Abstract
The reveal of the near wake structure and development is of vital importance to the understanding of the working principle and power coefficient of tidal turbines. In this paper, the detailed near wake flow field was visualized by high resolution particle image velocimetry (PIV) technique, and the time-averaged mean velocity (both axial and tangential) and phase-locked transient velocity were measured, respectively, for a counter-rotating horizontal axis tidal turbine in the wind tunnel. The vortex was captured by vorticity and swirling strength calculation of the vector field for each phase, and the vortex formation and development were analyzed based on the summation of the data from all the different phases. To achieve more information about vortex, the experiment was conducted for two different axis distances and three different tip speed ratios. The existence of the rear rotor affects the trajectory of vortices shed from the front rotor and enhances the velocity deficit of the free stream. The experimental result can serve as a reference for numerical simulations.