power performance of a marine current turbine under a free surface through a nite volume solver coupled with immersed boundary method. Two free surface methods have been applied and validated. Then the power coecient at dierent tip speed ratios was calculated and the results compared with reference experimental data exhibiting a general good agreement. Noruzi et al. [3] carried out a numerical simulations by means of Reynolds-averaged Navier-Stokes (RANS) with and without gravity waves and the results were compared to experimental data. The results showed that when the ratio between the installation depth and the total depth of the domain is below a critical value, the installation depth of the rotor inuences the performance of the turbine. A few studies concern the investigation of power performance of turbines arrays. Mycek et al. [4] carried out an experimental and numerical study both for a single turbine and for two turbines. A three-dimensional software was employed based on a Lagrangian vortex particle method for numerical