Propeller and vortex ring state for floating wind turbines during surge

Abstract

A floating wind turbine can experience propeller-like conditions when moving backwards quicker than the wind, where both the rotor thrust and torque are negative. This is known as propeller state. Under such cases, it will also interact with its own wake to enter the turbine into a vortex ring state (VRS). The behaviour of a wind turbine during both states still requires further research. In this work, the aerodynamics of a rotor during surge-only conditions favourable to these states have been studied, using OpenFOAM to solve the 3D URANS equations with the k-ω SST turbulence model for closure. The results showed that the torque always transitioned ahead of the thrust into propeller state, which is herein named the braking state, where the torque is negative whilst thrust is positive. Both braking and propeller state were shown to be due to a reduction in span-wise angle of attack during surge, where inboard regions were found to be most susceptible. It was also shown that propeller state occurs ahead of VRS, and that both 1D velocity and 2D quasi-steady BEM predictions agreed within 0.2 s of when these rotor-averaged states would initiate. Both 1D and 2D approaches, based on relative rotor velocities, assumed that the quarter surge period is small compared to the dynamic inflow timescale but high compared to the aerofoil timescale. Although surge was considered in isolation, it is hypothesised that similar findings should be had with pitch due to the assumptions based only on relative velocities. As a result, such simplified methods could be adopted to predict the onset of braking and propeller state during high wave conditions. Coupled to a suitable mitigation strategy, this could form a means to minimise system damage during such an event. This work further explains the causes of and behaviour during propeller and vortex ring state in the context of a floating wind turbine during surge.