FLOW MODAL DECOMPOSITION OF A VERTICAL-AXIS WIND TURBINE WITH THE MOVING BOUNDARIES OF ROTATING BLADES

Aiming at analyzing the wake evolution features of a 3-blade vertical axis wind turbine(VAWT), dynamic mode decomposition(DMD) is used to evaluate statistics of the flow field based on unsteady flow field computations. Considering the limitation that DMD can only be applied to problems with stationary boundaries, the technology to deal with the moving boundaries of rotating blades is investigated. First, the flow fields of VAWT are calculated using CFD, and computations are compared with the experimental results to verify the numeral calculations. Then the spectral analysis is performed on the VAWT wake at a tip speed ratio of 2.35, and different frequency characteristics are observed in the near wake, the transition region of the wake, and the far wake, respectively. Next, DMD is employed to investigate the dynamic stall and the wake evolution of the VAWT in the absolute frame and the relative frame, respectively. Details of different possessing methods are presented. Finally, depending on the corresponding frequencies obtained before, the dominant modes are extracted from the CFD results by DMD. The extracted modes corresponding to specific frequencies can capture the coherent structures with different scales, which reflecting the characteristics of the propagation of shedding vortices from blades and the bluff-body oscillation in the wake of the VAWT.