Abstract
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 Kárman 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.
Highlights
Harnessing wind energy and converting it in electric energy require the design and installation of large wind farms, constituted by hundreds of turbines
The proper orthogonal decomposition (POD) analysis provided is carried out on a dataset composed of M = 2361 snapshots of the velocity field, stored every 10◦ rotation, in a sub-domain enclosing the wake with dimensions [0 7.5] × [−0, 7 0.63] × [0.8 2.2], where the velocities have been downsampled with a 1:5 ratio with respect to the computational grid
The present work provides a numerical analysis of the dynamics of the wake developing behind a three bladed wind turbine, for laminar inflow conditions, using the proper orthogonal decomposition (POD) of the unsteady flow field
Summary
Harnessing wind energy and converting it in electric energy require the design and installation of large wind farms, constituted by hundreds of turbines. Understanding the dynamics of wind turbine wakes is crucial for the design and efficiency improvement of wind turbines and farms. This task is challenging, considering that the main features of atmospheric turbulence may spread over a wide range of spatial and time scales [4]. The far wake, instead, is characterized by turbulent structures derived from the break-down of coherent vortices and by the entrainment of the outer flow, up to wake recovery [7, 8] How these coherent structures in the near and far wake contribute to wake recovery, is still an open problem. The present work provides an analysis of the dynamics and recovery of the wake of a wind turbine using the proper orthogonal decomposition (POD) technique [9,10] applied to numerical data obtained by large
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