For tidal turbines mounted on floating structures the possibility of cavitation occurring on the blades is higher than for seabed mounted tidal turbines. In this study we present Reynolds-Averaged Navier-Stokes (RANS) solutions of the well-studied Southampton three bladed horizontal axis tidal turbine (HATT). The numerical simulations were carried out using the ReFRESCO viscous flow solver using three types of simulations:
steady wetted flow;
unsteady wetted flow and
unsteady cavitating flow.
The wetted flow simulations gave overall good prediction of thrust and power coefficients over the entire experimental range of tip speed ratios (TSRs), with the unsteady solution providing the better result. Low numerical uncertainties were obtained for medium to high TSRs and larger for low TSR values, where the flow is transitional and highly separated. The dynamic cavitation simulation was carried out for the case of a cavitation number of 0.63 at a TSR of 7.5. The simulations showed a good agreement of the extent of the sheet cavity. However, the dynamics of the sheet cavities have not been fully captured and the power and thrust coefficients are under predicted compared to the experiments. This is most likely due to lack of mesh resolution outside the wetted flow boundary layer where the cavity dynamics occur, and due to high numerical and experimental uncertainties for such a complex flow case. The simulations showed that existing methodology used for computing cavitation on marine propellers could be applied to HATTs, yielding reliable results. Importantly, simulation of cavitation on HATTs could be used as input for noise and erosion predictions.
While the literature on computational fluid dynamic (CFD) simulations of HATTs is vast and many highly sophisticated RANS and large eddy simulation (LES) studies have been put forward, very few studies have addressed the topic of possible occurrence of cavitation on the turbine blades. Cavitation and its negative effects - erosion, noise, vibrations and thrust breakdown - are frequently occurring in rotating machinery such as propellers and hydro turbines. Cavitation has been observed in laboratory experiments of HATTs (Bahaj, Molland, Chaplin and Batten, 2007), but traditionally it is said that the risk of cavitation is low under production conditions. This is true for bottom mounted tidal turbines but for tidal turbines mounted on floating support platforms this might change due to the smaller hydrostatic pressure as well as the influence of waves.