Wake impacting on a horizontal axis wind turbine

Abstract

Offshore wind is set to contribute a significant portion of the UK’s renewable energy production. In order to achieve this, installation costs must be reduced and energy density optimised, but this must be balanced with the increase in maintenance costs resulting from fatigue due to wake impact. The aim of this thesis is to investigate the effects of horizontal axis wind turbine wake impact on a downstream rotor. A force-free wake implementation of the unsteady vortex lattice method has been developed in order to simulate the flow around the downstream rotor, including the effects of an upstream rotor wake, uncorrelated wind field and the dynamic inflow response of the turbine wake. In addition, a series of wind tunnel experiments were undertaken to characterise the wake of a horizontal axis wind turbine and measure time histories of the turbine thrust and blade root bending moments in uniform and turbulent inflow and upstream rotor wake impact. Comparisons are made between the model and wind tunnel experiments for a range of flow cases: uniform inflow, turbulent inflow and operation in an upstream rotor wake at varying degrees of lateral offset. The upstream flow field is modelled on a Cartesian grid, following the assumption of frozen turbulence. For both the turbulent flow and upstream rotor wake, a simplified model is used as a starting point and then refined to better model the effect of turbulence. Ambient turbulence is found to have minimal impact on the mean response of the rotor, suggesting that a linearised approach can be taken in the numerical modelling of turbulence effects. The simple model better predicts the low frequency response, but does not capture the per revolution frequencies identified by the refined model, which also better predicts the admittance. The response of the rotor to an aligned upstream rotor wake is found to be dominated by the wake turbulence, although the proposed model does not reproduce the measured response. However, for laterally offset upstream rotor wakes the mean velocity deficit is the dominant factor and the model captures the response, including the shift to higher bending moment cycles which will contribute to increased fatigue.