Vertical Axis Tidal Turbine Behaviour under Sheared Flow Effects

ABSTRACT: Tidal turbines are often subjected to complex flow conditions that can affect their power output and the risk of failure. In this article, an experimental study on a vertical axis tidal turbine with twin counter-rotating rotors is carried out at 1/20 scale, submitted to a sheared turbulent (ST) flow and a sheared weakly turbulent (SWT) flow. The performance and wake development comparison indicates that the turbine behaves differently depending on the shear rate considered. A 7% decrease in performance is observed at the turbine’s nominal operating point between uniform and ST conditions. The asymmetry of the flow along the vertical axis is reflected in the angular and frequency distributions of the rotor torque, indicating a production asymmetry between the lower and the upper rotors. Analysis of wake development reveals that transport terms constitute the main mechanism of wake dissipation. In the case of SWT and uniform flow, vertical advection largely dominates the other terms, whereas in ST flow, transverse advection is initially predominant. This results in a higher average wake height and a lower average wake width in the ST case compared to the other flow conditions, and a faster wake recovery.