This paper presents results from numerical simulations of a 3-bladed horizontal axis tidal stream turbine (TST). A two-equation transient Reynolds Averaged Navier Stokes (RANS) k-ω Shear Stress Transport (SST) model was used for code validation and testing of a newly implemented sliding mesh technique for Code Saturne. Wall- and blade-resolved large-eddy simulations (LES) were then performed to study the complete geometry at various tip speed ratios (TSR). Thrust and power coefficients were compared to experimental measurements in a towing tank for a range of TSR (4, 5, 6, 7 & 8) at a fixed hub pitch angle. A strong flow meandering was observed with the wake recovery and velocity deficit showing a high sensitivity to upstream turbulence intensities. The mean thrust and power coefficients by RANS k-ω SST model were found to be more sensitive to the upstream turbulence than the LES. A comparison of all models are also presented for the mean sectional blade pressures and mean wake velocity profiles. The paper also presents an overview of modelling and numerical issues relating to fine LES for such rotating geometries.