Experimental Study on the Hydrodynamic Effects of Tidal Turbine Rotor Using Porous Discs

The hydrodynamic effect of tidal turbine has been a particular interest topic for current energy extraction. This study carried out a series of laboratory experiments to investigate the hydrodynamic properties of tidal turbine wake using two porous discs. Flow thrust on the rotor disc system was measured by a dynamic strain gauge located above water surface. Flow velocities were measured by an Acoustic Doppler Velocimetry within 10 times Rotor Diameter (RD) downstream. It was found that the hole distribution of porous disc had influence on both the hydrodynamic performance and the performance stability, with lower thrust coefficient and higher fluctuation for the non-uniform distribution. Wake velocity significantly reduced as flow passed through the rotor disc, while the bypass flow accelerated due to blockage effect. Immediately downstream, wake velocity deficit enlarged for the non-uniform disc. Shear layer occurred between the wake region and ambient flow, with a thickness being 0.25RD. The shear layer diffused towards wake core with limited expansion outwards at the initial stage of wake attenuation. Strong turbulence mainly focused in the shear layer just downstream, but it diffused rapidly into wake core within 4RD downstream. The wake zone expanded outwards and the wake core moved towards water surface further downstream. The study provided detailed measurement data of wake velocities to further investigate the hydrodynamic characteristics of wake attenuation.

Keywords: Laboratory experiments, Tidal turbine rotor, Hydrodynamic performance, Wake velocity, Wake turbulence