Effects of submergence depth on the performance of the savonius hydrokinetic turbine near a free surface

The influence of the submergence depth ratio on the Savonius hydrokinetic turbine and flow characteristics is numerically studied under different tip speed ratios. The tip speed ratio TSR (Ratio of the flow speed to blade speed) changes from 0.4 to 1.2, and the submergence depth ratio H*(Ratio of the depth to the turbine diameter) ranging from 0.22 to 1.1. The main conclusions are: (1) When the submergence depth ratio increases from 0.22 to 0.66, the flow field structure gradually becomes regular, and the power coefficient keeps increasing. (2) When the submergence depth ratio reaches up to 0.88, the curve of the power coefficient is similar to that of H* = 1.1, which means the effect of free surface can be ignored when H*>0.88. (3) According to the pressure distribution around the turbine, the free surface will increase the pressure difference on the blade due to the surface tension and blockage effect, thereby influencing the rotation of the turbine. (4) As the TSR increase, the submergence depth ratios of 0.55 and 0.88 are regarded as two demarcation points of the average power coefficient.