Performance of counter-rotating tandem propellers at oblique flow conditions

Tidal currents are a potential source of reliable and renewable energy. Various power generation systems have been proposed. The authors developed a counter-rotating type tidal stream power unit in which the tandem runners make the inner and the outer armatures counter-rotate. The unit is composed of tandem propellers with 3 front and 5 rear blades and has a promising advantage that the rotational torque between the front runner and the rear propellers, namely the inner and the outer armatures, is balanced in the unit. That is, this tidal stream power unit can be moored with only one cable and then its nose and tail will naturally line up in response to the tidal stream by yawing. In this paper, the performance of the propellers in oblique flow conditions were investigated experimentally and numerically. When the coming flow obliquely to the propeller by 30 degrees, the performance deteriorates by 20%.