Large Eddy Simulations of Energy Entrainment in Wind Turbine Wakes and Wind Farms

Wind turbines extract kinetic energy from the inflow, leaving a wake region behind the turbine which has a lower velocity and higher turbulence intensity than the freestream. When turbines are clustered into wind farms, this leads to a reduction in power output and increased structural fatigue for turbines operating in wake. Reducing wake effects in wind farms is therefore a topic of significant scientific relevance as it could lead to improved wind farm efficiency and power output. However, wind turbine wakes and wind farm flows are highly chaotic and complex. Turbulence is an integral part of wind farm flow physics, as turbulent mixing governs energy entrainment and wake recovery. Flow dynamics present in wind farms are related to a combination of individual wake development, wind farm layout and two-way interaction with the atmospheric boundary layer.

The aim of this thesis is to study the interaction between the dynamics of inflow, wind turbine and wake; specifically related to understanding the impact of inflow turbulent scales, both on wake development behind a single turbine and on energy entrainment into wind farms. The identification of beneficial time scales that enhance wake recovery could allow the development of methods to introduce such scales into wind farms, for example through active control or layout design, and hence could passively improve efficiency and power output. In order to study dynamic and complex phenomena such as these, robust and well-validated high fidelity numerical modelling tools are required. Therefore, the initial part of the thesis is an extensive verification and validation of large eddy simulations of wind turbines modelled using the aeroelastic coupled actuator disc and actuator line method, and of atmospheric boundary layer flows.

The influence of the scales of inflow turbulence is investigated in isolation from turbulence intensity, shear or buoyancy. In all types of inflow and scenario - idealised periodic flows or synthetic turbulence fields, single wakes or wind farm flows - the integral time scale of the turbulence has a significant impact on wake recovery. For a single turbine in periodic inflows, the predominant time scale affects both the persistance of tip vortices and the amount of meandering in the far wake; shorter time scales cause a faster near wake breakdown but induce little wake meandering. For full synthetic turbulence fields, using inflow time scales in the equivalent Strouhal number range of f D/U = 0.12 − 0.5, the amount of meandering is similar in all cases but shorter inflow time scales lead to a faster breakdown of tip vortices, earlier onset of energy entrainment and improved recovery. When investigating a row of 10 turbines, shorter time scales therefore lead to a substantial power output increase for the second turbine. The integral time scales impact mainly the first turbine wake, and hence the relative difference in cumulative wind farm power output decreases over the turbine row. In both single turbine wakes and in the entrance to wind farms, spectral analysis shows the breakdown of long inflow scales and the dominance of smaller-scale wake-generated turbulence. In the wind farm, flow structures associated with the turbine spacing are also seen to develop. Overall, the work presented in this thesis demonstrates that the turbulent scales of freestream flow have a significant impact on wake recovery and wind farm flows.