EXPLORATION AND NUMERICAL SIMULATION OF WIND TURBINE
WAKE
F. Massouh *, I.K. Dobrev**
Ecole Nationale Supérieure d'Arts et Métiers (ENSAM) bvd. L'Hôpital, 151, Paris, 75013, France
+33 1 44246256/+33144246266; *E-mail: fawaz.massouh@paris.ensam.fr **E-mail: ivan.dobrev@paris.ensam.fr
Received: 24 Sept 2007; accepted: 31 Oct 2007
In this article, the flow behind a horizontal axis wind turbine (HAWT) is investigated and the obtained data is compared to the results of numerical simulation. The aim is to test reliability of random averaged Navier-Stokes (RANS) solver to model the wake behind a wind turbine. The experimental investigations are carried out by means of the 2D PIV measurements. The flow field is obtained in rotating frame of reference in which the rotor appears fixed by means of the phase-locked technique. Explorations are carried out in different azimuth planes. Because of large dimensions of the flow field, each azimuth plane is divided into several windows. For each window, the instantaneous velocity field is measured and stored successively to enable obtaining the averaged velocity field. Then, the flow in each azimuthal plane is reconstructed by stitching the averaged velocity field of these windows. Finally, the 3D velocity field is reconstituted by treating the results of images resulting from the different explored azimuth planes. These results are compared with RANS calculations. In general the numerical results show agreement with experiment, but some inconsistency concerning obtained power is revealed.
Keywords: wind energy
Organization(s): Ecole Nationale Supérieure d'Arts et Métiers, Researcher, Assoc. Prof. Education: PhD - Paris-VI University (1984). Experience: ENSAM (1979 to now).
Main range of scientific interests: fluid mechanics, wind energy. Publications: 7 papers in international scientific journals.
Fawaz Massouh
Ш
Ivan Dobrev
Organization(s): Ecole Nationale Supérieure d'Arts et Métiers. Education: Tech. Univ.-Sofia, Faculty of Energetic Machines (1978-1983). Experience: Tech. Univ.-Sofia, assistant (1983-2003). ENSAM, researcher (2004 to now). Main range of scientific interests: wind turbine, aerodynamics. Publications: 2 papers in international scientific journals.
Introduction
The investigation of the wake development downstream wind turbines is required for the design of wind farms. In this case the wake is influenced by the presence of atmospheric boundary layer, atmosphere instabilities and "ground-wake" interaction. The flow is very complex
and it is very difficult to obtain the velocity field downstream the rotor even with most sophisticated Navier-Stokes solvers. In such case there is need of experimental results that can validate calculations and adjust some parameters. There exist experiments in-situ, which permit to understand the wake structure [1]. However, in the case of large-scale experiments and due
to the sensors applied for velocity measurements, it is difficult to acquire with sufficient spatial and temporal resolution the development of the wake. As results, the usefulness of the obtained velocity field to serve as reference for CFD modeling is limited. Fortunately, there exist investigations of wake flows, which are realized in wind tunnel. These measurements are carried out in controlled flow conditions and more precise methods for the velocity measurement are applied. Numerous studies were performed in wind tunnels in order to reveal the development of wake behind a wind turbine and to obtain precise results. These studies [2-6] were carried out using a Pitot tube or hot wire anemometry (HWA). The main disadvantage of these experiments is due to one-point measurement capabilities of applied sensors. Thus it is not possible to obtain instantaneous velocity simultaneously in the entire field of investigation. Also, due to limited directional sensibility, it is not possible to obtain the velocities in core of the blade tip vortices. However, the particle image velocimetry (PIV) is a non-intrusive method that permits to measure the instantaneous velocity vectors in plane. But researchers used PIV technique for the exploration of wind turbine wakes are few and their results are rather qualitative. Here we might mention the papers [7-12]. Recently, stereo PIV measurements are carried in out the case of HAWT with diameter of 4.5m and first results are published in [13]. The aim of this study is to present the quantitative information about the wake downstream of model wind turbine. This information is acquired by means of the PIV technique and cannot be obtained by techniques of HWA or pneumatic sensors. For example, the possibility of obtaining numerical data on velocity field around the tip vortices is of great interest. Moreover, the quality of data obtained in case of yaw and non-yaw flow conditions permits to use these results as reference in case of CFD computing of flow around the model wind turbine. Finally in this study, the numerical simulation is carried out in the case of non-yaw conditions. The comparison of obtained results with experiment permits to reveal some particularities of the used random averaged Navier-Stokes (RANS) solver.
Experimental results
The Fluid Mechanics Laboratory at ENSAM, Paris has a closed circuit wind tunnel with a semi-open test section. A settling chamber is equipped with a convergent nozzle which has contraction ratio of 12. This contraction ratio ensures a uniform flow and the turbulence intensity does not exceed 0.5 % for a velocity of 35 m/s. The test section has working dimensions 1.35 m by 1.65 m and 2 m of length. The investigation is carried out in wind tunnel using a modified commercial wind turbine Rutland 503.
This horizontal axis wind turbine has a three blade rotor with diameter of 500 mm and hub diameter of 135 mm. The blades are tapered and untwisted. They have a pitch
angle of 10° and a chord of 45 mm at tip and 65 mm at the root. The rotational speed is 1000 rpm with a free-stream velocity of 9.3 m/s. Hence, the TSR is equal to 3, which is lower than the case of market wind turbines. The wind turbine is mounted on a support tube of 37 mm of diameter ensuring a sufficient height in order to allow the lasers fixed above the transparent roof to illuminate the explored plane with a sufficient intensity. Fig. 1.
Laser beam
Fig. 1. Experimental test bench
The PIV technique is applied to obtain the velocity field in the wake downstream the turbine rotor. Here, a double cavity Quantel "Blue Sky" Nd:YAG pulsed laser (532 nm), which produce approximately 120 mJ per pulse, is installed above the transparent roof of the wind tunnel test section. A cylindrical lens is used to create a thin vertical sheet of laser light, which passes through the center of the rotor. As the test section is semi-open and without sidewalls, it is possible to place the camera outside the tunnel, Fig. 1. Olive oil droplets are introduced for seeding on the inlet of the wind tunnel diffuser.
In order to carry out the phase-locked measurements, the test bench is equipped with an optical sensor. This sensor synchronize the lasers pulses and a reference angular position of the blade. The sensor tracks a reflecting target fixed on the rotor hub and emits a signal, each time when the target passes. Then the emitted signal is sent via a delay circuit. The change of delay time permits to change the angular distance between the reference position of blade and the plane of exploration. As the plane of exploration passes trough the rotor axis and the PIV used in this study is planar, then it is only possible to obtain radial and axial velocities. However the tangential velocity of the flow downstream the rotor is not zero and time interval between two laser pulses is set to 150 ^s, it is probable that the tracked particles will be blown out of the illuminated volume. Therefore, it is needed to adjust the laser sheet thickness to nearly 3-4 mm.
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300 200 100 0
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0 200 400 600
Fig. 2. Flow map reconstruction
800
Exploration of flow downstream the rotor is carried out in four azimuth planes with angles 0°, 30°, 60° and 90°. Here, the plane of 0° corresponds to the vertical position of the reference blade, Fig. 2. Because of the laser output power limitation and the camera resolution of 1600x1200 pixels, it is not possible to obtain with sufficient precision a velocity field larger than 300 mm. As a consequence to widen the explored area, the investigated velocity field is divided into six windows (3 horizontal by 2 vertical) with a certain overlapping. The scale of windows and their relative positions are defined using calibration markers placed in known positions inside the interrogation area. For this purpose, the images of these markers are taken after each series of tests.
For each explored window the imagery is repeated 95 times synchronously with rotor rotation. Hence temporal sequence is acquired during approximately 12 seconds in order to improve the precision of averaged velocity calculation.
Totally, four series of 6 by 95 pairs of images were acquired for different planes with azimuth angles of 0°, 30°, 60° and 90°; with zero corresponding to the vertical position of the reference blade.
Fig. 3. Raw image taken immediately behind the rotor in the h1 window
The raw images have a resolution of 1600x1200 pixels and 12 bits of grayscale resolution. Thanks to the synchronization between the laser pulse and the rotor position detected by the optical sensor, we can distinguish a blade frozen in vertical position, on the left. It can be seen also the cores of vortex tubes emitted from
the tips of other blades, Fig. 3. Due to velocities induced by these vortices the seeding particles are turned around the vortex core center and the centrifugal forces carry out these particles outside of
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