Design of a new wind tunnel facility at Industrial Engineering School in Badajoz (Spain)

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1 Design of a new wind tunnel facility at Industrial Engineering School in Badajoz (Spain) F. Zayas Hinojosa Department of Electronics and Electromechanical Engineering, University of Extremadura, Spain Abstract The Industrial Engineering School, part of the University of Extremadura, located in Badajoz (Spain), has been founded by the European Community under the regional development program of research facilities, and it is actually under the process of construction. The aerodynamic channel is that of the Eiffel type, also called open flow. Thus, it is inside a 25 meter long and 6 by 6 meter wide room, taking the air from the room and evacuating it to the same room without temperature conditioning. The instrumentation included consists of that used to measure the air velocities, such as a pitot-static probe, ultrasonic and hot wire anemometers, differential pressure sensors to record the pressure distribution over solid bodies and an aerodynamic balance to measure forces and moments over solids inside the air flow. There are also other instruments such as high-speed cameras, a thermal camera and, if the budget allows it, a Particle Image Velocimetry system (PIV). As part of the development, an agreement has been signed between the University of Extremadura (Industrial Engineering School) and the Polytechnic University of Madrid (Aeronautical Engineering School) in order to gain experience by sharing knowledge and information. 1 Introduction It is very well known that the experimental data used to solve aerodynamic problems may be obtained by scale model simulation using wind tunnels

2 398 Advances in Fluid Mechanics V (subsonic, transonic, supersonic or hypersonic types), drop tests, water tunnels, ballistic ranges, shock tubes, rocket flights, rotating arms, etc., but wind tunnels are the most useful means when dealing with the air speed used in ground structures applications and moving vehicles in incompressible flow (mainly cars, trains, and airplanes), offering a relatively fast, economical and precise way to carry out research in aerodynamics and its industrial applications. This project is of great importance for the development of local industries working in the design of metal structures of antennas, electric towers, industrial constructions, urban conditions, and also related to studies of pollutant diffusion in the air, caused by traffic or industrial activities. In order to gain experience in the explained applications of aerodynamics, the Industrial Engineering School, located in Badajoz (Spain), is building a wind tunnel, that will also be useful to work in activities related to low speed applications, such as airfoils in incompressible flow, low speed drag, aerodynamic vibration excitation and flutter, etc. This project is founded by the European Community under the regional development program of research facilities. 2 Wind tunnel general arrangement When dealing with a moving body in a fluid field, forces appear due to the fluid viscosity, its elasticity, inertia of the body and also gravity forces over the mass of fluid affected and the acceleration induced on its particles [1]. Thus, using the dimensional analysis in order to compare the relative magnitude of these forces we can finally use as relevant ratios the following formulae: ρud Re = µ (1) U M = c (2) 2 U Fr = (3) gd In these relations the fluid density and viscosity are represented by ρ and µ respectively, while U is the speed of the incident stream, c is the speed of sound in the fluid, D is a representative length of the system, and g is the local gravity acceleration. The Froude number, eqn. (3), is only relevant if the test deals within the vicinity of the interface of liquids, or if a free dynamic model is to be tested in a wind tunnel, but as a general rule it is only necessary to consider the effect of the Reynolds, eqn. (1), and Mach, eqn. (2), numbers, unless this last parameter is low (typically below 0,3), the Reynolds number then being the one to deal with. Sometimes it will also be necessary to consider the influence of other nondimensional parameters such as, for example, the Strouhal number [2], relevant when testing structures under vibrations induced by wind turbulence.

3 Advances in Fluid Mechanics V 399 One can think of a wind tunnel as an analog computer the aerodynamic engineer can adapt to improve designs in the working areas where this facility is useful, checking the test results against the theoretical model predictions and thus improving the theoretical model for a better performance in future situations. Among the different types of wind tunnels an Eiffel type configuration, similar to the one shown in fig. 1, has been selected because of the main usage expected, that is: testing models of buildings and ground structures (RF antennas, electric towers, bridges, etc.). This means it is necessary to develop a boundary layer similar to the one of the wind in an open and natural atmosphere, in a free air stream with a velocity profile as close as possible to it. Figure 1: Example of an open circuit (Eiffel type) wind tunnel schematics, with closed test section. 1) contraction; 2) test section; 3) diffuser; 4) fan; 5) air exit. As a consequence, the selected configuration is that of an open circuit tunnel, and consists of an inlet adapter and contraction, a flow straightener, a long continuous section square channel (where the velocity profile is developed), followed by a closed test section (where the test model is set up), a divergent channel (diffuser) to reduce the air speed, and a final section where the fans used to move the air are installed. The air is taken from the same room where the air is discharged. The open circuit tunnel has also been selected due to the lower cost of the installation, although it has several disadvantages such as, for example, more energy needed to operate and a more noisy operation, but these conditions are relieved because a high rate of utilization is not expected. At the beginning of the flow channel (fig. 2) there is a contraction to attenuate the turbulence of the intake air and also to achieve as uniform a velocity profile of the air stream as possible, so taking a fluid flow ready to be adapted at the particular experiment to perform in each case. That means, for example, that in order to work in the testing of wind effects in a building it is necessary to use devices that modify the boundary layer in one of the sides of the wind tunnel (usually the floor) to develop the velocity profile founded under atmospheric conditions as close as possible to the real scale, excepting temperature because the wind tunnel is not intended to control temperature profiles.

4 400 Advances in Fluid Mechanics V Figure 2: Velocity profile development and test section. The contraction profile is based on the contours of Boerger [3] for axisymmetric contractions, modified in order to take into count that the wind tunnel section, in these cases, is not axisymmetric but squared. This implies that the contraction has to be at least 29% longer than the equivalent axisymmetric in order to avoid turbulent boundary layer separation. This section ends in a flow straightener built from a honeycomb panel used to align the air flow with the longitudinal axis of the wind tunnel. After the straightener, the boundary layer developed by a rough floor on the wind tunnel may be used to obtain the velocity profile, but in order to reduce the length of the system some devices can be attached to the floor to reach the required velocity profile in a much faster way. The controlled boundary layer is developed along a constant square section with the help of elliptic vortex generators at the beginning of the profile development section, and strakes of different sizes from there up to the test section, according to the Counihan [4] and Standen [5] methods. The turbulence created by these devices generates a boundary layer velocity distribution that resembles as much as possible the one present in the real atmosphere. In addition to the regular boundary layer on the top panel of the wind tunnel, plus a region with an almost uniform velocity profile, inside the atmospheric boundary layer the velocity distribution (fig. 3) may be modelled as a set of four sub-layers, as described by Plate [6]. The first one is the canopy layer, and inside it the flow is determined mainly by the local structures (buildings) vertical size and the layer is characterised by a highly turbulent and three-dimensional flow. The thickness of this region is the same order as the surface roughness.

5 Advances in Fluid Mechanics V 401 Top panel boundary layer Uniform velocity profile Potential law layer Logarithmic law layer Mixing or blending layer Canopy layer Figure 3: Sub-layers in the velocity profile. The second layer is a mixing or blending region, with a thickness twice that of the canopy layer, and immediately over it there is a region inside which the wind profile is scaled according to the logarithmic law * u z U( z) = ln κ z (4) 0 * where ( ) 1 2 u = τ ρ is the shear velocity, being τ 0 the shear stress on the 0 ground surface because of the wind, and ρ the density of the air. Also κ is the von Karman constant (about 0.4) and z 0 is a measure of the roughness of the surface, usually ranging from 0.1 to 1 meter. Over the logarithmic layer there is another one, with the velocity profile described by the potential law ( z) ( z ) α U z = U R z (5) R where the altitude of reference used to define the mean wind speed U is usually accepted to be measured at z R = 10 m, and α is a parameter ranging from 0.23 to 0.28, in neutral stability conditions. On top of the already described layers a uniform velocity profile exist until the flow gets close to de upper wall of the wind tunnel, where a new boundary layer exists. Following the velocity profile development section, the test section is found. In this area the model to be tested and the instrumentation used to measure pressure distributions over the model are installed. The model is mounted on a base that can be rotated around a vertical axis to allow different azimuths of the air stream incidence over the test model. In this section there are also two

6 402 Advances in Fluid Mechanics V windows, plus cameras and illumination sources to remotely observe and monitor the test, especially when smoke is used to draw stream lines. Immediately after the test section a diffuser drives the air towards the fan section, built as a matrix of nine axial fans driven by electric motors which speed of rotation is controlled with an electronic inverter, supplying electric current with the required frequency to achieve the desired wind velocity in the test section. This configuration ensures a lower turbulence level of the original air stream and as a consequence the flow does not need as much screening as in those systems with the fans installed before de contraction section, thus leading to a much more economical arrangement. The aerodynamic channel of the adopted design (fig. 4) is enclosed in a 25 meters long and 6 by 6 meters wide room, with a test section 2 by 2 meters and an exhaust section at the matrix of fans measuring 3 by 3 meters. The velocity profile will be created with the help of four 1.58 meters high vortex generators and a set of prismatic roughness elements distributed along the profile development section, 9 meters long. Contraction Profile development Fan matrix Straightener Test section Diffuser 3 Instrumentation Figure 4: General arrangement of the wind tunnel. In order to gather experimental data, the wind tunnel must provide the adequate instrumentation to record pressure, temperature and velocity of the air stream, and in relevant points of the test model, and sometimes will be also necessary to record forces and moments. The local barometric pressure and temperature in the laboratory must be recorded in order to calculate the density of the air used in the test. To measure the air velocity of the stream and in several points through the test section a set of pitot-static probes is used, each one connected to a solid state pressure electronic transducer, being possible to use them combined with ultrasonic and hot wire anemometers. There are also previsions to use a Particle Image Velocimetry (PIV) system in a second development phase of the system.

7 Advances in Fluid Mechanics V 403 Differential pressure sensors are used to record the pressure distribution over solid bodies and, in order to make the system more economical, a sequential reading system allows reading pressure in up to 50 points with only one sensor by using a locally designed pneumatic valves array, computer controlled, with the ability to install a maximum of four sequential systems giving up to 200 metering points. There is also other equipment such as a high-speed camera to follow dynamic tests, and also to be used as part of the PIV system. The data acquisition system is based on a set of three computers, each of them with a data acquisition card running commercial software to design a set of virtual instruments. Another computer is used to control two digital cameras in order to record the test model, especially when provisions to trace the streamlines with a smoke generator are convenient. And finally one more computer is dedicated to control the high-speed camera. Provisions for an aerodynamic balance to measure forces and moments over solids inside the air flow are made to be adapted sometime in the future. 4 Development program The tunnel is actually in the building construction contracting phase, but much of the instrumentation has already been acquired, and is expected to be fully installed and running by the end of may The pneumatic sequential system is in the assembly, calibration and test process, expecting to be ready by the same time the instruments installation in the wind tunnel is completed. After the installation, the test and calibration process of the complete system (including the tunnel itself with all the subsystems) will take place to determine the turbulence level and the mean velocity distribution across the test section. It will also be necessary to adjust the instrumentation and tune the turbulence generators to achieve the desired velocity profile for several tests cases, before the real experimentation takes place. In order to reduce this phase and to start working as soon as the facility is finished, a memorandum of agreement has been signed between University of Extremadura (Industrial Engineering School) and Polytechnic University of Madrid (Aeronautical Engineering School). This agreement will let us gain experience in wind tunnel operations, by helping in part of practical projects at the beginning of the process and increasing the cooperation in more partial works as our experience grows, until we are able to develop a whole test by ourselves, although the agreement and cooperation will follow from then and on by sharing knowledge and information. 5 Conclusions The Industrial Engineering School, located in Badajoz (Spain), has been founded by the European Community under the regional development program of research facilities to design an build a wind tunnel, mainly oriented to research the industrial applications of aerodynamics, such as experimentation on atmospheric boundary layer effects on buildings and ground structures, although

8 404 Advances in Fluid Mechanics V is also useful to work in activities related to low speed applications, such as airfoils in incompressible flow, low speed drag, aerodynamic flutter, etc. Acknowledgements The project is funded by the European Community FEDER program, to be used as a help for investigation and development infrastructure, assigned as project number UNEX We also give thanks to the colleagues in the Aerodynamic Laboratory of the Polytechnic University (Aeronautical Engineering School) of Madrid for their help through the conceptual and design phase of our wind tunnel. References [1] Rae, E. H. Jr. & Pope, A., Low-Speed Wind Tunnel Testing, John Wiley & Sons, New York, [2] Meseguer, J., Sanz, A., Perales, J. M., & Pindado, S., Aerodinámica Civil. Cargas de viento en las edificaciones, McGraw-Hill, Madrid, [3] Boerger, G. G., The optimisation of wind tunnel contractions for the subsonic range, National Agency of Space Administration, NASA TTF 18899, [4] Counihan, J., An improved meted of simulating an atmospheric boundary layer in a wind tunnel, Atmos. Environ., 3, pp , [5] Standen, M. M., A spike array for generating thick turbulent shear layers for natural wind simulation in wind tunnels, National Research Council of Canada, NAE Report LTR-LA-94, [6] Plate, E. J., Methods of investigating urban wind fields physical models, Atmospheric Environment, 33, pp , 1999.