Combined effects of yaw misalignment and inflow turbulence on tidal turbine wake development

Abstract

The development of wakes downstream of horizontal-axis tidal stream turbines is crucial because these devices, when installed in arrays, generate predictable renewable energy. Wake formation impacts both individual turbine performance and overall array efficiency, as upstream turbines can reduce the power output of downstream devices. Additionally, tidal flows are often asymmetric and vary in the predominant incoming flow angle over short time scales. This study presents methods and findings from a lab-scale experimental campaign that investigated wake structure under combined yaw and turbulent flow conditions. A 0.9 m lab-scale tidal turbine was tested under both low- and high-turbulence inflow conditions, with three yaw configurations: $\pm 20°$ and a no-yaw case. A three-component laser Doppler velocimeter measured the downstream wake to characterise its structure. Under low turbulence, yawing slightly improved wake recovery. While most yaw cases showed wake skew, the centre-line progression of the wake was complex, influenced by constrained inflow characteristics. Some degree of self-similarity in the wake was observed, likely increasing with downstream distance. Analysis of wake turbulence revealed a complex picture. Turbulent kinetic energy was particularly high at the blade tips under yawed flow conditions. In the near to mid-wake region, turbulence length scales were smaller than the rotor plane. The return to isotropic conditions depended on inflow turbulence anisotropy, challenging common assumptions in computational turbulence models. These findings add to a more complex understanding of wake dynamics. The findings of this research suggest that optimisation of tidal devices and arrays utilising devices yawing and wake steering could be fruitful, however the limits of this lab-scale experiment in terms scale, blockage and ambient flow conditions mean that further work is needed.