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Delegates are invited to meet and discuss with the poster presenters in this topic directly after the session 'Aerodynamics and rotor design' taking place on Wednesday, 12 March 2014 at 09:00-10:30. The meet-the-authors will take place in the poster area.

Valentín Sánchez Morales Universitat Rovira i Virgili, Spain
Co-authors:
Silvana Tourn (1) F P Robert Gilabert (1) Valentín Sánchez (1) Jordi Pallares (1) Ildefonso Cuesta (1) Uwe S.Paulsen (2)
(1) Universitat Rovira i Virgili, Tarragona, Tarragona, Spain (2) Department of Wind Energy. Technical University of Denmark, Roskilde, Denmark

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Abstract

Characterization of a new open jet wind tunnel to optimize and test vertical axis wind turbines using flow visualization and measurement

Introduction

Based on the increasing interest in urban environmental technologies the study of small scale of vertical axis wind turbines (VAWTs) presents motivating challenges. In this paper we present the characteristics and potentials of an open jet wind tunnel (OJWT) built in the laboratories of the ECoMMFiT1 research group in Tarragona (Spain). Wind tunnels are scientific-technology instruments designed to obtain reproducible real air flow conditions. On the other hand they are valuable research facilities to study the aerodynamics and improve the design of the small scale wind turbines.



Approach

Open wind tunnels present some favorable characteristics in comparison with closed loop wind tunnel. One advantage is that the cost is less expensive and second the easy access to the experimental section, allowing the use of measurement and visualization techniques in a simpler way. The experimental data obtained with de open jet wind tunnel represent a valuable tool for validation of numerical simulations of the flow induced by rotor blades of a vertical wind turbine. In addition these experiments could be a significant contribution to improve turbulence models for the near wake of the wind turbine blades. Complementary, the open jet wind tunnel is a teaching experimental equipment of the laboratory for the Master of the University Rovira i Virgili "Design and construction of wind turbine farms" in which students perform experiments as the flow visualization of the wake and the calculation of the performance of small scale wind turbines for different synthetic winds.

The scientific benefit of using a wind tunnel depends strongly upon information about the characteristics of the flow. For this purpose once a wind tunnel is built, the first step is the evaluation of the flow characteristics in the test section. In order to carry out the characterization of the flow in that section, calibrated static pitot tubes with ellipsoidal fit, cup anemometers and hot wire anemometry based on Constant Temperature Anemometry (CTA) methodology have been used. Therefore, mean velocities, turbulence intensities and blockage effects have been determined.


Picture of the open jet wind tunnel installed in the laboratory.

Main body of abstract

Firstly, the main characteristics of the wind tunnel have been presented. The overall length of the tunnel is 4.82 m and its maximum height and width are 2.24 m. The 4 axial fans of total power 30 kW have an intake from the interior of the building and the flow of air then passes through a settling chamber where a honeycomb and a series of three screens remove the disturbances from the room air and produces a uniform velocity profile. The flow then passes through a contraction which further reduces the turbulence and also aids in producing an uniform velocity profile. The flow then is passing on to test section as an even jet stream. The contraction or nozzle has an area ratio of 2.22:1. The maximum volumetric flow is 129600m3/h.

The coordinate system (x,y,z) is defined before to design and carry out the experiments in the test section of the wind tunnel presented. The origin of coordinates is located at the nozzle centre. Figure 1 provides side and front views with: x, y and z axis, the main dimensions and labelling the major sections. The measures with pitot tubes and hot wires have been done in the empty wind tunnel.

Figure 1- Side and front views with: x, y and z axis, the main dimensions and labelling the major sections. All dimensions are in mm.

These measures describe the velocity profile of two cross sections (plane y-z) in two different distances from the exit of the flow (x direction). Figure 2 shows the velocity profiles of the plane perpendicular to the jet. In the first profile (circle symbols), x is located at 1m of distance of the nozzle exit (x1). In the second profile (star symbols), x is set to 2m of distance (x2). In both profiles, the distance along the z direction from -0.7 to 0.7 m with increments of 0.05 or 0.1 m.

The turbulence intensity (Ti) has been estimated considering the root mean square (rms) of the signal of the single hot wire of the probe which is oriented perpendicularly to the flow during the experiments. Figure 3 shows the distribution of the turbulence intensity along the z direction of the free jet at flow velocities of: 3.1 m/s, 6.7 m/s and 10.3 m/s. These measurements were obtained at x1 = 1m (star symbols) and x2 = 2m (circle symbols) away from the nozzle exit.

Figure 2. Vertical velocity profiles at x1 =1m (circle nodes) and x2= 2m (star nodes) for 3.1, 6.7 and 10.3 m/s and Figure 3. Distribution of Ti of the free jet along z direction, at y = 0 and x1= 1m and x2= 2m for same mean velocities.

As expected the first figure shows that, the flow mean velocity slows down along the x direction. The turbulence intensities show a good behavior in the central section of the flow.

Cup anemometer measurements have been completed at five different positions in the cross section and at five different distances from the exit flow. The validatation of average velocities in the flow and determination of the blockage effects are possible with this instrument.

The laboratory is also equipped with flow visualization equipment instruments and with the instruments required to perform flow velocity measurements using the Particle Image Velocimetry technique. Therefore, flow visualization and measurement of the flow are possible to develop.

Conclusion

The wind tunnel can be operated with exit velocities from 3 m/s to 14 m/s. Measurements show, for the range of exit velocities available, that at the cross section where the velocity and turbulence intensities are constant has an area of 0.8x0.8 m2 and a streamwise dimension of 2 m from the exit of the tunnel. In this region the maximum turbulence intensity is 3%.

The size of the test section with high flow quality (i.e. uniform velocity and low turbulence intensity) indicates that the wind tunnel can be used to test small scale models of wind turbines and determine their power curve. The cup anemometer enforce blockage into the jet and into calibration results, which are compared with calibrations from other wind tunnels.

The methologies presented here represents specific experiments conducted on the flow quality and on the wind tunnel performance, which is used as a validating tool on top of results derived by CFD.

ECoMMFiT research group has extensive experience in CFD simulation. This was the reason because previous computational studies were performed in order to predict the behaviour of the air flow which passes through all sections of the tunnel. Fluent 12.0 with the RNG k-epsilon turbulence model was used for the simulations. Three-dimensional meshes with approximately 945,000 nodes were used for calculations. Results showed good flow velocity profiles in the open test section. The future objective is to make a comparison between experimental and numerical studies in the empty test section. The characterization of the test section using CDF could be useful tool for future studies with obstacles in the test section.



Learning objectives
The flow visualization and measurement of the flow in the internal and external wake of a vertical axis wind turbine is crucial to understand and analyse the flow effects and changes in the performance of the turbine, by changes in the geometry or operating conditions of the turbine and, thus, decisive to successfully optimize its efficiency. Currently, we are performing these analyses in a small scale model of a commercial vertical axis wind turbine.


References
[1] Low-Speed Wind Tunnel Testing by J.Barlow, W. H. Rae and Jr. A. Pope, 3rd ed., EEUU.

[2] Design rules for Small Low-Speed Wind Tunnels by Mehta and Bradshaw, Royal Aeronautical Society, UK.

[3] Deutsche WindGuard Closed-circuit Calibration Wind Tunnel, Varel, Germany.

[4] Design and Characterisation of a Quiet, Low Turbulence Open Jet Blow Down Wind Tunnel in ISVR, University of Southampton, UK.

[5] The Design of an Open-Jet Wind Tunnel for Model Testing, Durham University, UK.