Abstract
AbstractThe wake produced by a model wind turbine is investigated using proper orthogonal decomposition (POD) of numerical data obtained by large eddy simulations at a diameter‐based Reynolds number . The blades are modeled employing the actuator line method and an immersed boundary method is used to simulate tower and nacelle. Two simulations are performed: one accounts only for the blades effect; the other includes also tower and nacelle. The two simulations are analyzed and compared in terms of mean flow fields and POD modes that mainly characterize the wake dynamics. In the rotor‐only case, the most energetic modes in the near wake are composed of high‐frequency tip and root vortices, whereas in the far wake, low‐frequency modes accounting for mutual inductance instability of tip vortices are found. When tower and nacelle are included, low‐frequency POD modes are present already in the near wake, linked to the von Karman vortices shed by the tower. These modes interact nonlinearly with the tip vortices in the far wake, generating new low‐frequency POD modes, some of them lying in the frequency range of wake meandering. An analysis of the mean kinetic energy (MKE) entrainment of each POD mode shows that tip vortices sustain the wake mean shear, whereas low‐frequency modes contribute to wake recovery. This explains why tower and nacelle induce a faster wake recovery.
Highlights
In order to limit the negative impact of energy production by fossil fuels on the environment and climate equilibrium, in the upcoming years, a larger fraction of energy demand must be satisfied by renewable sources
This work provides a detailed analysis of the wake produced by a three-bladed wind turbine and of the main embedded coherent structures, which are identified by means of proper orthogonal decomposition (POD)
Analysis of the phase-averaged RO case flow shows that tip vortices spirals are advected downstream almost undisturbed until the end of the computational domain, whereas in the TN case, breakdown of the tip vortices occurs at the bottom and left side of the wake, suggesting a correlation between the tip vortices breakdown and the increased wake recovery
Summary
In order to limit the negative impact of energy production by fossil fuels on the environment and climate equilibrium, in the upcoming years, a larger fraction of energy demand must be satisfied by renewable sources. The wake of the tower appears to promote the breakdown of the tip vortices, increasing the MKE flux into the wake.[34] to background turbulence, tower–wake interactions induce an asymmetry of the wake deficit, leading to a decreased efficiency and increased blade stress levels for a turbine placed downstream.[35] the role of the nacelle on the origin of wake meandering in the near field of a single turbine has been recently investigated.[30] The helical hub vortex that forms behind the turbine nacelle interacts with the tip vortices while expanding radially, leading to turbulent coherent structures in the far wake that can be connected to the wake meandering phenomenon.[36] For this reason, incorporating the presence of the nacelle in wind farm simulations is crucial for accurately predicting the dynamics of wake meandering as well as the loads on downwind turbines.[37] most of these studies including the effect of tower and nacelle focus on the features of the mean flow only, failing to capture unsteady phenomena due to the interaction of coherent structures with different frequencies and structures, which may have a strong impact on wake entrainment and recovery.
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