Influence of seabed irregularity on the wake of a tidal turbine

Bathymetric features at tidal sites can introduce complex flow phenomena, such as pressure gradients and bed-induced turbulence. Understanding how these modify tidal-stream turbine wakes is critical to the design and operation of turbine arrays. Large-eddy simulations are performed to characterise the wakes of a tidal-stream turbine operating over a rough bed generated as a self-affine surface with roughness statistically similar to those found at a real tidal site. Comparison is drawn to smooth bed conditions, both without and with a localised Gaussian ridge. The rough bathymetry enhances near-bed turbulence generation and increases vertical shear. Different wake centre propagation and vertical wake 18 expansion are observed for four transverse turbine positions, whilst lateral wake expansion is similar. Disc-averaged velocities and turbulence intensity in the wake region are always higher for the rough bed cases. The ridge with a smooth bed leads to coherent turbulence structures, but these are suppressed by the higher turbulence occurring over the rough bed. Similar wake recovery rates are seen for all cases. However lateral wake expansion is larger for the smooth bed, especially in the far-wake. The irregular bathymetry leads to an uneven streamwise variation of pressure transport and streamwise convection terms of turbulent kinetic energy budget. The ridge’s adverse pressure gradient increased the contribution of the vertical convection term, counterbalanced by larger vertical turbulent transport and turbulent production. These findings highlight that realistic bathymetry notably impacts turbine wake recovery, with smooth bed conditions underestimating downstream velocity and overestimating wake expansion.