In this paper, the quantitative investigation of the free-surface effect on tidal turbine performance by using a multi-phase flow formulation is presented. The computational formulation is briefly introduced. The free-surface effect is rigorously studied by the simulations performed for a single turbine and two back-to-back turbines using different inflow conditions and immersion depths. The thrust and power coefficients of the tidal turbines in free-surface flows are quantified. It is found that the presence of free-surface has a significantly negative effect on tidal turbine performance by decreasing the thrust and power coefficients in idealized inflow conditions, such as uniform inflow and Airy wave inflow conditions. The comparison with pure hydrodynamics simulation results shows that pure hydrodynamics simulations are unable to provide accurate predictions and the free-surface effect must be taken into account in the modeling and simulations of tidal turbines, especially for shallow immersion depth.
Keywords —tidal energy, computational fluid dynamics, free-surface flows