Wind-tunnel and full-scale comparisons on the change of local wind characteristics due to an open cut

Jottrnal of VCind Engineering and Industrial Aerodynamics, 32 (1989) 51-61

51

Elsevier Science Publishers B.V., A m s t e r d a m - - P r i n t e d in T h e N e t h e r l a n d s

WIND-TUNNEL AND FULL-SCALE COMPARISONSON THE CHANGEOF LOCAL WIND CHARACTERISTICS DUE TO AN OPEN CUT H. UTSUNOMIYAI , F. NAGAO1 and S. YOSHIMURA2 Iconstruction Engineer, Tokushima U n i v e r s i ty , Minami-josanjima, Tokushima (Japan) 2Central Consultant Co. L td . , Meguro 2-23-18, Tokyo (Japan)

ABSTRACT The effects of open cuts f o r road construction on the local wind environment in a complex t e r r a i n are discussed using wind-tunnel simulation, the results of which are examined by f u l l - s c a l e measurements. Wind-tunnel tests show that the ridge l i n e of an open cut worsens the wind environment downstream. As a countermeasure to t h i s , porous wind fences set up at proper positions near the open cut are very e f f e c t i v e . Some simple ridge models with trapezoidal cross-section are used to obtain s i g n i f i c a n t t e r r a i n parameters which influence on the downstream flow. The r e s u l t shows that the p r i n c i p a l parameters are the width of the open cut and the wind v e l o c i t y at i t s centre. INTRODUCTION This study had the p r a c ti c a l purpose of estimating changes in the wind environment due to a large-scale open cut f o r road construction, and to find some proper countermeasures to improve the wind environment and prevent any trouble.

Because the area studied is very complex rough t e r r a i n and there are

no previous wind data, the wind-tunnel simulation method was chosen to estimate the e f f e c t of an open cut.

In r e l a t i o n to this problem, there is a well-known

study by P. Jackson and J. Hunt ( r e f . I) which discusses a turbulent wind over a low h i l l .

Similar studies were undertaken by H.W. Teunissen et a l . at

Askervein H i l l and Kettle H i l l (refs. 2,3,4), in which they compared the results of wind-tunnel simulations with those of f u l l - s c a l e measurements and mathematical models.

A.J. Bowen ( r e f . 5) also gives the r e l a t i o n s h i p between some

t e r r a i n parameters such as escarpments or ridges and the wind-flow over them. In these studies, comparatively simple t e r r a i n s l i k e isolated or two-dimensional h i l l s were chosen.

For very complex t e r r a i n and complex wind structures,

authors have concentrated t h e i r a t t e n t i o n on the wind near ground l e v e l , which has an e f f e c t on d a i l y l i f e and on crops.

For methodology on an estimation of

the wind environment within a l i v i n g space, the idea of the formulation of a nuisance parameter, put forward by J. Gandemer ( r e f . 6), is very useful, and

52

J.D. I v e r s e n ' s study ( r e f .

7) on sand d r i f t

was used in our i n v e s t i g a t i o n .

• S~'!

Fig. I.

a:a.IT

Te.l :Nn

' "~£~

General topography and modeled area.

SITE DESCRIPTION The small open area in question is shown in Fig. I .

I t i s the c e n t e r o f

a small i s l a n d p o s i t i o n e d a t the n o r t h - e a s t end o f Shikoku I s l a n d in Japan. About 700 m to the north and south o f t h i s small open area t h e r e are two peaks o f 123 m and 198 m h i g h , r e s p e c t i v e l y .

The r i d g e l i n e s of the peaks run down

to the west side o f the area and are connected w i t h each o t h e r in a r i d g e about 50 m h i g h , l i k e a wind fence f o r a n o r t h - w e s t wind.

A small bay extends

about 3 km to the n o r t h - w e s t , where t h e r e i s another small i s l a n d .

The peaks

and the r i d g e are covered by shrubbery, h e i g h t less than I0 m, and are roughly r o l l i n g . shore.

The open area in question spreads g e n t l y down to the east

The p o s i t i o n o f the open cut i s the w e s t - c e n t e r o f t h i s small open area

and resembles a c o r r i d o r f o r n o r t h - w e s t wind. WIND TUNNEL SIMULATIONS Wind t u n n e l , equipment and models An o p e n - c i r c u i t wind tunnel from Tokushima U n i v e r s i t y was used f o r t h i s simulation.

The t e s t s e c t i o n is 1.5 m x 1.5 m c r o s s - s e c t i o n and 5 m long.

53 The maximum wind speed is 18 m s -I .

A h o t - w i r e anemometer was used to

measure mean wind v e l o c i t y and turbulence.

To measure the wind d i r e c t i o n

near the ground surface, many small f l a g s made with t h i n polystyrene foam were prepared. shown in Fig. I .

The t e r r a i n model (scale 1:1500) f o r the e n c i r c l e d area is To f i l l

the t e s t - s e c t i o n o f the wind t u n n e l , i t was

necessary to model some a d d i t i o n a l t e r r a i n upstream of the t e s t area (see Photo I ) .

A skew-scaled model ( h o r i z o n t a l scale I : I 0 0 0 and v e r t i c a l scale

1:500) was also made to c l a r i f y

the e f f e c t of the open cut on local wind

characteristics. The s i m i l a r i t y

law used is based on the c o n d i t i o n of consistency f o r eddy

Reynolds number between a model and a f u l l - s c a l e

s i t u a t i o n ( r e f . 8).

The

r e l a t i o n s h i p VM / VN = ( LM / LN ) I / 3 a p p l i e s , where subscripts M and N correspond to the model and the f u l l - s c a l e

situation, respectively.

Approaching f l o w s i m u l a t i o n As there were no previous wind data which could be simulated in a windtunnel t e s t , and as the approach surfaces were very d i f f e r e n t f o r each wind direction,

i t was assumed t h a t the wind v e l o c i t y p r o f i l e of the approaching

f l o w obeyed the power law.

D i f f e r e n t types of flows were prepared, e.g. the

exponent ~ = 0.12-0.15 f o r north-west and east winds over the sea, and = 0 . 3 - 0 . 4 f o r north or south winds over the mountains.

To generate t h i c k

boundary layers w i t h o u t a long up-wind f e t c h , spires and roughness blocks were used f o l l o w i n g the r e p o r t by H.P. I r w i n ( r e f . 9).

A t y p i c a l example o f 6O

(cm] 5O 40 30 20

~:0.13 Q (5.6)

6(s.s) (7.3)~

10

0

1 2 3 4 5(m/s) 'O.Ci5 ' 0.'10

Photo. I .

T e r r a i n model in t e s t section

Fig. 2. An example of wind s t r u c t u r e : w , wind v e l o c i t y p r o f i l e ; . . . . , i n t e n s i t y of turbulence; ( ) , scale of turbulence.

54 the simulations is shown in Fig. 2. approaching the target.

This is very close

The power spectra obtained f i t

to the flow the empirical

expression of von Karman (shown in the next a r t i c l e ) . Results of wind-tunnel simulations The strong winds experienced in this area can be classified roughly into two groups, one of which is due to typhoons.

\

In this case, the wind direction changes

/

depending on the course of each typhoon.

\si

The other case is the dry monsoons in winter, the direction of which is almost north-west.

Moreover,the effect of an

/

•

open cut on the wind environment of this area could be seen for north-west winds from the results of preliminary windtunnel tests. The measuring points on the model are shown in Fig. 3, where one mesh size corresponds to lO0 m f u l l scale.

Fig. 3. Measuring points. Encircled numbers show f u l l scale measurement points.

Encircled points show the full-scale

measurement positions.

The wind v e l o c i t y at point 35 was chosen as a

reference value and a l l other data were normalized using t h i s value.

co) :

ot 3~,o~ ~,8 oo~

>'

/ VG = 50.4 mls

~

~ o,~

/ j

~ j ~ / ~

L~-,~ot3~

~:,~ ~ _ ~

Fig. 4. Results of wind-tunnel simulation f o r north-west winds; FIN F3 show the wind fences and VG is gradient wind speed (~=0.13). (a) Before open cut. (b) A f t e r open cut. (c) A f t e r fence construction.

55 Figure 4 shows the experimental r e s u l t s , where the mean wind v e l o c i t y and wind d i r e c t i o n (with contours) at 5 m above the ground surface are indicated in (a) and (b), corresponding to before and a f t e r the open cut, respectively.

Numerical values given in parentheses show the wind v e l o c i t y

r a t i o (in %) of after-open-cut to before-open-cut.

Af t er road construction,

the region of strong winds spread to the south, and windspeeds at some points increased more than 150% in the southern area. From these r e s u l t s , i t was decided to build some wind-control fences along the road.

Using the same model, the proper p o s i t i o n , height and length

of the fences were tested with a t r i a l - a n d - e r r o r method, the results of which are shown in Fig. 4(c).

A l l fences were 57% porous fences; the heights of

F1 and F2 were 5 m and the height of F3 was 2.5 m as i t was b u i l t into a slope.

A f t e r the b u i l d i n g of these fences, the approaching winds turned

more to a southerly d i r e c t i o n , and wind speeds at Pts. 73 and 83 increased considerably compared to those before construction.

From t h i s i t can be seen

that the e f f e c t of the fence on the wind here is too strong, although the fences perform t h e i r task of wind control well at a l l other points. In the process of these wind-tunnel simulations, the effectiveness of the a d d i t i o n a l t e r r a i n shown in Photo. 1 was examined.

In the case of a north-

west wind d i r e c t i o n , two peaks were put facing the i n l e t which were not included in the o r i g i n a l t e r r a i n model, and i t was confirmed that the presence of these peaks strongly influenced the wind v e l o c i t y d i s t r i b u t i o n in the open f i e l d in question.

This shows that the use of optional t e r r a i n models is very

e f f e c t i v e in m i t i g a t i n g the r e s t r i c t i o n s of scale problems even a wind-tunnel with a small t e s t section has to be used. FULL-SCALE MEASUREMENTS Measuring instruments and data c o l l e c t i o n As i t was impossible to observe the wind over a long period of time because of the on-site circumstances and the high cost, f u l l - s c a l e measurements were done on the days of a strong wind from the north-west d i r e c t i o n in the winter. Fifteen measuring points were selected in the open f i e l d corresponding to those of the wind-tunnel simulations (encircled in Fig. 3).

An u l t r a s o n i c

anemometer was set at Pt. 35 and the data obtained were used as the reference values f o r a l l other measurements.

Another reference point was chosen at

Pt. 83 where a G i l l - t y p e anemometer with two components was set.

Three

t r a n s i s t o r - t y p e non-directive anemometers were used at the other measuring points and t h e i r positions were changed in each run.

All anemometers were set

at the top of poles 5 m high. The data length of one run was I0 minutes, and at Pts. 35 and 83 wind

56 data were recorded on magnetic tapes during each observation time. F u l l - s c a l e measurements were c a r r i e d out four times, from January to March 1986.

As there was no proper method to estimate the wind d i r e c t i o n of the

g r a d i e n t wind above t h i s area, the measured d i r e c t i o n at Pt. 35 was used f o r convenience.

However, there were some problems about i t s accuracy, because

the wind d i r e c t i o n at Pt. 35 is u s u a l l y north-west. Results o f measurements Two p a r t i c u l a r sets of observations are shown in Fig. 5(a) and (b), in which a l l measured values f o r the d i f f e r e n t runs are c a l i b r a t e d with the values at Pt. 35. Pt. 35.

The numerical values in the f i g u r e are wind v e l o c i t y r a t i o s to

On 22 January, fence F3 had not been constructed on the slope, and

t h e r e f o r e the wind speed at Pt. 64 became very strong (wind speed r a t i o 99%). Comparing t h i s with the data f o r 19 February, we can see t h a t the e f f e c t i v e n e s s o f F3 is apparent.

Fig. 5. D i s t r i b u t i o n s of wind v e l o c i t y and wind d i r e c t i o n f~r f u l l - s c a l e measurements: (a) Observation on 22 Jan. 1986; V35=9.83 m s - ' . (b) Observation on 19 Feb. 1986; V35=8.17 m s - l .

57 Figure 6 gives c o r r e l a t i o n s of wind speeds between the f u l l - s c a l e and the wind-tunnel measurements.

Figure 6(b) shows high p o s i t i v e c o r r e l a t i o n .

In t h i s case, i t is expected that the gradient wind d i r e c t i o n s almost coincide. In Fig. 6(a) the measured values give low c o r r e l a t i o n , the cause of which w i l l be the mismatching of wind d i r e c t i o n s at Pt. 35. Figure 7 shows the power spectra of t u r b u l e n t winds in f u l l - s c a l e and wind-tunnel measurements at Pt. 83.

Model 1.0

(a)

Both of them correspond to the w e l l -

Model 1.0 (b)

/

/

/

O A

A O

0.5

/ I

I

0.5

1.0

Full scale

J

0

L

0.5

1.10 Full sco.le

Fig. 6. Correlation of wind v e l o c i t i e s between f u l l - s c a l e and wind-tunnel measurements: (A,VG=5Om/s; O,VG=25m/s). mixed t u r b u l e n t flow near the ground, and

Io fS(f )/o"z

(b)

agree with yon Karman's spectra (shown by the smooth l i n e in the f i g u r e ) . According to the scaled-model simu-

05

l a t i o n s , the wind speed at Pt. 73 is not decreased by the construction of fences. The f u l l - s c a l e data also show a s i m i l a r

0.1

1.0

fL/U

tendency; the wind-speed r a t i o at Pt. 35 is 72%, and is comparatively large compared to other points because of the

i.o

fS(f )/0-~

(a )

leading e f f e c t of the fences on the flow. At t h i s time, we have no proper standard of judgement to use to evaluate the r e s u l t s of wind-tunnel simulations L=I0.I rn

and f u l l - s c a l e measurements, but in estimating the general features of a wind

i

i

0.1

1.0

fLtTJ

environment, i t is expected that s a t i s f a c t o r y r e s u l t s can be obtained with the use of a small t e r r a i n model l i k e t h i s .

Fig. 7. Examples of normalized power spectra. (a) Wind-tunnel simulation. (b) F u l l - s c a l e measurement.

58 ESTIMATIONS OF LOCAL WIND PROPERTIESWITH SIMPLE TERRAIN MODELS Simple open-cut models with trapezoidal cross-section were used to estimate the change of local winds due to an open cut.

The t e r r a i n parameters

to be examined were the height and slope of the ridge, H and ~, respectively, and the width and slope of the open cut, W and e, respectively, which are shown in Table 1 and Fig. 8.

Approaching flow was generated with a spire as

in the case of t e r r a i n models, and the exponent was ~ = 0.15. Table 1 Dimensions of trapezoidal models

Height of model: H (cm)

=

2.5,

5.0,

7.5,

Angle of slope: (deg)

=

35,

45,

55

Width of open cut: W (cm)

=

0.0,

Angle of open cut-slope: (deg)

=

35,

I0.0

5.0, I 0. 0, 40,

20.0,

45,

50,

30.0 55,

60

Figure 9 shows the wind v e l o c i t y d i s t r i b u t i o n s down stream from an open

Z

cut, which correspond to the change of height H under a constant base-length B.

:_w i

I t is clear that the wind v e l o c i t y

convergence is increased with the increase of height H. This r e s u l t also suggests that a skew model w i l l overestimate changes in the wind environment.

Fig. 8. Notations of a simple t e r r a i n model.

The influence of open cut parameters W and o on the wind speed is given in Fig. I0, where measured wind speed is normalized by the gradient wind speed.

The horizontal l i n e in each figure corresponds to the wind speed of

the approaching flow at height Z/H=O.I.

When the open-cut area is small, the

wind speed-up r a t i o at the center of the open cut is about 150% of the approaching flow and, as the area becomes wider, the r a t i o is decreased to about 120%. On the other hand, the downstream spread of a strong wind from a small open-cut area is very small compared to that of a large open-cut area. For a small open-cut area, the pressure difference between upstream and downstream of the ridge becomes large, hence the convergence e f f e c t of the wind flow is stressed and i t s down-wash v e l o c i t y g r e a t ly increases.

However,

because of the small volume, the flow diffuses r a p i d l y in the cross-wind d i r e c t i o n behind the ridge.

59 ~ ;lr7cI

X/ B -3

~..~.~W_ndi

X/H

~ . 6

6 ,4

-2

-3

X/H X/B

X/H -3

,•,l'l

.6

_L}

-1

-2

-1

0

.4

i nd

r3

0

02

0

o

)

1 4-

p

2

3

3

4

i4

6 L~_2 .:_.

10 i

|

i

A

I

I

0 2 4 6 (a) " ~ ~ ...... 35~

I

i

8Y/H

I

1

(b)

i

2

1 I

2 H :

i

3 I

1

3

4

8 f 'B

0

I

l.S cm

1 I

i

0

v/H

1

;

I

Y/B I

2

I

3

H : I0

~'C~

!,=:~=45°

2

V/H

cm

dj=9=55~

F i g . 9. Wind v e l o c i t y d i s t r i b u t i o n s down stream o f an open c u t (W = 5 cm; Z = O.2H; B = 19 cm; Vmean = 5 m s - l ) .

I 0 U/U0

\

wAo

~-~

1.0 U/Uo

T =o,~

o. 5.~

~

1.0

~I~

O.

U/Uo W:3Ocm

O. 51 X/H

,

0

1

5 I0 0 5 I0 ( o : ',=59 °. z ~ : ~ = 5 1 . 3 ° 0 : 0 = 39.8 °

' .5 ' 1 6 Z/H=O.I

Fig. I0. Change o f wind v e l o c i t y a l o n g the c e n t e r l i n e o f an open c u t due t o t h e change o f o p e n - c u t a r e a : Uo i s g r a d i e n t wind speed.

To s p e c i f y characteristics

the particular

wise method i s used. coordinate

terrain

around an open c u t , The c r i t e r i o n

t o the wind

regression analysis

variable

i s the wind v e l o c i t y ,

(X, Y, Z ) , and e x p l a n a t o r y v a r i a b l e s

speed a t the c e n t e r o f the open c u t , The r e s u l t

parameters r e l a t i n g

multiple

o f the s t e p U, a t any

are H, ~, W, e, the wind

Uc, and t h e i r

2nd-order combinations.

i s as f o l l o w s :

U = 1.736 + 0.0289W - 0.0387Y + 0.0257Z + 0.442(W - 15)(U c - 2.64) -0.0553(U c - 2 . 6 4 ) ( Y - 9 . 3 2 ) + O.O019(W - 9 . 2 3 ) ( X - 24.98)

60 The m u l t i p l e c o r r e l a t i o n c o e f f i c i e n t d e t e r m i n a t i o n is 0.763.

is 0.873 and the c o e f f i c i e n t

of

The parameters H, ~ and e vanish in t h i s a n a l y s i s

because t h a t i n f o r m a t i o n is a l l

included in parameter Uc.

CONCLUSION Because the t e r r a i n in question is very complex, the wind v e l o c i t y distribution

in an open f i e l d

g r a d i e n t wind d i r e c t i o n . case. I.

changes remarkably f o l l o w i n g a s l i g h t

change o f

Therefore, t h i s p r o j e c t was not completed f o r every

A summary o f the r e s u l t s is shown below. The r e s u l t s o f wind-tunnel t e s t s are g e n e r a l l y in agreement w i t h those of full-scale

measurements.

To make a more p r e c i s e comparison, i t

necessary to determine the g r a d i e n t wind d i r e c t i o n

is

in the f u l l - s c a l e

situation. 2.

As an open cut in a r i d g e g e n e r a l l y has a d i r e c t e f f e c t on the wind p r o p e r t i e s downwind, wind environmental assessment sometimes becomes necessary.

When the wind-tunnel s i m u l a t i o n method i s chosen f o r t h i s

purpose, t h e r e should be a r e s t r i c t i o n

of the scale o f the model.

In

t h i s case, the use of o p t i o n a l t e r r a i n models is very e f f e c t i v e . 3.

The power spectra measured near the ground o f a complex t e r r a i n agree well w i t h Karman's type.

The r e s u l t s o f wind-tunnel t e s t s also show the

same tendencies. 4.

The r e s u l t s w i t h some simple t e r r a i n models show t h a t the e f f e c t o f an open cut on the downwind area can be estimated from the w i d t h o f the open cut and the wind v e l o c i t y a t the c e n t e r o f the open c u t . expected t h a t a l l

I t can be

t o p o g r a p h i c a l i n f o r m a t i o n i s included in the wind

v e l o c i t y a t the c e n t e r of an open c u t . ACKNOWLEDGEMENTS The authors wish to thank the Honshu-Shikoku Bridge A u t h o r i t y f o r o b t a i n i n g the f u l l - s c a l e

measurements.

The authors are also g r a t e f u l f o r the

support o f Science Research Grant No.60025038 from the M i n i s t r y o f Education, Japan. REFERENCES (I) (2) (3)

P. Jackson and J. Hunt, T u r b u l e n t f l o w over a low h i l l , Quart. J. R. Met. Soc., I01, 1975. H. W. Teunissen, Wind-tunnel and f u l l - s c a l e comparisons o f mean f l o w over an i s o l a t e d low h i l l , Journ. Wind. Eng. and Indust. Aero. Vol. 15, 1983. H. W. Teunissen and P. T a y l o r , The Askervein H i l l p r o j e c t : F u l l - s c a l e measurements and model comparisons o f wind f l o w over an i s o l a t e d h i l l , Proc. 5th U.S. Nat. Conf. on Wind Eng., Nov., 1985, Texas.

61 (4) (5) (6) (7) (8) (9)

H. W. Teunissen and M. Shobr, Wind tunnel/Full scale comparisons of boundary-layer flow over Askervein h i l l , Scotland, Proc. Asia Pacific Sympo. on Wind Eng., Dec., 1985, India. A. J. Bowen, The prediction of mean wind speed above simple 2D h i l l shapes, Journ. Wind Eng. Indust. Aero., VoI. 15, 1983. J. Gandemer, Simulation and measurement of the local wind environment, Proc. I n t . Workshop on Wind Tunnel Modeling C r i t e r i a and Tech. in C i v i l Eng, A p p l i . , A p r i l , 1982, USA. J. D. Iversen, Small scale modeling of snow-drift phenomena, Proc. Int. Workshop on Wind Tunnel Modeling C r i t e r i a and Tech. in C i v i l Eng. A p p l i . , A p r i l , 1982, USA. S. Nemoto, S i m i l a r i t y between natural wind in atmosphere and model wind tunnel Part I . , Pap. Met. Geophys. 12, 1961. H. P. Irwin, Design and use of spires for natural wind simulation, National Research Council of Canada, NAE. Report, LTR-LA-233, 1979.