EW
E.I. Wiegant
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The recovery of wind turbine wakes is governed by a combination of incoherent and coherent structures that transport momentum into the wake. The understanding of the role of coherent structures in wake recovery is largely qualitative and therefore difficult to extrapolate to how any particular coherent structure, such as those induced by the motions of floating offshore wind turbines, accelerates the wake. This work presents analyses that primarily consider turbulent momentum transport rather than the behaviour of coherent structures. These analyses are used to apply emphasis and nuance to various concepts within the leading paradigm on wake recovery. We analyse the wakes of a scale model wind turbine subject to surging or pitching motions at two different frequencies, in a wind tunnel and using large eddy simulations. We then introduce a simple vortex sequence model, which we use to attribute our results to specific flow structures. We identify wake recovery as a two-step process; the process of momentum transport that accelerates the mean flow, and a preceding process that causes gradual buildup of momentum transport from the rotor onwards. This preceding process is attributed, using our vortex sequence model, to the pairing of vortices. This process being present shortly behind the rotor suggests no notion of the wake being inhibited. Our vortex sequence model also shows that tip vortices shed from a surging or pitching floating offshore wind turbine collectively induce vortices on the scale of the rotor/floater motion, which we refer to as macrovortices. We suggest that we may attribute differences in wake recovery between floating and stationary wind turbines to the behaviour of these macrovortices.
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The recovery of wind turbine wakes is governed by a combination of incoherent and coherent structures that transport momentum into the wake. The understanding of the role of coherent structures in wake recovery is largely qualitative and therefore difficult to extrapolate to how any particular coherent structure, such as those induced by the motions of floating offshore wind turbines, accelerates the wake. This work presents analyses that primarily consider turbulent momentum transport rather than the behaviour of coherent structures. These analyses are used to apply emphasis and nuance to various concepts within the leading paradigm on wake recovery. We analyse the wakes of a scale model wind turbine subject to surging or pitching motions at two different frequencies, in a wind tunnel and using large eddy simulations. We then introduce a simple vortex sequence model, which we use to attribute our results to specific flow structures. We identify wake recovery as a two-step process; the process of momentum transport that accelerates the mean flow, and a preceding process that causes gradual buildup of momentum transport from the rotor onwards. This preceding process is attributed, using our vortex sequence model, to the pairing of vortices. This process being present shortly behind the rotor suggests no notion of the wake being inhibited. Our vortex sequence model also shows that tip vortices shed from a surging or pitching floating offshore wind turbine collectively induce vortices on the scale of the rotor/floater motion, which we refer to as macrovortices. We suggest that we may attribute differences in wake recovery between floating and stationary wind turbines to the behaviour of these macrovortices.