Modulation of turbulence scales passing through the rotor of a wind turbine

Abstract

The modulation of boundary-layer turbulence across scales by passage through the rotor of a model wind turbine is assessed experimentally in a wind tunnel. Synchronous measurements of the flow were performed using hotwire anemometers located directly upwind and at various locations downwind of the turbine along the horizontal axis of symmetry. The synchronous data allowed for the distinct quantification of added and suppressed turbulence by considering the temporal correlation between upwind and downwind time series. The suppressed turbulence exhibited larger characteristic time scales than the added turbulence which decreased slightly with distance, while the integral time scale of the added turbulence increased substantially with downwind distance. The intensity of the suppressed turbulence did not change substantially with downwind distance, suggesting that the turbine had a much stronger effect on the dampening of the turbulence than simple natural evolution. The cross spectra between upwind and downwind velocity measurements suggest a dispersion relation for different time scales. In the near wake, lower-frequency components appear to be advected at velocity lower than the local wake velocity, and this advection velocity asymptotically approaches the local velocity at high frequency. This trend diminished in magnitude with downwind distance.