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    *Corresponding author.

    Journal of Wind Engineering

    and Industrial Aerodynamics 84 (2000) 307}320

    Characteristics of the low-speed wind tunnelof the UNNE

    Adrian R. Wittwer*, Sergio V. Mo  K 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 UNNE

    Wind Tunnel to verify its applicability to similarity studies and to experimental simulations of 

    the 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 was

    performed 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 as

    power spectral distributions of the axial component of the velocity   #uctuations. Results of 

    measurements in the empty tunnel showed a uniform velocity   "eld and low turbulence

    intensities. Analysis of atmospheric boundary-layer simulations by means of Counihan and

    Standen methods showed the adequecy of the tunnel for natural wind simulations.     2000

    Elsevier 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 that

    similarity studies can be performed, with the con"dence that actual operational

    conditions will be reproduced. Once a wind tunnel is built, the   "rst step is the

    evaluation 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-

    siste H 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

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    Nomenclature

     f    Frequency (Hz)

    I   Local turbulence intensityk   Von Ka H rma H n constant (k"0, 4)

    ¸

      Longitudinal integral length parameter (m)

    Re Reynolds number

    S   Model scale factor of a boundary-layer simulation

    t   Time (s)

    u

      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

    )

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

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

      Power-law exponent

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

      Density (kg/m)

      Variance of  u   (m/s)

      Standard deviation of  u   (m/s)

      Kinematic viscosity (m/s)

    tunnel, built with the aim to perform aerodynamic studies of structural models. The

    distribution of the   #ow impinging on the structural model must be such that the

    atmospheric boundary layer at the actual location is reproduced. This is obtained

    with help of turbulence promoters and vortex generators, so that natural wind

    simulations are performed.

    The open literature presents many evaluation studies of wind tunnels, some of 

    which are the tunnel in Garston, Watford, UK [1], the closed-loop wind tunnel in

    London [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 of 

    Langby, Denmark [6]. A partial mapping of the velocity  "eld in the UNNE Wind

    Tunnel was presented by De Bortoli et al. [7].

    In general, tunnel evaluation is performed at the highest  #ow velocity, the results

    being presented in terms of mean velocity distributions, turbulence intensities and

    scales. Boundary-layer simulations are performed with help of grids, vortex

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    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 to

    evaluate the characteristics of the UNNE Wind Tunnel and to verify its applicabilityto similarity studies in structural models and to simulate the atmospheric boundary

    layer, 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 long

    channel. The air enters through a contraction, passing a honeycomb and a screen

    prior to reach the test section, which is a 22.8 m long rectangular channel (2.40 m

    width, 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 pressure

    gradient boundary layers. The test section is connected to the velocity regulator and

    this in turn to the blower, which has a 2.25 m diameter and is driven by a 92 kW

    electric motor at 720 rpm. The air passes through a di! user before leaving the wind

    tunnel.

    The simulation of natural wind on the atmospheric boundary layer was performed

    by 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 or

    urban 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 for

    velocity distribution is given by

    ;  M(z)/ ;  M(z

    )"(z/ z

    ), (1)

    and

    ;  M(z)/ ;  M(10)"(z/10)   (2)

    with suitable values for the exponent   between 0.23 and 0.28 [12]. This law is of good

    application in neutral stability conditions of strong winds, typical for structural

    analysis.

    For Counihan full-depth simulation, where the complete boundary-layer thickness

    is simulated, four 1.42 m high elliptic vortex generators and a 0.23 m barrier were

    used, together with prismatic (3030 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 roughness

    elements used in Counihan method and two 1.5 m spires as vortex generators, to

    simulate the lowest part of the boundary-layer thickness.

    Scale factors of both atmospheric boundary-layer simulations is determined

    through the procedure proposed by Cook [13], by means of the roughness length

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    Fig. 1. The Wind Tunnel of the UNNE.

     3   1    0   

    A  .R  . W i   t   t   w er  , S  . V  .M

     o(         l   l   er    /     J   . W i   n d E 

     n  g .I   n d  .A 

     er  o d   y n . 8  4 

      (  2  0  0  0   )   3  0  7 }

     3 2  0 

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    Table 1

    Data acquisition conditions for spectral analysis

    Low frequency Mean frequency High frequency

    Ampli"cation rate 50 50 50

    Low-pass "lter (Hz) 100 300 1000

    High-pass "lter (Hz) 0.3 0.3 0.3

    Sampling frequency (Hz) 300 900 3000

    Sample size 32 000 32 000 32 000

    Sampling time (s) 106.7 35.6 10.7

    Number of blocks 124 124 124

    Block size 256 256 256

    Bandwidth (Hz) 1.132 3.516 11.719

    z

     and the integral scale ¸

     as parameters. The values of the roughness length are

    obtained 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 the

    design spectrum. The height of the roughness elements is constant, but the integral

    scale depends on the height z and the roughness length z

    , which according to ESDU

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

    ¸"25(z!z

    )z

      . (3)

    Substituting full-scale values by the product of the scale factor  S by the model-scale

    values, results in

    S¸"25[S(z!z

    )

    ][Sz

    ]. (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 the

    hot wire probe was calibrated. Velocity and longitudinal velocity  #uctuations were

    measured by a Dantec 56 constant temperature hot wire anemometry bridge, with

    a true-RMS voltmeter, connected to an Stanford SR560 ampli"er with low and

    high-pass analogic  "lters. Data acquisition of hot wire signals was made with help of 

    a Keithley DAS-1600 A/D converter board controlled by a personal computer which

    was 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 same

    location, each with a sampling frequency, as given in Table 1, as low, mean and high

    frequencies. The series were divided in blocks to which an FFT algorithm was applied

    [16].

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    Table 2

    Flow 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 2.06610   2.11610   2.08410

      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 left

    of this line are presented in Fig. 2. The boundary layer has a thickness of about 0.3 m

    and the velocity values have a maximal deviation of 3%, by taking the velocity at the

    center of the channel as reference.

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    Fig. 3. Turbulence intensity at Table 2.

    Turbulence intensity distribution at the same locations, presented in Fig. 3, shows

    values around 1% outside the boundary layer increasing, as expected, inside the

    boundary 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 hydraulical

    diameter, 3.6710.

    3.2. Counihan method 

    Measurement of the mean velocity distribution was made along a vertical line

    on 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 by

    the values of the exponent    obtained.

    Turbulence intensity distribution at the same locations are shown in Fig. 5. The

    values are lower than those obtained by Cook [13] and by using Harris}Davenport

    formula for atmospheric boundary layer [12]. Values are reduced as the distance from

    the lower wall is increased. This is also observed in the spectra of the velocity

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    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 clear

    region with a!

     declivity, 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 

      and of the frequency   fz

    / ;. The usual design spectrum is the

    so-called von Ka H rma H n spectrum [12], given by (Fig. 7)

     f 

    "

    1.6 fz

    / ;  M

    [1#11.325( fz

    / ;  M)]. (5)

    This dimensionless spectral function is de"ned by dividing the autospectral densityfunction by the variance    of the velocity  #uctuation. Kolmogorov's spectrum will

    have, therefore, a!

     exponent instead of !

    . The agreement is very good, except for

    the highest frequencies a! ected by Heisenberg's viscous dissipation subrange or by the

    action of the low-pass  "lters.

    A scale factor of 250 for this boundary layer simulation was obtained through the

    method proposed by Cook [13].

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    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 line

    on 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 are

    presented in Table 3. There is, again, a good similarity among the velocity pro"les

    given by the values of the exponent    obtained.

    Values of turbulence intensity distribution at the same locations, shown in Fig. 9

    are similar to those obtained by the Counihan method, with values lower than

    those given by the Harris}Davenport formula for atmospheric boundary layer[12], which are reduced as the distance from the lower wall is increased. This is

    also observed in the spectra of the velocity   #uctuation, Fig. 10, with a clear region

    with a!

     declivity. The same comments can be made about von Ka H rma H n spectrum,

    Fig. 11.

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

    method proposed by Cook [13].

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    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 at

    z"23.3 cm and Von Ka H rma H n design spectrum.

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

    Table 3

    Flow 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 2.06610   1.94010   2.01410

      0.249 0.244 0.225

    4. Conclusions

    The purpose of this research work was the evaluation of the low-speed Wind

    Tunnel of the UNNE to verify its adequacy for structural analysis applications.

    Measurements of velocity and turbulence intensities in the empty tunnel showed an

    uniform velocity  "eld and low turbulence intensities.

    Results of Counihan and Standen natural wind simulations for an ABNT-NBR

    6123 Class IV ground (Brazilian Standards), similar to a CIRSOC 102 class III

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    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.

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    Fig. 11. Autospectral density of the longitudinal velocity  #uctuation by the Standen method obtained at

    z"23.3 cm and Von Ka H rma H n design spectrum.

    ground (Argentinian Standards) showed good reproduction of the velocity pro"lesand turbulence intensities. Turbulence spectra of the longitudinal velocity #uctuations

    present, in general a very clear region with a !

     exponent (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 Ka H rma H n spectrum, is also, in

    general, 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 as

    other 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 4m2m industrial aerodynamics wind tunnel, J. Ind.Aerodyn. 4 (1979) 43}70.

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

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    [8] M.M. Standen, A spire array for generating thick turbulent shear layers for natural wind simulation

    in wind tunnels, National Research Council of Canada, NAE Report LTR-LA-94, 1972.

    [9] A.R. Wittwer, Ana H lisis Experimental de las Characterm Hsticas del Escurrimiento Turbulento en la Ca H pa

    Lm Hmite de un Tu H nel de Viento, Tesis de Maestria, Facultad de Inegenieria de la Universidad Nacional

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    em edi"cac  7  o  es, Rio de Janeiro, 1988.

    [11] Centro de Investigacio H 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,

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    [13] N.J. Cook, Determination of the model scale factor in wind tunnel simulations of the adiabatic

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