POD analysis of the structure of vertical axis wind turbine wakes

Proper orthogonal decomposition (POD) is used to investigate the structure and evolution of the wake behind a model vertical axis wind turbine. The analysis reveals the high level of energy contained in the coherent structures that mark the vortex shedding due to the dynamic stall on the blades and the vortex shedding downstream of the tower, and the dynamics of the wake interactions. Vortex pairing is shown to be an important feature of the wake evolution. Changing the inflow conditions from a uniform flow to a simulated atmospheric boundary layer flow reduced the relative energy contained in the coherent motions, because of the reduced level of dynamic stall. Similarly, changing the blade sweep angle reduces the modal energy associated with dynamic stall, which is desirable in terms of mitigating fatigue due to torque and force variations on the blades.