindrical structures of different geometrical cross-sections, J. Wind Eng. Ind. Aerodyn., 29 (1988) 263-272. 8 E. Savory, N. Toy, S. Dalley and J. Trussler, Wind loading on a portal frame agricultural building, Proc. 8th Colloquium on Industrial Aerodynamics, Aachen, September, 1989. 9 E. Savory and N.Toy, Hemispheres and hemisphere-cylinders in turbulent boundary layers, J. Wind Eng. Ind. Aerodyn., 23 (1986) 345-364.