Assessing intermittency characteristics via cumulant analysis in near-wake of offshore wind turbines.

Turbulence intermittency in the wake behind a single floating wind turbine (FWT) as well as merging wakes of a pair of floating turbines is investigated using cumulant analysis. Hotwire data are collected through wind tunnel experiments. First order cumulant cascades show the pitch motion effect to be apparent at the rotor tips and beyond the swept area of the rotor. At the rotor tips, a shorter inertial subrange is observed affecting the large scales and causing them to drift away from the inertial subrange. Intermittency in FWT is more pronounced in a fixed wind turbine wake. Intermittency coefficients for single FWT are higher than the typical range c2=−0.025±0.003. The pitch oscillations appear as fluctuations in the large scales portion of the cascade within the first cumulants. Also, other less frequent fluctuations exist in both fixed and floating WT tips caused by vortex shedding and formation of coherent structures. Coherent structures grow in size moving above the top tip (ATT) and beyond the horizontal tips (BHT). This observation is noted as larger fluctuations in the flow above and beyond the rotor tips in addition to longer inertial subrange. For two FWT, pitch fluctuations vanish in between the two turbines as wakes meet.