scheurich_frank_-_glasgow - Modelling the Aerodynamics of...

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Modelling the Aerodynamics of Vertical-Axis Wind Turbines Frank Scheurich PhD Student, University of Glasgow, Glasgow, UK E-Mail: [email protected] In recent years, there has been a resurgence of in- terest in both large-scale and small-scale vertical-axis wind turbines. The accurate aerodynamic modelling of these devices poses a signi±cant challenge. The cyclic motion of the turbine induces large variations in the angle of attack on the blades during each revo- lution of the rotor that result in signi±cant unsteadi- ness in their aerodynamic loading and can manifest as dynamic stall (see Fig. 1). In addition, an aero- dynamic interaction occurs between the blades of the turbine and the wake that is generated by the rotor (Ref. 1). Interactions between the blades of the tur- bine and, in particular, tip vortices that were trailed in previous revolutions produce impulsive variations in the blade aerodynamic loading, but these interac- tions are notoriously di²cult to simulate accurately. The aerodynamic characteristics of vertical-axis wind turbines are thus somewhat more complex compared to those of horizontal-axis con±gurations. Vertical- axis wind turbines are therefore still relatively poorly understood devices but the accurate numerical predic- tion of their performance is essential if their design is to be improved. A summary will be presented from a current doc- toral research that aims to increase the understand- ing of the ³ow ±eld surrounding vertical-axis turbines. The Vorticity Transport Model (VTM) has been used in this study to analyse the performance of di´erent vertical-axis turbine con±gurations when their rotors are operated in di´erent operating conditions. -25 -20 -15 -10 -5 0 5 10 15 20 25 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 Geometric angle of attack, α [deg] Sectional tangential force coefficient, C t VTM + dynamic stall model Experiment Figure 1: VTM-predicted tangential force coeFcient on a NACA 0015 aerofoil compared with experimental mea- surements of dynamic stall (Ref. 2) when the aerofoil was operated at Re=800,000. Taken from Ref. 3. The VTM enables the simulation of wind turbine aerodynamics by providing a high-±delity representa- tion of the dynamics of the wake that is generated by the turbine rotor. In contrast to more conventional computational ³uid dynamics techniques in which the ³ow variables are pressure, velocity and density, the VTM is based on the vorticity-velocity form of the unsteady incompressible Navier-Stokes equation. The bound vorticity distribution on the blades of the ro- tor is modelled using an extension of lifting-line the- ory. The lifting-line approach has been appropriately modi±ed by the use of two-dimensional experimental data in order to represent the real performance of any given aerofoil. The reader is referred to Ref. 4 for a detailed description of the computational method and to Refs. 1 and 3 for comparisons between VTM pre-
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scheurich_frank_-_glasgow - Modelling the Aerodynamics of...

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