characteristics of the low-speed wind tunnel of the unne

*Corresponding author.

Journal of Wind Engineeringand Industrial Aerodynamics 84 (2000) 307}320

Characteristics of the low-speed wind tunnelof the UNNE

Adrian R. Wittwer!,*, Sergio V. MoK ller"!Facultad de Ingenieria de la Universidad Nacional del Nordeste, Av. Las Heras 727,

3500 Resiste&ncia (Chaco), Argentina"PROMEC, Universidade Federal do Rio Grande do Sul, Porto Alerge, RS, Brazil

Abstract

This paper presents the evaluation of the characteristics of the open-loop low-speed UNNEWind Tunnel to verify its applicability to similarity studies and to experimental simulations ofthe atmospheric boundary layer. For this purpose a hot wire anemometry system was imple-mented for the measurements of mean velocity and velocity #uctuations. Data acquisition wasperformed by means of an A/D converter board connected to a personal computer. Experi-mental results are presented in form of velocity pro"les and turbulence intensities as well aspower spectral distributions of the axial component of the velocity #uctuations. Results ofmeasurements in the empty tunnel showed a uniform velocity "eld and low turbulenceintensities. Analysis of atmospheric boundary-layer simulations by means of Counihan andStanden methods showed the adequecy of the tunnel for natural wind simulations. ( 2000Elsevier Science Ltd. All rights reserved.

Keywords: Low-speed wind tunnel; Wind tunnel; Natural wind simulation

1. Introduction

Wind tunnels are equipment designed to obtain air #ow conditions, so thatsimilarity studies can be performed, with the con"dence that actual operationalconditions will be reproduced. Once a wind tunnel is built, the "rst step is theevaluation of the #ow characteristics and of the possibility of reproducing or achiev-ing the #ow characteristics for which the tunnel was designed.

The UNNE Wind Tunnel, located at the Northeast National University at Re-sisteH ncia (Chaco), Argentina, is a low velocity atmospheric boundary-layer wind

0167-6105/00/$ - see front matter ( 2000 Elsevier Science Ltd. All rights reserved.PII: S 0 1 6 7 - 6 1 0 5 ( 9 9 ) 0 0 1 1 0 - 5

Nomenclature

f Frequency (Hz)I6

Local turbulence intensityk Von KaH rmaH n constant (k"0, 4)¸96

Longitudinal integral length parameter (m)Re Reynolds numberS Model scale factor of a boundary-layer simulationt Time (s)u`1

Mean dimensionless velocity (;/;.)

u`'

Dimensionless velocity (;/;(z'))

;M Time-averaged local #ow velocity (m/s);M

.Reference velocity (m/s)

;M (z') Velocity at gradient height (m/s)

;M (10) Velocity at 10 m height (m/s)x Coordinate in #ow direction (m)y Coordinate transverse to #ow direction (m)z Vertical coordinate (m)z'`

Gradient height (m)z'

Dimensionless height (z/z')

z0

Roughness length parameter in law of the wall (m)z$

Zero-plane displacement in law of the wall (m)a Power-law exponentU

uAutospectral density of the longitudinal velocity #uctuation (m/s)2/Hz

o Density (kg/m3)p2u

Variance of u (m/s)2pu

Standard deviation of u (m/s)l Kinematic viscosity (m2/s)

tunnel, built with the aim to perform aerodynamic studies of structural models. Thedistribution of the #ow impinging on the structural model must be such that theatmospheric boundary layer at the actual location is reproduced. This is obtainedwith help of turbulence promoters and vortex generators, so that natural windsimulations are performed.

The open literature presents many evaluation studies of wind tunnels, some ofwhich are the tunnel in Garston, Watford, UK [1], the closed-loop wind tunnel inLondon [2], where the so-called Counihan-method for boundary-layer simulationwas developed [3], Oxford, UK [4], the TV2 Wind Tunnel at Porto Alegre [5] and ofLangby, Denmark [6]. A partial mapping of the velocity "eld in the UNNE WindTunnel was presented by De Bortoli et al. [7].

In general, tunnel evaluation is performed at the highest #ow velocity, the resultsbeing presented in terms of mean velocity distributions, turbulence intensities andscales. Boundary-layer simulations are performed with help of grids, vortex

308 A.R. Wittwer, S.V. Mo( ller / J. Wind Eng. Ind. Aerodyn. 84 (2000) 307}320

generators and roughness elements, to facilitate the growth of the boundary layer.This is used in the most applied simulation methods, namely the full-depth simulation[3] and part-depth simulation [8]. The use of jets and grids is also applied [5].

The purpose of this paper is to present results of measurements performed toevaluate the characteristics of the UNNE Wind Tunnel and to verify its applicabilityto similarity studies in structural models and to simulate the atmospheric boundarylayer, as described in greater detail in Wittwer [9].

2. Experimental technique

Fig. 1 shows a schematic view of the UNNE Wind Tunnel, which is a 39.56 m longchannel. The air enters through a contraction, passing a honeycomb and a screenprior to reach the test section, which is a 22.8 m long rectangular channel (2.40 mwidth, 1.80 m height) where two rotating tables are located to place structural models.The upper wall can be displaced vertically to allow conditions of zero pressuregradient boundary layers. The test section is connected to the velocity regulator andthis in turn to the blower, which has a 2.25 m diameter and is driven by a 92 kWelectric motor at 720 rpm. The air passes through a di!user before leaving the windtunnel.

The simulation of natural wind on the atmospheric boundary layer was performedby means of the Counihan and Standen methods with velocity distributions corre-sponding, according to Brazilian Standard NBR-6123 [10], to a Class IV ground,de"ned as `ground covered by several closely spaced obstacles in forest, industrial orurban territorya. The mean height of the obstacles is considered to be about 10 m,while the boundary layer thickness is z

'"420 m. Similar classi"cation is given by

Argentine Standards CIRSOC 102 [11] as a class III ground. The potential law forvelocity distribution is given by

;M (z)/;M (z')"(z/z

')a, (1)

and

;M (z)/;M (10)"(z/10)a (2)

with suitable values for the exponent a between 0.23 and 0.28 [12]. This law is of goodapplication in neutral stability conditions of strong winds, typical for structuralanalysis.

For Counihan full-depth simulation, where the complete boundary-layer thicknessis simulated, four 1.42 m high elliptic vortex generators and a 0.23 m barrier wereused, together with prismatic (30]30 mm base, 22 mm height) elements, 80 mm apartplaced on the test section #oor along 17 m.

Standen part-depth simulation method was implemented with the same roughnesselements used in Counihan method and two 1.5 m spires as vortex generators, tosimulate the lowest part of the boundary-layer thickness.

Scale factors of both atmospheric boundary-layer simulations is determinedthrough the procedure proposed by Cook [13], by means of the roughness length

A.R. Wittwer, S.V. Mo( ller / J. Wind Eng. Ind. Aerodyn. 84 (2000) 307}320 309

Fig

.1.

The

Win

dTunn

elof

the

UN

NE

.


Table 1Data acquisition conditions for spectral analysis

Low frequency Mean frequency High frequency

Ampli"cation rate 50 50 50Low-pass "lter (Hz) 100 300 1000High-pass "lter (Hz) 0.3 0.3 0.3Sampling frequency (Hz) 300 900 3000Sample size 32 000 32 000 32 000Sampling time (s) 106.7 35.6 10.7Number of blocks 124 124 124Block size 256 256 256Bandwidth (Hz) 1.132 3.516 11.719

z0

and the integral scale ¸96

as parameters. The values of the roughness length areobtained by "tting experimental values of velocity to the logarithmic law of the wall,while integral scale is given by "tting the values of the measured spectrum to thedesign spectrum. The height of the roughness elements is constant, but the integralscale depends on the height z and the roughness length z

0, which according to ESDU

(Engineering Sciences Data Unit) data, given by Cook [13], follows the expression

¸96"25(z!z

$)0.35z~0.063

0. (3)

Substituting full-scale values by the product of the scale factor S by the model-scalevalues, results in

S¸96M

"25[S(z!z$)M]0.35[Sz

0M]0.063. (4)

Subscript M denotes model values.Thus, scale factor is determined as a function of model-scale values.Mean velocity measurements were performed by means of a Pitot}Prandtl tube

connected to a van Essen Betz-type manometer. Before starting each measurement thehot wire probe was calibrated. Velocity and longitudinal velocity #uctuations weremeasured by a Dantec 56 constant temperature hot wire anemometry bridge, witha true-RMS voltmeter, connected to an Stanford SR560 ampli"er with low andhigh-pass analogic "lters. Data acquisition of hot wire signals was made with help ofa Keithley DAS-1600 A/D converter board controlled by a personal computer whichwas also used for the evaluation of the results.

Voltage output from hot wires was evaluated to obtain velocity and velocity#uctuations [14,15]. Prior measurements in a pipe #ow showed the adequacy of thecalibration and evaluation technique [9].

Spectral results from longitudinal velocity #uctuations were obtained by juxtaposi-ng three di!erent spectra from three di!erent sampling series, obtained in the samelocation, each with a sampling frequency, as given in Table 1, as low, mean and highfrequencies. The series were divided in blocks to which an FFT algorithm was applied[16].


Table 2Flow characteristics for Counihan simulation

y"0 y"0.30 m y"!0.30 m

z'

(m) 1.164 1.164 1.164;

'(m/s) 27.507 28.183 27.755

Re 2.066]106 2.116]106 2.084]106

a 0.2697 0.2649 0.2699

Fig. 2. Mean dimensionless velocity at Table 2.

3. Results

3.1. Empty tunnel

Mean dimensionless velocity pro"les measured with the empty tunnel along a verti-cal line on the center of the rotating Table 2 and at positions 0.6 m to the right and leftof this line are presented in Fig. 2. The boundary layer has a thickness of about 0.3 mand the velocity values have a maximal deviation of 3%, by taking the velocity at thecenter of the channel as reference.


Fig. 3. Turbulence intensity at Table 2.

Turbulence intensity distribution at the same locations, presented in Fig. 3, showsvalues around 1% outside the boundary layer increasing, as expected, inside theboundary layer. The measurements at the central position show values of about 3%turbulence intensity near the upper wall but outside of the boundary layer.

Reference velocity for both Figs. 2 and 3 is the velocity at the center of the channel,27 m/s, the resulting Reynolds number being, calculated with the tunnel hydraulicaldiameter, 3.67]106.

3.2. Counihan method

Measurement of the mean velocity distribution was made along a vertical lineon the center of rotating Table 2 and along lines 0.3 m to the right and left of this line.Fig. 4 shows the velocity distribution along the central line. Flow characteristics arepresented in Table 2. There is a good similarity among the velocity pro"les given bythe values of the exponent a obtained.

Turbulence intensity distribution at the same locations are shown in Fig. 5. Thevalues are lower than those obtained by Cook [13] and by using Harris}Davenportformula for atmospheric boundary layer [12]. Values are reduced as the distance fromthe lower wall is increased. This is also observed in the spectra of the velocity


Fig. 4. Mean velocity distribution at central position with the Counihan method.

#uctuation, Fig. 6. An important characteristic of the spectra is the presence of a clearregion with a !5

3declivity, characterizing Kolmogorov's inertial subrange, which is

of great importance in the structural analysis.The comparison of the results obtained through the simulations with the atmo-

spheric boundary layer is made by means of dimensionless variables of the autospec-tral density f/

u/p2

uand of the frequency fz

'/;. The usual design spectrum is the

so-called von KaH rmaH n spectrum [12], given by (Fig. 7)

fUu

p2"

1.6fz'/;M

[1#11.325( fz'/;M )2]5@6

. (5)

This dimensionless spectral function is de"ned by dividing the autospectral densityfunction by the variance p2 of the velocity #uctuation. Kolmogorov's spectrum willhave, therefore, a !2

3exponent instead of !5

3. The agreement is very good, except for

the highest frequencies a!ected by Heisenberg's viscous dissipation subrange or by theaction of the low-pass "lters.

A scale factor of 250 for this boundary layer simulation was obtained through themethod proposed by Cook [13].


Fig. 5. Turbulence intensity distribution at central position with the Counihan method.

3.3. Standen method

Measurement of the mean velocity distribution was made along a vertical lineon the center of rotating Table 2 and along lines 0.6 m to the right and left of this line.Fig. 8 shows the velocity distribution along the central line. Flow characteristics arepresented in Table 3. There is, again, a good similarity among the velocity pro"lesgiven by the values of the exponent a obtained.

Values of turbulence intensity distribution at the same locations, shown in Fig. 9are similar to those obtained by the Counihan method, with values lower thanthose given by the Harris}Davenport formula for atmospheric boundary layer[12], which are reduced as the distance from the lower wall is increased. This isalso observed in the spectra of the velocity #uctuation, Fig. 10, with a clear regionwith a !5

3declivity. The same comments can be made about von KaH rmaH n spectrum,

Fig. 11.For this boundary-layer simulation, a scale factor of 150 was obtained through the

method proposed by Cook [13].


Fig. 6. Autospectral density of the longitudinal velocity #uctuation by the Counihan method.

Fig. 7. Autospectral density of the longitudinal velocity #uctuation by the Counihan method obtained atz"23.3 cm and Von KaH rmaH n design spectrum.


Fig. 8. Mean velocity distribution at central position with the Standen method.

Table 3Flow characteristics for the Standen simulation

y"0 y"0.60 m y"!0.60 m

z'

(m) 1.214 1.214 1.214;

'(m/s) 25.602 24.761 25.716

Re 2.066]106 1.940]106 2.014]106

a 0.249 0.244 0.225

4. Conclusions

The purpose of this research work was the evaluation of the low-speed WindTunnel of the UNNE to verify its adequacy for structural analysis applications.

Measurements of velocity and turbulence intensities in the empty tunnel showed anuniform velocity "eld and low turbulence intensities.

Results of Counihan and Standen natural wind simulations for an ABNT-NBR6123 Class IV ground (Brazilian Standards), similar to a CIRSOC 102 class III


Fig. 9. Turbulence intensity distribution at central position and two lateral positions with the Standen method.

Fig. 10. Autospectral density of the longitudinal velocity #uctuation by the Standen method.


Fig. 11. Autospectral density of the longitudinal velocity #uctuation by the Standen method obtained atz"23.3 cm and Von KaH rmaH n design spectrum.

ground (Argentinian Standards) showed good reproduction of the velocity pro"lesand turbulence intensities. Turbulence spectra of the longitudinal velocity #uctuationspresent, in general a very clear region with a !5

3exponent (Kolmogorov's inertial

subrange) which is important from the structural analysis point of view. The repro-duction of a typical design spectrum, this being von KaH rmaH n spectrum, is also, ingeneral, very good.

The Wind Tunnel of the UNNE is, therefore, a very tool for natural wind simula-tions for structural analysis.

Future work will consider the evaluation of the #ow conditions in Table 1 as well asother types of grounds.

References

[1] N.J. Cook, A boundary layer wind tunnel for building aerodynamics, J. Ind. Aerodyn. 1 (1975) 3}12.[2] D.M. Sykes, A new wind tunnel for industrial aerodynamics, J. Ind. Aerodyn. 2 (1977) 65}78.[3] J. Counihan, An improved method of simulating an atmospheric boundary layer in a wind tunnel,

Atmos. Environ. 3 (1969) 197}214.[4] M. Greenway, C. Wood, The Oxford University 4m]2m industrial aerodynamics wind tunnel, J. Ind.

Aerodyn. 4 (1979) 43}70.[5] J. Blessmann, The boundary layer TV2 wind tunnel of the UFRGS, J. Eng. Ind. Aerodyn. 10 (1982)

231}248.[6] S. Hansen, E. Sorensen, A new boundary layer wind tunnel at the Danish Maritime Institute, J. Wind

Eng. Ind. Aerodyn. 18 (1985) 213}224.[7] M. De Bortoli, M. Natalini, M. Paluch, Relevamiento en vacio del tunel de viento de la UNNE, XV,

J. Argentinas Ingenieria Estrutural 2 (1996) 360}368.


[8] M.M. Standen, A spire array for generating thick turbulent shear layers for natural wind simulationin wind tunnels, National Research Council of Canada, NAE Report LTR-LA-94, 1972.

[9] A.R. Wittwer, AnaH lisis Experimental de las CharactermH sticas del Escurrimiento Turbulento en la CaH paLmHmite de un TuH nel de Viento, Tesis de Maestria, Facultad de Inegenieria de la Universidad Nacionaldel Nordeste, UNNE, Resistencia (Chaco), Argentina, 1997.

[10] Associac7 a8 o Brasileira de Normas de Normas TeH cnicas (ABNT) NBR 6123: Forc7 as devidas ao ventoem edi"cac7 o8 es, Rio de Janeiro, 1988.

[11] Centro de InvestigacioH n de los Reglamientos Nacionales de Seguridad para las Obras Civiles(CIRSOC), INTI, Reglamento CIRSOC 102, 1982.

[12] J. Blessmann, O Vento na Engenharia Estrutural, Editora da Universidade, UFRGS, Porto Alegre,1995.

[13] N.J. Cook, Determination of the model scale factor in wind tunnel simulations of the adiabaticatmospheric boundary-layer, J. Ind. Aerodyn. 2 (1978) 311}321.

[14] L. VosaH hlo, Computer programs for evaluation of turbulence characteristics from hot-wire measure-ments, KfK 3743, Kernforschungszentrum Karlsruhe, Karlsruhe, 1984.

[15] S.V. MoK ller, Experimentelle Untersuchung der VorgaK nge in engen Spalten zwischen den Unter-kanaK len von StabbuK ndeln bei turbulenter StroK mung, Dissertation, UniversitaK t Karlsruhe (TH),Karlsruhe, RFA, 1988, also KfK 4501, 1989.

[16] W.H. Press, B.P. Flannery, S.A. Teukolsky, W.T. Vetterling, Numerical Recipes: The Art of Scienti"cComputing, Cambridge University Press, New York, 1990.


characteristics of the low-speed wind tunnel of the unne

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