A new actuator disc model for oscillatory and steady flow that predicts reductions in jacket drag loads and enhancements in turbine power

Structures like marine wind and current turbines, and offshore jackets, may be most simply represented as porous obstacles. These exert a retarding force opposing the relative motion of the surrounding fluid, thus slowing the fluid velocity that passes through the structure. It has been shown experimentally that this reduction in fluid velocity is greater for combined oscillatory and steady flow than for steady flow alone, due to the extra mean drag force (or thrust force, for a turbine) applied over an oscillation period. In this paper, the interaction of oscillatory and steady flow with a porous structure is explored by combining two existing actuator disc models — one for a disc in a bounded steady flow and a second for an unbounded disc in oscillatory and steady flow. The new combined model overcomes previous limitations in modelling cases with high drag loading, and may be used to correct for geometric blockage associated with laboratory testing of model structures in wave and current flumes. Further, application of the model to turbines indicates that the time-averaged power removed from the flow exceeds that for steady flow, which may have implications for energy extraction by floating wind turbines undergoing oscillatory motion and tidal turbines placed in environments with waves and current.