Wake characteristics behind a tidal turbine with surface waves in turbulent flow analyzed with large-eddy simulation

Abstract

To design tidal stream turbine arrays, turbine wakes need to be fully characterized to assess the adequate row spacing considering environmental factors such as onset turbulence, velocity shear, and surface waves. The role of waves on wake development varies depending on their characteristics, such as wavelength and amplitude, and needs to be carefully understood. Here large-eddy simulations are performed to analyze the instantaneous and time-averaged wake characteristics developed downstream of a tidal stream turbine for four wave-current conditions ranging from nearly deep to intermediate waves and compared to current-only results. The tip-vortex convection near the free surface is highly influenced by the waves. During the period of increased surface elevation, there is an upwards motion of the vortices which have merged into a single vortical structure during the wave trough period. Downwards vertical transport of tip vortices occurs after the pass of the wave crest, enabling vertical entrainment of momentum into the turbine wake region. Disk-averaged velocity deficit recovery increases for wave-current cases compared to the current-alone condition, with all intermediate wave cases showing a very similar wake recovery rate throughout the wake length. The nearly deep-water waves lead to a slightly slower recovery rate up to 12 turbine diameters (D) downstream while, after this distance, the recovery rate is the highest among the simulated wave conditions. The integration of the mean kinetic energy (MKE) budget in the region up to 12D shows there is a similar distribution in the balance between terms irrespective of the wave-current case. However, in the far-wake region covering from 12D to 20D, the nearly deep waves have a larger contribution for MKE replenishment from the transverse convection and turbulence transport terms, balanced with their vertical counterparts and the streamwise convection term. Analysis of the wake self-similarity shows that a Gaussian model description holds for the recovery rate and lateral profile of the wake of a tidal turbine operating in coupled wave-current conditions, with a similar rate of velocity recovery after 12D irrespective of the wave characteristics. Lateral wake spreading is smaller for the shortest wavelength, which relates to an increased contribution of transverse fluxes in the MKE budget. Published by the American Physical Society 2024