Experimental study of the wake characteristics of an axial flow hydrokinetic turbine at different tip speed ratios

The wake characteristics of an axial flow hydrokinetic turbine were investigated for various tip speed ratios using velocity measurements and a flow visualization technique. The experimental results showed that the structures of the wake within six rotor diameters downstream of the turbine are significantly affected by a change in the tip speed ratio. As a tip speed ratio decreases, the core region of the wake that is featured by low axial velocity near the turbine rotor hub became more unstable, producing higher turbulence levels in the radial and azimuthal directions. It resulted in different growth rates of the shear layer that expands from the core region toward the outer part of the wake and that interacts with the tip vortices to trigger wake meandering. It turns out that the swirl number of the wake is a key factor that determines the stability of the core region in the near wake region. At locations more than six rotor diameters away from the turbines, the mean and turbulence characteristics became nearly independent of the change in the tip speed ratio. This was due to large-scale turbulent mixing whose size was in the order of the rotor diameter.