Flume experiments for studying the effects of the tip speed ratio on the flow pattern in the wake of a horizontal-axis hydrokinetic turbine

Abstract

Effects of the tip speed ratio on the wake of a hydrokinetic turbine were experimentally investigated using acoustic Doppler velocimetry and flow visualization. Systematic comparisons were conducted to analyze the effects of the tip speed ratio on three dimensional mean velocity and turbulent flow field, wake dynamics, and bed shear stress. In addition, an actuator disk that has the same diameter as the turbine rotor was used to further examine the origin of the low frequency motions in the wake. Two distinct low frequency motions were observed in the wake: the inner wake meandering observed in the near wake due to the instability of the hub and tip vortices, and the wake meandering detected in the far wake, which has similar wake characteristics with the von karman vortex shed from a bluff body. Results show that the distributions of the momentum flux are largely sensitive to the tip speed ratio, and the momentum flux that originates from the blade tips governs the wake meandering phenomenon in the transition and the far wake. Because the distributions of the momentum flux that are observed in the two tip speed ratio conditions become identical in the far wake, the low frequency motions exhibit a constant dominant low frequency peak in the far wake regardless of the tip speed ratio. The measurements also elucidate that the distributions of the mean streamwise velocity and turbulence kinetic energy become independent on the tip speed ratio after six and nine rotor diameter, respectively. Moreover, greater bed shear stress is obtained in the higher tip speed ratio condition due to the increased blocking effects of the rotating blades. The installation of a turbine was found to have a trivial impact on the bed shear stress further downstream than six rotor diameter.