Influence of the inflow conditions on the dynamics of a floating wind turbine wake under harmonic surge motion

Abstract

Floating offshore wind turbines (FOWTs) are projected to undergo substantial expansion in the coming decades. However, the high compliance of their floating foundations, coupled with aerodynamic, hydrodynamic and mooring forces, leads to complex platform motions that make it difficult to predict their wake dynamics. The vortex ring structure produced during surge motion has been the subject of study for nearly a decade now but there are still many features to bring to light. As most studies have been under idealised uniform flow, there is little knowledge on how this structure behaves under atmospheric boundary layer (ABL) flow. In this work, the authors propose to study this structure under three different inflow conditions: laminar and lowturbulence no-shear flows and ABL flow. Large eddy simulations are carried out in combination with an actuator disc (AD) as a wind turbine model, with a focus on surge motion and a Strouhal number ranging between 0 and 0.47. The velocity values are extracted at a vertical plane parallel to the AD axis, which is subsequently analysed by means of proper orthogonal decomposition (POD) and phase averaging. In the POD analysis, vortex ring structures are identified under all inflow conditions, though their energy decreases as turbulence increases. Additionally, a dependence of energy on frequency is observed for low-turbulence no-shear and ABL flows, with the maximum energy occurring at Strouhal numbers 0.30 and 0.32, respectively. Furthermore, vertical meandering is detected in both cases. In low-turbulence no-shear flow, meandering and vortex rings act as decoupled phenomena. Replicating the analysis on a horizontal plane at hub height, it is observed that lateral meandering is uniformly intense under this inflow condition. Conversely, under ABL conditions, the surge motion interacts with the turbulent shear flow to actively induce a coupled vertical meandering. Vertical and lateral meandering in ABL conditions rely on entirely distinct mechanisms, the latter being unrelated to the vortex ring structure. Finally, phase-averaging analysis indicates that the wake is modulated by the surge motion, manifesting as expansions and contractions, for laminar and low-turbulence no-shear cases. Conversely, an inclination of the structures towards the flow direction is identified in the ABL conditions, attributable to the shear flow.