Analysis of an Actuator Disc under Unsteady Loading

Abstract

Wind turbines operate in the earth's atmospheric boundary layer - an environment where the climate is turbulent and unsteady. In addition, wind turbines usually operate in clusters (wind farms) and are thus subjected to the unsteady wakes generated by upstream turbines. Because of the unsteady inflow, wind turbines suffer from structural fatigue damages. There is therefore motivation to better analyse and model the unsteady loading in order to design better turbines and reduce the cost of energy. To study the unsteady aerodynamics phenomena, an actuator disc model under unsteady loading was simulated using experimental and numerical methods. Experimentally, a porous disc rotor was constructed and tested in the Open Jet Facility, TU Delft. The design involved a novel method to dynamically control the rotor's thrust coefficient through its porosity. Numerically, a dynamically loaded actuator disc was implemented in the RANS equations and solved by the commercial CFD solver Star-CCM+. Results from the experiment, CFD simulations, as well as a free wake vortex ring model were used to benchmark the performance of dynamic inflow engineering models. Results show that the wake of a wind turbine under steady loading is also turbulent and meandering. The Strouhal number of the wake fluctuation closely agrees to that from a previous experiment of a 2-bladed wind turbine (Medici & Alfredsson, 2006). This suggests that the wakes of blade rotors are comparable to that from disc rotors. The tower shadow effect also bears a major influence on the inner wake profile which causes further velocity deficit and higher turbulent intensity. For rotors with higher CT , the velocity recovery process occurs relatively earlier. This is because of the enhanced momentum entrainment arising from a steeper velocity gradient at the shear layer. For the disc rotor under unsteady loading, the wake is affected by unsteady aerodynamics or the dynamic inflow effect. The transient velocity profile experiences an overshoot due to the passage of the 'old' and 'new' shed vortices, which are generated at the wake edge as a consequence of the uniform load profile on the rotor. The subsequent decay of the velocity in the wake or inflow at the rotor plane to the 'new' equilibrium state is a result of the rate of convection of the 'new' shed vortices to the far field. From this research, numerical results showed lower inflow velocity decay rates (velocity response due to the change in loading on the rotor) than that predicted by engineering models. While this is so, it was found that the decay rate is highly influenced by the ambient flow's turbulence. At high ambient turbulence, the 'new' equilibrium state is achieved in a shorter time due to the enhanced momentum mixing process. It is hypothesised that this is the primary reason behind the higher decay rates predicted by empirical-based engineering models.