The effects of inflow conditions on vertical axis wind turbine wake structure and performance

The behavior of wind turbines wakes is of considerable interest, especially in the context of wind farms where wake-turbine interactions are important. Here we investigate experimentally the wake structure of a straight-bladed vertical axis wind turbine (VAWT), both in uniform flow, and immersed in a thick turbulent boundary layer. We use high resolution experimental data obtained with particle image velocimetry (PIV) over a range of tip speed ratios typical of the operation of vertical axis wind turbines. A highly three-dimensional wake was observed, with strong deflections in both the spanwise and wall-normal directions. The deflection toward the wall may help explain the larger equivalent roughness length observed for VAWT wind farms. Three distinct regions were identified in the wake, marked by the presence of vortex streets, including a previously unreported vortex street downstream of the advancing blade at the blade passing frequency. Increasing the tip speed ratio decreased the spacing between the vortices in vortex streets downstream of the advancing and retreating blades, causing them to appear more as vortex sheets, with increased velocity deficits and increased turbulent kinetic energy. The wake decays rapidly downstream of the turbine, with significant momentum recovery and turbulence reduction after only three turbine diameters. Finally, inflow conditions do not affect the overall wake structure, though an increase in the wake velocity deficit and a decrease in the near wake turbulence were observed in the boundary layer inflow condition, despite the associated increase in the inflow turbulence levels.