POD analysis of the recovery process in wind turbine wakes

The wake produced by a single three-bladed wind turbine is investigated using the proper orthogonal decomposition (POD) of numerical data obtained by a large eddy simulation. The rotor blades are modeled using the actuator line method, whereas tower and nacelle are simulated through an immersed boundary method. The POD is performed in a three-dimensional subdomain enclosing the wake after conducting a convergence test, which demonstrates that the first ten modes are well converged. Most energetic POD modes identify and isolate different flow features characterising the wake dynamics, such as the tip-vortices spirals, the von Karman vortices shed by the tower, the Kelvin-Helmholtz instability linked to the wake shear layer. Very low frequency modes are also found, which could be related to the wake meandering phenomenon. Moreover, the wake recovery process is studied by computing the contribution of each POD mode to the mean-kinetic-energy entrainment. This analysis indicates that tip vortices negatively affect the wake recovery, since they provide a negative entrainment. On the contrary, flow structures related to the tower wake are found to be beneficial to wake recovery, demonstrating the importance of including tower and nacelle in numerical simulations.