Numerical study of the effect of a ridge on the wake and loading of a tidal stream turbine

Understanding the impact of bathymetry features on the wake and loading of a tidal stream turbine is crucial to inform deployment of tidal turbine farms. This study investigates the influence of a Gaussian ridge on a single turbine of diameter ($D$) using high-fidelity large-eddy simulations. The ridge height is 0.33$D$ and turbine locations at ridge centre and at six upstream and six downstream distances are analysed. The analysis elucidates the important role of bathymetry on wake recovery and fatigue design providing valuable insight for real-world planning of turbine arrays. The rate of wake recovery is increased both for turbine locations beyond 1.5$D$ upstream of the ridge due to the favourable pressure gradient over the upslope, and for locations beyond 3$D$ downstream of the ridge due to elevated turbulence intensity. For locations, close to and atop the ridge, the higher flow-speed and adverse pressure gradient of the downslope of the ridge were found to reduce the rate of wake recovery. When unaffected by the ridge wake meandering is similar to the flat bed case and characterised by Strouhal number but modulated by the frequency of ridge shedding downstream of the ridge. Damage equivalent loads are slightly increased at upstream locations due to flow speed-up and further increased at downstream locations due to a combination of increased turbulence intensity and variation over the rotor plane of both onset flow and turbulence.