Misaligned sheared flow effects on a ducted twin vertical axis tidal turbine

Abstract

Most tidal turbines experimental tests are carried out in idealised conditions with uniform velocity profiles and without flow misalignment. However, at sea, the velocity profiles are mostly sheared and the flow direction spreads out around the main direction. In this study, we address experimentally the effect of those realistic conditions on the response of a twin counter-rotating vertical axis tidal turbine in Ifremer’s wave and current flume tank. At the whole turbine scale, the current shear hardly affects the drag and the average power coefficient but it leads to a 35 % increase of the power standard deviation. The flow misalignment does not affect the power production but it raises the average drag coefficient by 15 % for a ±15°angle of incidence. At the rotor column scale, both the shear and the misalignment modify the torque distribution with regard to the blades angular position, with a torque asymmetry up to 30 % between the upper and lower rotors, among other effects. Absolute rotor angular position as well as blades local flow visualisation would be needed to explain the complex torque distribution on a rotor column, which must be taken into account in the turbine design. This paper also provides a wide experimental database for the validation of numerical models applied to ducted twin vertical axis tidal turbines.