Numerical Simulation of Cavitation on a Horizontal Axis Tidal Turbine

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: (i) steady wetted flow; (ii) unsteady wetted flow and (iii) 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.

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