High Reynolds number Wind Tunnel Studies of a Wind Turbine's Wake

Abstract

Wind energy is expected to serve as an integral player in the campaign to replace fossil fuels with renewable energy sources. The use of wind farms, groups of individual wind turbines, is a common way to harness wind energy. Wind turbine wakes extract energy from the flow and reduce the energy available for downstream turbines, compromising the energy density of wind farms. Experimental campaigns have been conducted to understand these turbulent and complex flows, but none have been able to conduct these experiments while accounting for scale effects. In the following dissertation, wind turbine wake experiments conducted over an unprecedented Reynolds number, tip speed ratio, and downstream distance range are presented. A unique pressurized wind tunnel facility was used to acquire this novel dataset. Furthermore, to gain insight into some of the smallest of turbulent lengthscales, a nanoscale thermal anemometry probe was used to measure the wake. An investigation of Reynolds number and tip speed ratio effects was conducted through an analysis of the flow statistics and spectra. Special attention is dedicated to understanding how turbine-dependent structures, such as the tip vortex, wake meandering, and hub vortex, evolve and influence wake behavior. The tip vortex is shown to have a strong and long-lasting effect on a wide range of structures. Wake meandering is found to be one of the only structures that survives into the intermediate wake and is an integral part of the wake dynamics. Finally, a hub vortex — hypothesized to be caused by the high solidity of the studied rotor — was found, indicating that turbine geometry plays a critical role in wake dynamics and is an avenue for future research.