The purpose of this paper is to show the feasibility of a large-scale unsteady flow computation. This computation consists in the simulation of the rotating stall appearing into a pump-turbine operating at part load condition (Q/QBEP=76%) in pumping mode. The computations are performed using an Overset Finite Element Large Eddy Simulation (LES) code with the dynamic Smagorinsky turbulence model (DSM). To understand the onset and the propagation of the stall patterns a fine mesh is required in order to compute the small turbulence scales present in the turbulent boundary layer (TBL). In the present study such fine mesh features 8 billion elements.