Large Eddy Simulation of an isolated vertical axis wind turbine

A computational study is presented on the wake features of a Vertical Axis Wind Turbine (VAWT), with a particular focus on the way stall phenomena lead to the formation of large coherent structures. In our earlier work (Int. J. Heat Fluid Flow 61: 75-84, 2016) numerical results were compared with experiments carried out in similar conditions. The operation of VAWTs is characterized by the generation of large vortices, especially at low Tip Speed Ratios (TSRs), corresponding to larger angles of attack. In the present study the use of a Large Eddy Simulation (LES) approach, coupled with an immersed-boundary (IB) formulation, allowed the solution of such coherent structures, which were found to affect substantially the flow within the rotor and downstream of the turbine. Two values of TSR are investigated here, in order to discuss its influence on the wake structure. Lower TSRs are associated to boundary layer separation closer to the leading edge of the blades and larger rollers, during both upwind and downwind stall. Upwind stall affects more substantially wake properties, producing larger structures populating the leeward side of the overall wake.