This paper presents numerical results for the wake behind a three bladed horizontal axis turbine in a uniform free upstream current. A three-dimensional software, initially issuing from Pinon et al. [6], taking into account the non stationary evolution of the wake emitted by turbine blades is developed in order to assess the disturbances generated on the sea-bed and on the free surface. So an unsteady Lagrangian method is considered for these simulations using Vortex Method. The vortex flow is discretised with particles carrying vorticity, which are advected in a Lagrangian frame.