Onset of wake meandering for a floating offshore wind turbine under side-to-side motion

Abstract

Wind turbine wakes, being convectively unstable, may behave as an amplifier of upstream perturbations and make downstream turbines experience strong inflow fluctuations. In this work, we investigate the effects of the side-to-side motion of a floating offshore wind turbine (FOWT) on wake dynamics using large-eddy simulation and linear stability analysis (LSA). When the inflow turbulence intensity is low, simulation results reveal that the turbine motion with certain Strouhal numbers $St = fD/U_\infty \in (0.2,0.6)$ (where $f$ is the motion frequency, $D$ is the rotor diameter, and $U_\infty$ is the incoming wind speed), which overlap with the Strouhal numbers of wake meandering induced by the shear layer instability, can lead to wake meandering with amplitudes being one order of magnitude larger than the FOWT motion for the most unstable frequency. For high inflow turbulence intensity, on the other hand, the onset of wake meandering is dominated by the inflow turbulence. The probability density function of the spanwise instantaneous wake centres is observed being non-Gaussian and closely related to that of the side-to-side motion. This complements the existing wake meandering mechanisms, that the side-to-side motion of an FOWT can be a novel origin for the onset of wake meandering. It is also found that LSA can predict the least stable frequencies and the amplification factor with acceptable accuracy for motion amplitude $0.01D$ . Effects of nonlinearity are observed when motion amplitude increases to $0.04D$ , for which the most unstable turbine oscillations shift slightly to lower frequencies and the amplification factor decreases.

Revealing inflow and wake conditions of a 6MW floating turbine

“…They state that the side-to-side motion amplifies the wake meandering when the ambient TI is low (< 3%), leading to a faster recovery of the wake deficit. However, in the TI regime studied here 4 % -8 %, the wake recovery of a floating wind turbine is similar to that of a fixed wind turbine, as reported by (Li et al, 2022). This similarity in wake recovery could explain the agreement of the wake characteristics presented in this analysis with results from previous numerical and wind tunnel studies of fixed wind turbine wakes.…”

“…In the case of floating wind turbines, the wind turbine motion that results from the interaction of the floating structure with the wind and sea has to be considered, in addition to the atmospheric conditions. The wake characteristics of floating wind turbines have been studied using numerical (Wise and Bachynski, 2020;Kleine et al, 2021;Nanos et al, 2021;Chen et al, 2022;Li et al, 2022) and wind tunnel (Fu et al, 2019;Schliffke et al, 2020) experiments. Those studies focussed on the impact of the sway (Fu et al, 2019;Nanos et al, 2021;Li et al, 2022), surge (Fu et al, 2019;Schliffke et al, 2020;Chen et al, 2022) and heave (Kleine et al, 2021) motions on the turbine operation.…”

“…The wake characteristics of floating wind turbines have been studied using numerical (Wise and Bachynski, 2020;Kleine et al, 2021;Nanos et al, 2021;Chen et al, 2022;Li et al, 2022) and wind tunnel (Fu et al, 2019;Schliffke et al, 2020) experiments. Those studies focussed on the impact of the sway (Fu et al, 2019;Nanos et al, 2021;Li et al, 2022), surge (Fu et al, 2019;Schliffke et al, 2020;Chen et al, 2022) and heave (Kleine et al, 2021) motions on the turbine operation. Yet, the absence of the study of the motion of floating turbine under realistic atmospheric turbulence conditions limits our understanding on how the impact of the motion of the wind turbine on the wake properties can be utilized in control strategies that optimize the operation of wind turbines and subsequently wind farms.…”

Numerical Study on the Yaw Control for Two Wind Turbines under Different Spacings

“…The total number of grids is about 13 million. As Li et al [28] selected uniform inflow in his research, an idealized uniform inflow is used in our study to focus only on the effects of yaw control and spacing, and the wake fields under the turbulent inflow with the turbine spacing of 7D are investigated to analyze the influences of the turbulent intensity on the yaw control. The uniform inflow speed is 11.4 m/s.…”

Investigation of the floating IEA wind 15‐MW RWT using vortex methods Part II: Wake impact on downstream turbines under turbulent inflow

“…Moreover, they used their aero‐elastic solver to study half‐wake effects with a one‐way and two‐way wake interactions of two NREL 5‐MW RWT. Recently, Li et al 21 investigated the effects of a floating turbine on the wake dynamics using LES. In line with the findings in the present work, they reported that low turbulence levels were key to the appearance of rotor‐induced wake meandering, which they linked to the side‐to‐side motion of a floating turbine.…”