The wake of an unsteady actuator disc

Blade Element Momentum (BEM) is the most important aerodynamic analysis method for wind turbines. BEM is derived assuming stationary conditions, which limits its ability to model the unsteady aerodynamic effects. This becomes increasingly relevant for the flexible blades of current large-scale turbines, and the employment of passive and active aerodynamic control strategies, such as yaw, pitch control and smart rotor control. Currently, sub-models are included to consider the unsteady aerodynamic effects for wind turbine design. Previous research developed several dynamic-inflow engineering models to be integrated into BEM, to account for the unsteady flow acceleration. However, their applicability for unsteady load and the relative performance between the models are not fully known. The development of the dynamic wake of an actuator disc under unsteady load needs further understanding, to improve the engineering prediction of dynamic-inflow effect. This research aims to evaluate the accuracy of BEM with current dynamic-inflow engineering models; to further understand the dynamic wake flow-field of an actuator disc undergoing unsteady load; to improve current dynamic-inflow engineering models for wind turbine design using numerical and experimental approaches. A free wake vortex ring (FWVR) model is firstly developed. The accuracy of BEM with current dynamic-inflow engineering models of Pitt-Peters, Øye and ECN in predicting the induction of an actuator disc with unsteady load is verified using the developed FWVR model. The wake flow response of an actuator disc undergoing unsteady loads is studied experimentally by using a disc model with variable porosity. The unsteady load is generated by a ramp type variation of porosity of the disc, at several reduced times of the ramp motion. The wake development of an actuator disc undergoing the same unsteady load tested in the experiments is further studied using the FWVR model. The steady actuator-disc model is extended to unsteady load. Results from this linear actuator-disc model are compared with those from the FWVR model. Finally, a new engineering model is developed using the differential form of the Duhamel’s integrals of indicial response of the actuator-disc type vortex-models. The time constants of the indicial functions are obtained by the indicial responses of a linear and a nonlinear actuator-disc model, respectively. The work provides more insights into the wake development of an unsteady actuator disc. The experimental results create a database for validation of unsteady numerical models, in prediction of the dynamic induction in the near wake of an actuator disc or a rotor. The limitation of current dynamic-inflow engineering models are evaluated and discussed. The new engineering model, which is developed based on the indicial response of the nonlinear actuator-disc model, can better predict the dynamic-inflow effects, especially for the radial distribution of the dynamic-inflow effect.