Wake meandering of a model wind turbine operating in two different operating regimes

Daniel Foti, PhD Candidate, Mechanical Engineering, University of Minnesota

The flows behind a model wind turbine under two different turbine operating regimes (Region 2 for turbine operating at optimal condition with the maximum power coefficient and 1.4 degrees of pitch angle, and Region 3 for turbine operating at sub-optimal condition with a lower power coefficient and 7 degrees of pitch angle) are investigated using large-eddy simulation (LES). Simulations with and without a nacelle model are carried out for each operating condition to study the influence of the nacelle on the formation of the hub vortex. The simulations with the nacelle model reveal an unstable helical hub vortex expanding outwards towards the outer wake; while the simulations without a nacelle model show a stable and columnar hub vortex. Because of the different interactions of the inner region of the wake with the outer region of the wake, a region with higher turbulence intensity is observed in the tip shear layer for the simulation with a nacelle model. The hub vortex for the turbine operating in Region 3 remains in a tight helical spiral and intercepts the outer wake a few diameters further downstream than the turbine operating in Region 2. Wake meandering, a low frequency large-scale motion of the wake, commences in the region of high turbulence intensity for all the simulations with and without a nacelle model indicating that neither a nacelle model nor unstable hub vortex is a necessary requirement for the existence of wake meandering. However, the unstable hub vortex promotes a high amplitude energetic meandering which cannot be predicted without a nacelle model. The turbine operating regime affects the shape and expansion of the hub vortex altering the location of the onset of the wake meandering and wake meander oscillating intensity.

NOTE: Due to the content of the presentation (unpublished material), the speaker was not comfortable posting the recording of this presentation.