Unsteady aerodynamics with applications for wind turbines

Abstract

Wind energy plays a crucial role in clean energy generation. Currently, horizontal axis wind turbines dominate the wind energy sector. Nevertheless, vertical axis wind turbines (VAWTs) have some advantages over traditional horizontal axis wind turbines.However, their performance is difficult to predict and they are prone to failure over time. This is because the axis of rotation of a VAWT is perpendicular to the wind direction, which causes the individual blades to experience rapidly fluctuating flows, even when the wind direction is constant. These fluctuations can lead to unsteady flow phenomena that are not well understood. In this dissertation, new insights into the unsteady flow over an individual blade cross-section are presented. In particular, a flow phenomenon known as dynamic stall is investigated. Due to the experimental challenges of studying high Reynolds number flows, most experimental wind turbine research is conducted at lower Reynolds numbers. The experiments presented in this dissertation were conducted in a pressurized wind tunnel that allowed for high Reynolds numbers at low velocities. The effects of Reynolds number on static and dynamic stall are investigated and the scaling of the dynamic stall delay is studied. The insights from this research are relevant not only for improving the durability and performance of VAWTs, but also to wind turbines and aircraft in general. Lastly, the role of onshore and offshore wind energy in future energy scenarios is investigated using the integrated assessment model WITCH.