Controlled low-frequency modulation of tip vortex instability to enhance turbine wake recovery

This study investigates the impact of periodically modulating the strength of tip vortices on the instability and recovery mechanisms of a wind turbine wake. A simplified representation of tip vortices as a nominally two-dimensional vortex street in isolation is used, allowing for high-resolution large-eddy simulations for a range of control scenarios. An actuator fence model is adopted to harmonically impose resistance on the flow field, forming discrete vortices with a controlled strength modulation. It is found, in agreement with previous studies, that depending on the modulation frequency, coherent flow structures consisting of groups of several vortices may appear post-instability. This leads to delayed but increased production of turbulent kinetic energy (TKE), which in turn affects the wake recovery. The size of coherent structures can be controlled by the choice of modulation frequency, with lower 'frequency ratios' (relative to blade passing frequency) leading to larger coherent structures and thus higher production of TKE where they break down. This suggests that low-frequency tip vortex modulation through e.g. flap actuation could enhance not only near-wake breakdown but also far-wake recovery, depending on the frequency ratio.