Tidal stream turbines were widely deployed at tidal energy sites with high turbulence levels. The harsh environment can significantly affect the wake dynamics, which in turn affects the overall efficiency of tidal turbine arrays/farms. Current studies have demonstrated a faster wake recovery and increased fatigue loads perceived by rotors prompted by incoming turbulence. However, the wake dynamics and recovery mechanism in turbulence were not fully understood. This paper presents a detailed analysis of the near-wake characteristics of a three-bladed, horizontal-axis tidal turbine. The results revealed a strong correlation between the wake recovery process and the tip vortex instability. Specifically, the onset of the instability was greatly accelerated by incoming turbulence, speeding up the wake expansion, shear layer development, and production of turbulence kinetic energy. The momentum recovery process was further quantified using the streamwise momentum equation, which manifests the important role of turbulence transport and vertical advection in the near wake region. The entire measurements and analysis provided a comprehensive insight into the wake recovery process of axial-flow hydrokinetic turbines, especially in turbulent environments.

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• Homogeneous turbulence
• Transition to turbulence
• Turbulence
• Wakes & jets