Influence of Tip Speed Ratio on wake features of a Vertical Axis Wind Turbine

Dependence of wake features of a Vertical Axis Wind Turbine (VAWT) on Tip Speed Ratio (TSR) is studied in the present work, using a computational approach, based on Large-Eddy Simulation (LES). The overall methodology was validated in an earlier study, via comparison with Particle Image Velocimetry (PIV) wind-tunnel measurements on the same turbine. Results are analyzed in terms of both ensemble-averaged and phase-averaged statistics. They demonstrate a strong dependence of wake properties on TSR. Increasing values of such parameter produce a higher momentum deficit in the wake, but also a faster wake recovery, triggered by the instability of the shear layers at both leeward and windward edges of the wake of the turbine. Both turbulent kinetic energy and turbulent production display their maxima at the leeward and windward sides of the wake, where the largest velocity gradients occur. Turbulent production was found higher at higher TSRs, making turbulent kinetic energy few diameters downstream of the turbine an increasing function of TSR. The level of coherence of the wake was instead usually a decreasing function of TSR, with the largest cylindrical vortices populating the leeward side of the wake.