Sandrine Aubrun

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 low-turbulence 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.

As the number of floating offshore wind farm projects grows across the world, several questions remain unanswered in the domain of floating wind turbine wakes. Previous studies observed that the motions impact the wake under laminar inflow conditions, and that the modifications are observable even under high turbulent inflow conditions, that are more realistic. However, these spatiotemporal behavior modifications still require precise characterization, especially under turbulent conditions. Thus, the present article aims to extend the previous work by studying the wake dynamics induced by surge motion under turbulent inflow, and to find the frequency and the spatiotemporal characteristics related to this motion. For this purpose, a porous disk is placed in a wind tunnel with a high level of turbulent inflow, and some experiments are performed at 8.125D (D the turbine diameter) downwind of the disk. First, hot-wire measurements are used to identify a 'critical' frequency at which wake dynamics are the greatest, and, then, stereo-PIV measurements are conducted for this frequency configuration to characterize the spatiotemporal wake dynamics. The results indicate a critical Strouhal number of approximately St = 0.24, for which the surge motion induces periodic contraction and expansion of the wake at the same frequency as the imposed motion. A quasi-steady-state analysis reveals that these modifications are larger than the predictions of basic wake models, highlighting the dynamic nature of the wake response. Moreover, inclined surge-generated periodic coherent structures are observed during this work, which are supposed to be related to shear effects. Additionally, an unexpected subharmonic frequency peak appears in the far wake velocity spectra. POD and coherence analyses suggest that this is related to a far wake meandering phenomenon, locked onto this subharmonic frequency.

Wind tunnel testing is essential for the advancement of wind energy, providing the "ground truth" necessary to validate numerical and data-driven models. However, the increasing size of modern turbines introduces significant challenges, including aerodynamic scaling, tunnel blockage, and the need to simulate complex atmospheric boundary layers. This paper reviews the state-of-the-art in wind energy experimental aerodynamics, covering inflow conditions, airfoil performance, aeroacoustics, and aeroelasticity. It further examines the integration of wind tunnel data with CFD modeling, field measurements, and data science. By synthesizing current progress and identifying inherent limitations, this review provides a critical outlook on future directions for the experimental wind engineering community.

Wind tunnel testing is essential for the advancement of wind energy, providing the "ground truth" necessary to validate numerical and data-driven models. However, the increasing size of modern turbines introduces significant challenges, including aerodynamic scaling, tunnel blockage, and the need to simulate complex atmospheric boundary layers. This paper reviews the state-of-the-art in wind energy experimental aerodynamics, covering inflow conditions, airfoil performance, aeroacoustics, and aeroelasticity. It further examines the integration of wind tunnel data with CFD modeling, field measurements, and data science. By synthesizing current progress and identifying inherent limitations, this review provides a critical outlook on future directions for the experimental wind engineering community.

Floating Offshore Wind Turbines (FOWTs) are projected to undergo substantial expansion in the coming decades. However, the motion of their supporting platforms due to mooring lines and wave interaction makes it difficult to predict their wake dynamics. The vortex ring structure produced during surge motion has been subject of study for nearly a decade now but there are still many features to bring to light. As most of the studies have been under idealized, 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 turbulent uniform flows, and ABL flow. Large Eddy Simulations are carried out in combination with an Actuator Disk (AD) as a wind turbine model, with a focus on surge motion and a Strouhal number ranging between 0 and 0.47. In order to quantify the energy of the vortex ring structure, Proper Orthogonal Decomposition (POD) is applied to a vertical plane parallel to the AD axis. A good visualization of the structure is achieved for all cases, and the energy of the vortex ring structure exhibits a local maximum for turbulent and ABL flows, with the highest energy at Strouhal number 0.30 and 0.32, respectively. However, the local maximum is narrower in the ABL case than in the turbulent one. Also, due to the presence of strong turbulent structures in ABL flow, the energy present in the vortex ring structure is considerably lower in this case than under uniform turbulent flow. For the laminar case, the POD method allocates almost 99.9 % of the energy to modes related to the vortex ring structure, as no other strong structures arise in the wake. A meandering phenomenon is detected under low-turbulence and ABL flows. In the first scenario, meandering is initiated by inflow conditions, while in the second, it is the consequence of the interplay between shear flow and surge motion. The study is replicated in a horizontal plane at hub height, thereby demonstrating that for the low-turbulence flow, meandering occurs with equal intensity in all directions. Conversely, in ABL conditions, lateral meandering is unrelated to the vortex ring structure. Finally, phase average is carried out to evaluate how the structure propagates in each case. The results obtained indicate a faster decay of the structure for the non-laminar inflow cases, although the impact on the growth rate is contingent on the energy content of the vortex structure for each surge frequency. Further analysis indicates that the wake is modulated by the surge motion, manifesting as expansions and contractions, for the laminar and low-turbulence cases. In contrast, an inclination of the structures towards the flow direction is identified in the ABL conditions, attributable to the shear flow.

As the number of floating offshore wind farm projects grows across the world, several questions remain unanswered in the domain of floating wind turbine wakes. Previous studies observed that the motions impact the wake under laminar inflow conditions, and that the modifications are observable even under high turbulent inflow conditions, that are more realistic. However, these spatiotemporal behavior modifications still require precise characterization, especially under turbulent conditions. Thus, the present article aims to extend the previous work by studying the wake dynamics induced by surge motion under turbulent inflow, and to find the frequency and the spatiotemporal characteristics related to this motion. For this purpose, a porous disk is placed in a wind tunnel with a high level of turbulent inflow, and some experiments are performed at 8.125D (D the turbine diameter) downwind of the disk. First, hot-wire measurements are used to identify a 'critical' frequency at which wake dynamics are the greatest, and, then, stereo-PIV measurements are conducted for this frequency configuration to characterize the spatiotemporal wake dynamics. The results indicate a critical Strouhal number of approximately St = 0.24, for which the surge motion induces periodic contraction and expansion of the wake at the same frequency as the imposed motion. A quasi-steady-state analysis reveals that these modifications are larger than the predictions of basic wake models, highlighting the dynamic nature of the wake response. Moreover, inclined surge-generated periodic coherent structures are observed during this work, which are supposed to be related to shear effects. Additionally, an unexpected subharmonic frequency peak appears in the far wake velocity spectra. POD and coherence analyses suggest that this is related to a far wake meandering phenomenon, locked onto this subharmonic frequency.

A comprehensive meteorological data set from an operational wind farm, consisting of six 2 MW turbines, has been made available. A meteorological mast, equipped with sonic anemometers at four different heights, was installed at the center of the farm and has collected data over 3 years. The data set is further supplemented with radiometer measurements for atmospheric stability analysis. Simultaneously, supervisory control and data acquisition (SCADA) data were acquired to provide operational information about the wind turbines, including inter alia power production and wind direction. Additionally, the turbine blades were scanned to support aerodynamic simulations. This unique and comprehensive database has been made accessible to the research community through the AERIS platform.

The complex dynamics introduced by floating platforms present new challenges in the study of wind turbine wakes, and numerous questions remain unresolved due to the early stage of this technology and the limited operational experience. Some previous studies showed that harmonic motions with realistic amplitude and frequency under a modeled atmospheric boundary layer have no significant impact on time-averaged values due to the relatively high background turbulence, but they also show that frequency signatures are still visible in spectra of wake parameters. The purpose of this work is to shed light on the spatiotemporal behavior of the wake imposed by surge, heave, and pitch harmonic motions. Wind tunnel experiments on the wake of a porous disk immersed in a modeled marine atmospheric boundary layer were performed, and a phase-averaging method with kernel smoothing was applied to the data to extract the harmonic response of the wake. A quasi-steady-state analysis was carried out, showing that the phase-averaged observations appear to be larger than simple steady-wake model predictions and revealing the dynamic nature of the wake responses to the motion. Thus, distinct wake dynamic hypotheses are formulated depending on the nature of the motion: (i) for heave, the wake is translated vertically while maintaining its integrity and containing the same power; (ii) for surge, the wake contracts and expands without any displacement of its center localization, accompanied by in-phase power modulation; (iii) and for pitch, the wake dynamics include both heave and surge impacts, with a vertical translation of the wake synchronized with crosswise wake surface and power modulations.