409_EWEC2008fullpaper - 3D STEREO PIV STUDY OF TIP VORTEX...

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3D STEREO PIV STUDY OF TIP VORTEX EVOLUTION ON A VAWT Claudia Hofemann, Carlos Sim˜ao Ferreira, Kristian Dixon Gerard van Bussel, Gijs van Kuik, Fulvio Scarano DUWIND - Delft University Wind Energy Research Institute Kluyverweg 1, 2629 HS, Delft, The Netherlands c.j.simaoferreira@tudelft.nl (corresponding author) Abstract The increased application of Wind Turbines in the built environment has lead to the recurrence of Vertical Axis Wind Turbines (VAWT). In highly turbulent urban flow fields, VAWT present several advantages over Horizontal Axis Wind Turbines, mainly due to its insensitivity to yaw and aesthetics. The complex aerodynamics of the VAWT are dominated by three-dimensional unsteady flow effects, including phenomena such as wake-blade inter- actions, wake deformation in the rotor volume, tip vortex effects, dynamic stall and time-varying induction. The present research paper aims at understanding and quantifying the development of the near wake of a VAWT, by experimentally measuring by 3D-Stereo Particle Image Velocimetry the evolution of the tip vortices and imme- diately adjacent flow. A two bladed H-Darrieus VAWT model was tested in the low speed/low turbulence wind tunnel at the Technical University of Delft. Stereo-PIV measurements were used to visualize the flow in the near wake focusing on the flow field around four different tip geometries at two different tip speed ratios. The mea- surement planes cover several section of the rotor volume, allowing for a reconstruction of the evolution of the tip vortex and nearby dynamic stall shed wake. The formation, convection and dissipation for each tip vortex were located and quantified. A comparison the Stereo-PIV Data for the four different tip geometries shows a clear influence of the tip shape on the generation and release of the tip vortex and the influence of the tip speed ratio on the interaction between the dynamic stall and the tip vortices. The experimental results provide a new insight into the development of dynamic stall and vortex formation on VAWTs. The visualization of the flow field for the different cases provides an important database for the validation of numerical dynamic stall and vortex models Keywords: Vertical Axis Wind Turbine, PIV, Tip Vortex, Wake, Darrieus 1. * Nomenclature c airfoil/blade chord, m D rotor diameter, m R rotor radius, m Re Reynolds number Re = c.λ.U U Unperturbed velocity m/s U Local flow velocity m/s α angle of attack λ tip speed ratio Ω .R/U θ azimuth angle Γ circulation m 2 /s 2. Introduction One of the results of the development of solutions for the built environment is the reappearance of Verti- cal Axis Wind Turbines (VAWTs). Extensive research
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was conducted until the end of the 1980’s, when, due to the increasing success of the application of Hori- zontal Axis Wind Turbines (HAWTs), it was discon- tinued. Yet, in the built environment, VAWTs present several advantages over the (presently) more common HAWTs, namely: low sound emission (consequence
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This note was uploaded on 11/13/2011 for the course AEE 495 taught by Professor O.uzol during the Spring '11 term at Middle East Technical University.

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409_EWEC2008fullpaper - 3D STEREO PIV STUDY OF TIP VORTEX...

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