Survey of the near wake of an axial-flow hydrokinetic turbine in quiescent conditions

Flow field results are presented for the near-wake of an axial flow hydrokinetic turbine in quiescent flow conditions. The turbine is a 1/25 scale, 0.8 m diameter, two bladed turbine modeled after the Sandia National Laboratory Reference Model 1 Tidal Current Turbine. All measurements were obtained in the large towing tank facility at the United States Naval Academy with the turbine towed at a constant carriage speed and a tip speed ratio corresponding to maximum power production. The turbine is scale independent with respect to lift and very slightly dependent with respect to drag for these conditions (Rec@0.7R≈4×105). The wake velocity field data was obtained using a two-dimensional particle image velocimetry (PIV) system. PIV ensembles were obtained for phase locked conditions. This paper focuses on characterizing the velocity and the mean flow structure in the near wake. Specifically, the downstream evolution of coherent tip vortices shed by the rotor blades were examined. Vortex aperiodicity was shown to increase with downstream distance. The streamwise spacing between adjacent vortex cores was shown to be constant within a diameter downstream of the rotor. Further downstream, significant vortex filament interaction was observed, including leapfrogging. This interaction is thought to be the primary mechanism for wake breakdown and re-energization.