Survey of the near wake of an axial-flow hydrokinetic turbine in the presence of waves

Abstract

Flow field results are presented for the near-wake of an axial-flow hydrokinetic turbine in the presence of surface gravity waves. The turbine is a 1/25 scale, 0.8 m diameter, two bladed turbine based on the U.S. Department of Energy's Reference Model 1 tidal current turbine. Measurements were obtained in the large towing tank facility at the U.S. Naval Academy with the turbine towed at a constant carriage speed and a tip speed ratio selected to provide maximum power. The turbine has been shown to be nearly scale independent for these conditions. The selected wave form was intended to represent oceanic swell encountered off the U.S. eastern seaboard. The resulting model wave is a deep water wave, in terms of relative depth, traveling with the “current”, in the opposite direction of the towing carriage. Velocity measurements were obtained using a submersible, planar particle image velocimetry (PIV) system at streamwise distances of up to two diameters downstream of the rotor plane. PIV ensembles were obtained for phase locked conditions with the reference blade at the horizontal position. Phase averaged results for no-wave and wave conditions are presented for comparison showing further expansion of the wake and shear layer in the presence of waves as compared to the no-wave case. When ensembles are selectively sampled on the wave phase, a high degree of coherency is shown to remain and the wake width is shown to undulate with the passing of the wave, with the vertical displacement range on the same order as that of a particle under similar conditions. The impact of waves on turbine tip vortex helical structure is also examined. Waves are shown to change the location of adjacent helices. In the streamwise direction, this changes the pitch of the helix, shown in previous studies to affect the downstream wake recovery distance. In the vertical direction, depending on the wave-induced flow field at the time they were created, vortices are forced outward, into the mean flow or inward, into the wake core, potentially enhancing kinetic energy transport and accelerating the re-energization process.