Journal of Wind Engineering and Industrial Aerodynamics, 41-44 (1992) 959-970 Elsevier

959

ACTIVE GENERATION OF WIND GUST IN A

TUNNEL

Hiroshi Kobayashi I and Akihide Hatanaka 2

TWO-DIMENSIONAL WIND

1 Professor, Dept. of Civil Engineering, Ritsumeikan University, Tojiin Kitamachi 56-1 Kitaku, Kyoto, 603 Japan 2 Graduate student, Dept. of Civil Engineering, Ritsumeikan University

ABSTRACT

In this paper simulation of wind gust was tried in a Eiffel type wind tunnel by an active control technique. Along-wind and vertical gusts were obtained by driving arrays of plates and airfoils, respectively, in a two-dimensional wind tunnel. By this technique a large-scale turbulence was obtained with the velocity spectra well fitting target spectra.

NOTATION

c = chord length(mm);

f = frequency(Hz);

fu = upper cutoff frequency(Hz);

Iu,I w = turbulence intensity of horizontal and vertical velocity, respectively;

Lu,L w = integral scale of horizontal and vertical velocity respectively, calculated from the integral of auto correlation;

Lx,u = windward spatial scale calculated from the integral of space correlation at two points along a windward direction;

N = number of subdivisions on wind spectra;

s = space of airfoils(mm);

Su(f),Sw(f) = one-sided power spectral density of and vertical velocity, respectively;

horizontal

t = time;

Rx,Ry,R z = spatial correlation coefficient of horizontal velocity at two points along windward, lateral and vertical directions, respectively;

016%6105/92/505.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.

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= mean wind velocity;

u(t),w(t) = time series of horizontal fluctuating velocity;

and vertical

df = frequency increment;

Zt = time increment;

~j = random phase angles, uniformly distribute spanning from 0 through 2 = ;

SUBSCRIPT

u,w = horizontal and vertical component veiocihy, respectively;

of wind

x,y,z = orthogonal coordinates, the longitudinal axis x lies in the windward direction, y and z are the lateral and vertical axes;

I. INTRODUCTION

In order to examine a safety against aerodynamic excitations of long-span bridges a wind tunnel test is carried out usually, in a smooth flow.

On the other hand, natural wind is turbulent. In general the aeroelastic instabilities, observed in a smooth flow, are stabilized due to the effects of turbulence. However, additional buffeting response is generated by turbulence[l].

In order to examine more rationally the safety of long-span bridges in a natural wind, wind tunnel tests should be carried out in a turbulent flow similar to natural wind. For that reason the turbulent flow similar to natural wind is desired to be accurately simulated in a wind tunnel.

In a field of wind engineering there are several techniques of turbulence generation. Boundary layer turbulence is produced by roughness blocks and spires, which are placed on an upstream of the test section. Such flow is used in three dimensional wind tunnel tests[2]. The mean wind profile can be modified through change in an arrangement of the blocks or the shape of spires[3,4]. However, simulation of a scaled boundary layer flow, similar to that of the natural wind, requires a large wind tunnel with a long test section. In a small wind tunnel with a short test section grid turbulence is typically used[5]. Turbulence generation by a grid is easy. The data on the properties of grid have been accumulated[6]. Typical grid turbulence has too small length scale, required to properly model wind. Recently a few active generation techniques have been tried instead of the above-mentioned passive techniques. B. Bienkiewicz et al. presented an active gust generating device, which was composed of two arrays of airfoils[7]. They also used a pulsating grid[8]. These active techniques are based on a quite new concept, additional

961

energy, to generate turbulence, is supplied to the air flow by placing active gust generating devices. The results show that the active techniques can simulate large scale turbulence in a short test section and that the turbulence parameters can be arbitrarily controlled.

The authors have tried to simulate wind gust in a two- dimensional wind tunnel by using an active gust generator, consisting of arrays of plates and airfoils. This device provided a large scale t~rbulence compared with passive techniques. The measured spectra of wind gust well fitted the given spectra[9,10].

This paper describes the active turbulence simulation developed by the authors and discusses the turbulence properties of the simulated wind gust.

2. PROCEDURE OF ACTIVE TURBULENCE SIMULATION

A schematic diagram of active turbulence simulation is shown in Fig.l. The turbulence properties, which are taken into consideration in a two-dimensional wind tunnel test, include the mean wind velocity and the power spectra. Other turbulence properties, turbulence intensity and turbulence length scale, are derived from power spectra. There are several experimental expressions for power spectra of natural

Potensiometer

\\\\\\\\%\\\

][[~ Hot-wire

,•tor Target series spectra

# ~ Measured "-k ~ spectra

Comparison

7

Wind speed

1

Fig.l ~| ...... atlc diagram of active simulation

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wind velocity[ll]. In this study K~rmAn's expressions are chosen as the target spectra of simulated turbulence. K~rmAn's expressions give the power spectra of horizontal and vertical velocity component as follows:

Su (f) =4iu2~Lu{ i+70.8 (fLu/U) 2 } (-5/6) (l)

Sw(f)=4iw2ULw{l+755.2(fLw/U)2}{ (l+283.2(fLw/U)2} (-11/6) (2)

Gaussian random process u(t) is specified by the one-sided wind spectra Su(f), defined for 0<f<f.. Then the time series of random process can be produced by u the discrete inverse Fourier transform as follows[12]:

N U(tm)=,/ '~" Re [ Z [ S u ( f j ) / l f ] l / 2 e i (2:~jm/N+ ~bj) ] m=i} , l , 2 . . . . .N- I ( 3 )

j : l

The time, tm, is defined by mz~t. zJt is the time increment. The frequency, fj, is defined by jz~f. zJf is the frequency increment, defined by z~f=2f /N. Phase angle informations ~j

• U

for each frequency are glven by uniformly distrlbuted rando~ numbers spanning from 0 through 2~ To reduce computation time, Eq. (4) is computed by using the FFT algorithm[13]. Time series of process w(t) can be obtained from Eg.(4) by replacing Su(f j) by Sw(f ).

Time series'are transformed into voltage data to drive an AC servomotor which actuates an array of plates. An angle change of the plates gives time varying wind velocity. The spectra of the measured fluctuating wind velocity don' t coincide with the tr2get spectra during the initial simulation. Therefore, the time series data have to be modified by considering the difference between the target and the measured spectra. The difference is estimated by the ratio of the target to the measured spectra. The modified spectra is computed by the target spectra multiplying by the above ratio. Again time series data are generated from the modified spectra using Eq.(4) and transformed into the voltage data for the second turbulence simulation. This modification is repeated several times until the measured spectra nearly coincide with the target spectra. After geveral modifications, Turbulence intensity and turbulence length scale also nearlv coincide with the target values.

3. EXPERIMENTAL FACILITIES

The experiments are carried out in an Eiffel type wind tunnel with a working section of 700mm x 1000mm. A wind duct whose cross section is 700mm x 600mm and whose length is 1200mm, is installed in the wind tunnel(as shown in Fig.2). The arrays of airfoils and plates are respectively placed at the outlet and the entrance of the wind duct as shown in Fig.2. The array of 12 plates consists of fixed plates and driving plates. The plates are intended to control the horizontal wind gust by changing the blockage ratio of cross section.

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The array of airfoils contains 16 airfoils with a chord length of 100mm. A space between adjacent airfoils is 40mm. The airfoils are intended to control the vertical wind gust. To reduce the turbulence by the vortices which separate from the array of plates, a grid ( rectangular bars 5mm x 5mm with 50mm center-to-center spacing) and meshes (A : 1/8 inch mesh size, B : 1/12 inch mesh size) are installed between the arrays of airfoils and plates.

A fluctuating flow, generated by the driving of plates, is ejected as an open jet from the outlet of the wind duct.

The arrays of airfoils and plates are driven by AC servomotors controlled by a micro computer. The fluctuating vertical and horizontal components of the flow are measured by a hot wire anemometer.

O )ening

Nozzle

i i

W ~ q) q)

Mesh A Mesh B

Grid 1400

1200

" i ! :

" i

, E i !

f 20. .0 _ 4 0 0 _ _j 2 0 0

1 0 0

2o9

Z

O

\ w~na a,~nt Array of airfoils

Measurement point

.-----D- X ® (X,Y,Z)

I Unit:mm Array of plates

Fig.2 Experimental facilities

4. EXPERIMENTAL RESULTS

4.1 Fundamental properties of gust generator

(I) Relation between angle of airfoils and mean wind angle

Properties of the flo~ downstream of the array of airfoils depend on a geometrical size of the a~[~i ~ of airfoils, a chord length of the airfoils and a airfoil spacing. The flow downstream of airfoils was investigated

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through changing the chord length and the space. As a result, the chord length and the space, which were appropriate for the control of vertical fluctuating flow, were determined.

The mean wind angle is shown in Fig.3 as a function of the angle of attack of the airfoils. The chord length, the space and the chord/space ratio are given in the figure. The flow is measured at the distance 1000mm downstream from the array of airfoils.

It is recognized that the chord length and the space play an important role for the mean wind angle. Namely, the longer the chord length or the narrower the space (defined in Fig.3) , the larger the wind angle. However, the chord length should not be too long, in view of consideration of wake turbulence (by a development of laminar boundary layer along a surface of airfoil) and the increase of airfoil mass moment inertia.

Based on the above results, the array of airfoils was made with chord c=100mm and space s=40mm. Wind angle for this array of airfoils was measured again. The mean wind angle is shown in Fig.4. Thi~ figure shows that the farther the measured points from the array of airfoils, the higher the reduction

3.75( [50/40 ) - ~ s.oo(2ooj~o~

~ L _ _ . . . . ~ Q e.5o(zooleo)

0 t0 20 ~), (de,)

~ (deg)

20

15

10

5

0

c=0. I s= 40 mm

--o-- x=~ooj U=2.0 m/s - . , ,(]p- - x ~ : ' o o I

J ' I I I I

10 20 30 e , (dee)

Fig.3 Mean wind angle versus angle of airfoils

Fig.4 Mean wind angle at difference points

in the mean wind angle. It can be seen that the mean wind angle is roughly proportional to angle of airfoils up to the airfoil angle of attack 0a=15~

(2) Turbulence due to wake vortex behind an airfoil

Vertical distribution of wind velocity in downstream of the array of airfoils is disturbed by the wake vortex produced by an airfoil. In order to confirm the above phenomenon and take a countermeasure, a flow-visualization (by smoke-wire method)

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of the wake behind the array of airfoils was carried out. airfoils were kept horizontally.

The

(a) in a smooth flow (b) in a turbulent flow

Fig.5 Flow visualization of the wake behind the array of airfoils

At first the array of airfoils was installed in flow. Fig.5 (a) shows the wake behind the array of Wind flows from the left to the right ( U=2.0 m/s photograph shows that alternating vortex sheets are

a smooth airfoils. ). This produced

Z(cm)

6

0

-5

0 ~mooth

O T u b u l e n t

Z(cm) g o §

9) o

O N

m ~

g o 0

2 , , o 3 0(~) o o I

OCD

I O 0 0 I l l -~

© g o

o •

o I

l

oo I

cD 00

- D

o

I0 o o

0 Smooth

O Turbu len t

o

! I

20 lug)

o o

(a) Distribution of mean wind velocity

(b) Distribution of turbulence intensity of horizontal component

Fig.6 Vertical distribution

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at vertical regular intervals. The intervals of the sheets coincide with the airfoil spacing. Fig.6 shows a vertical distribution of the horizontal velocity component. O and • are respectively experimental value in the smooth and the turbulent flow. The reduction in the mean wind velocity and the increase in turbulent intensity behind the airfoils can be found from the figures. These changes are caused by the alternating vortex sheets. It is known that it should be possible to eliminated such vortices by a boundary-layer control.

A grid ( rectangular bars 10 mm x 10 mm with 100 mm center- to-center spacing ) was installed upstream of the array of airfoils and thus the airfoils were exposed to the turbulent flow generated by the grid.

Again the flow-visualization of the wake behind the array of airfoils is carried out. Fig.5(b) shows the result. It is shown that the vortex sheets, observed in a smooth flow, were transformed into an uniform turbulence. The mean velocity U and the turbulence intensity I u are shown in Fig.6. Both U and I, are nearly uniform. The effect of wake vortex behind the a~rfoil is not observed.

(3) Relation between angle of plates and mean wind velocity

A relation between wind velocity and an angle of plates is investigated next. A flow was measured at the distance 600mm downstream from the array of airfoils. Fig.7 shows the mean wind velocity and the turbulence intensity I u versus angle of plates. U/U 0 in the ordinate axis means a rate of reduction in mean wind velocity, where U 0 is mean wind velocity when the control angle Op=O%

Mean wind velocity is reduced monotonously with an increase in angle of plates. Turbulence intensity has a tendency of an increase with the angle of plates.

01~0 I l, (~) 0/00 _~

I.o~ i o ~ / ~..a2' Measurement point

°I

o 5 Io 15 2o 25 30 35 40OD (deg)

Fig.7 Rate of drop in mean wind velocity and turbulent intensity of horizontal velocity component versus angle fo plates

967 4.2 Turbulence simulation

Mean velocity U=2.0m/s, turbulence intensity I~=12.0%, Iw=6.0% and the integral scale of turbulence Lu=120cm, Lw=36cm were set up as target parameters of the K~rm~n's power spectra, and the wind gusts were simulated. The flow is measured at the 600mm downstream from the array of airfoils. The convergence to the target values of turbulence properties and the number of modification is shown in Fig.8. The measured values of I u and I w are in good agreement wlth the target values after the first modification. The

I~ Measurement point (X,Y,Z)=(600,O,O) ,, \ . . . . _ . . ~ . . . . , . . . . . . . . . . .

ha

<

?

Iu

• • iw

LW

J I I I

Number of modification

measured values of L u and Fig.8 Convergence to target values also agree well with of turbulence versus number of

~e target values after modification the first modification.

Fig.9 shows the measured spectra of the horizontal and the vertical velocities together with the target spectra. For Iu=12.0%, ( Fig.9 (a) ), the measured spectra are in good agreement with the target ones over a wide range of frequency.

When Iu=7.0%, ( Fig.9 (b)), the measured power spectra have larger values than the target ones in a high frequency region. The turbulent energy excess in this region is due to the turbulence by the vortices which separate from the arrays of plates and airfoils. The turbulence can be reduced by some meshes adequately installed.

4.3 Spatial properties of simulated wind gust

The two-point spatial correlation coefficients for the horizontal velocity in lateral, vertical and windward directions (R., R z, R x) were calculated. The results are shown in Fig.10. ~he reference point is located at the distance 600mm downstream from the array of airfoils. The lateral correlation is high over a wide region. The distribution of the mean velocity is nearly uniform. The vertical correlation is high over a limited region. The vertical distribution of the wind velocity is not uniform.

The spatial correlation coefficient in the windward direction was fitted by an exponential function Rx( ~,0)=exp( -

/Lx, u) by means of the least square method. Where, Lx, u

968

I 0 "

IO-a

ID \

iO-J

~1 10' '

~Target - ' - - - - ' ~ ~ # lu:12,0I

Lu=12Ocm

Measured hl:12.01 LU=IIS~B

lO-S

Horizontal tom ~onent tO.e. ! L IH,'.,,I r ,,,,.,! , ,,,,,,,I , ,tt,ttJ

I0 "~ tO' ' I0 ' I0 ' tO z

f (Xz)

l O ' i

iOiZ eu

I;D \

IO "3

i I0""

i i0.~ '

i0-~

/

Measured Iw:f l .Ol Lw:41ci

Target Iw=6.Ol Lw=36cB

I i ertical component

I I IIIIIII I Iiii1111 I IIIIIIII I llllll

IO "a tO' ' I0 ' IO' IO 2

f (Hz)

(a) Target value : Iu=12.0%, Iw=6.0%, U=2.0m/s, Lu=120cm Lw=36cm (7th modification)

N

ID \ .... I 0 "

I 0 "

lU:7,0%

L.:120¢m

i0.~ ..... " %

FlorlzontaL component I O" S LLI/U~.._

I0"~ I0 ' ' IO t 101 I0 e -~lXz)

lO*i

i0-~

I0 " n

3 I 0 " I/)

IO-Q

Target / I v : 3 .5 l

,.... / I,v:3BCll

Measured % Iv=] ,81 tvT33cI

Vertical component lO'l J LLIIIIILI I IIIIIII I I llllll| I I IIIII

I0 "a I0 "t IO* 101 IO ~ f(Hz)

(b) Target value : Iu=7.0%, Iw=3.5%, U=2.0m/s, Lu=120cm Lw=36cm (4th modification)

Fig.9 Measured and target spectra of turbulent velocit~

denotes the spatial scale. L x , was found to be about 107cm. On the other hand, integral s6~le in this direction calculated from auto correlation coefficient was 124cm. The ratio of spatial scale to integral one was about 1 : 1o16. This value roughly agrees with the observed values in the natural wind, e.g. wind from the west; 1 : (0.75-~1.20) at the northern

969

part of Osaka Harbor, Japan, J14].

5. CONCLUSION

In the paper, the turbulent flow similar to a natural wind has been simulated by using an active gust generator, ( consisting of two arrays of airfoils and plates ) in a two- dimensional wind tunnel. The conclusions of the study are summarized as follows.

i) Large turbulence scale was obtained by the active gust generator. Power spectra of the simulated wind gust agreed well with the target spectra over a wide range of frequency. In the case of the wind gust with sma] intensity (Iu=7.0%, Iw=3.5%), the measured power spectra had larger values than the target ones in a high frequency region. The turbulence by the vortices which separate from the arrays of plates and airfoils has to be reduced.

2) The lateral correlation of the horizontal wind velocity was high over a wide region and the lateral distribution of the mean velocity was nearly uniform. However the vertical correlation of the wind velocity was not uniform.

3) Along wind spatial correlation was similar to that of the natural wind.

ACKNOWLEDGMENTS

The authors wish to thank Assoc. Prof. M. Kawatani of Osaka University for his valuable advice. They also wish to thank Mr. H. Kim and Mr. K. Ota(Presently, Officer, Dept. of Urban Development, Mitsubishi Co.), graduate students at Osaka University, for their cooperation. Contributions of students at

U ( r n ~ " s )

eoooo • • o 2 r o • • o o e o o

1.5f Ry

I ooooo o o o'I° o o 0% 0 0.5

J t t i i I J I I i I I i I

-35-3o-2s-2o-15-Io-5 o 5 1o ~5 20 2~ 30 3s < e. rn)

(a) Lateral direction

~: < c r n ) 30 25 O •

o 2O 15 0 _ ~ ,o o0% 5 0 "

-5

-I0 I -15 -20 -25 -~0 Rx !

0 0

0.5 ~ 1 1,5 ~2

o • O •

(b) Vertical direction

(cu)

(c) Windward direction

Fig.10 Spartial correlation coefficients

970

Ritsumeikan University are greatly appreciated.

REFERENCES

1 R.L. Wardlow, H. Tanaka, H. Utsunomiya, "Wind Tunnel Experiments on the Effects of Turbulence on the Aerodynamic Behaviour of Bridge Road Decks," Proc. of 6th International Conference on Wind Engineering, Australia, March, 1983.

2 R.L. Wardlaw, S.J. Zan, "A Wind Tunnel Investigation of the Theodore Roosevelt Lake Steel Arch Bridge," Proc. of Canada-Japan Workshop on Bridge Aerodynamics, Canada, September, 1989, pp73-82. J.E. Cermak, "Laboratory Simulation of the Atmospheric Boundary Layer," A.I.A.A.J., Vol. 9, 1971, pp1746-1754 H.P.A.H. Irwin, "The Design of Spires for Wind Simulation," Journal of Wind Engineering and Industrial Aerodynamics, 7, 1981, pp361-366

5 T. Mori, "Simulation of Atmospheric Turbulence in a Wind Tunnel by a Grid," Proc. of 5th Symposium on Wind Effects on Structures, 1978, pp.275-281 (in Japanese)

6 H. Okanan, "Study on The Effect of Fluctuating Flow Exerting Wind Effects on Structures," Doctoral thesis, Kyoto University, 1988 (in Japanese)

7 B. Bienkiewicz, J.E. Cermak and J.A. Peterka, "A New Technique of Modeling Atmospheric Turbulence for Wind- Tunnel tests of Bridge Models," Proc. of the Fourth U.S. National Conference on Wind Engineering Research, University of Washington, Seattle, U.S.A., 1981, pp.i13- 116

8 B. Bienkiewicz, J.E. Cermak, J.A. Petrka and R.H. Scanlan, "Active Modeling of Large-Scale Turbulence," Proc. of 6th International Conference on Wind Engineering, Australia, March, 1983.

9 A. Hatanaka, H. Kobayashi and Y. Mishima, 1989, "Two -Dimendional Turbulence Simulation in an Eiffel Type Wind Tunnel," Proc. 44th Annual Conference of the Japan Society of Civil Engineers, 1-402, pp.856-857(in Japanese)

10 H. Kobayashi and A. Hatanaka, "Active Simulation of Turbulence in an Eiffel type Wind Tunnel," Memories of Research Institute of Science and Engineering, Ritsumeikan, Univ., Kyoto, Japan, 1989, No.48, pp.35-54 (in Japanese)

ii E. Simiu and R.H. Scanlan, "Wind Effects on Structures - An Introduction to Wind Engineering,", Second edition, John Wiley & Sons, New York, 1986, Chap. 2

12 J.G. Belivieau, R. Vaicaitis and M. Shinozuka, "Motion of Suspension Bridge Subject to Wind Loads," Journal of the Structural Division, ASCE, Vol. 103, No. ST6, June, 1977.

13 J.S. Bendat and A.G. Piersol, "Random Data, Analysis and Measurement processing,", Second edition, 1986, John Wiley and sons, New York

14 S. Komatsu, H. Kobayashi, M. Kawatani and M. Kamei, "Wind Characteristics of the Osaka North Harbour," Proc. of 9th National Symposium on Wind Engineering, 1986, pp. 13-18, (in Japanese)

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