The effect of freestream turbulence (FST) on a wing-tip vortex was investigated at a chord-based Reynolds number of 105. Experiments were conducted at four FST intensities, generated by an active grid, 0.30%, 1.84%,7.70%, and 13.25%, with integral scales ranging from 0.5 to 2.6 times the wing chord length. Stereoscopic particle image velocimetry measurements document the effects of FST on the meandering motion of the wingtip vortex in the near field and middle field of a NACA0012 wing, i.e., planes 𝑥∕𝑐 = [2, 5, 12] downstream from the trailing edge of the wing. Conditional averaging based on recentring the coordinate system on the vortex center has been used to eliminate the influence of vortex motion. When the analysis was conditionally averaged on the vortex’s core position, a reduction in vortex strength with increasing FST was observed, along with a slight increase in diffusion for the highest FST cases (FST levels > 6%). Snapshot proper orthogonal decomposition (POD) analysis on the coherent component of the streamwise vorticity field revealed two dominant modes associated with meandering displacement for all FST cases. POD analysis further reveals turbulence accelerates the onset of spatial modes associated with vortex deformation, which typically emerge in the far wake (𝑥∕𝑐 ≥ 36) under non-turbulent conditions but appear in the near wake (𝑥∕𝑐 = 5) when FST is present. The vortex deformation modes contribute more significantly to the total enstrophy in turbulent cases in the near wake than in the far wake under non-turbulent conditions.

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.

Wakes and the dynamic interactions of multiple wakes have been a focal point of numerous research endeavours. Traditionally, wake interaction studies have focused on wakes produced by similar bodies. In contrast, the present study positions a non-shedding porous disc adjacent to periodically shedding solid discs of varying diameters and dimensional shedding frequencies. Using hot-wire measurements, we explore the intriguing interaction between these wakes. Remarkably, our findings reveal that the wake of the non-shedding disc acquires oscillations from the wake of the shedding disc, irrespective of their distinct frequencies. We demonstrate high receptivity of the porous disc’s wake and connect our findings to real-life applications.

Ten international teams participated in a second round-robin test with wake measurements behind two different types of actuator discs focusing on comparability of the experimental boundary conditions. Results show that even for such simple experiments small differences in the set-ups and inflow can lead to variations in the measurements. Nevertheless, a clear reduction in the deviations in maximum velocity deficit could be achieved for one disc compared to the first round-robin test from 2018.

The impact of freestream turbulence (FST) on the aerodynamic performance of a flexible finite wing and the produced wingtip vortex was investigated. The wing had a NACA 4412 airfoil profile and the chord-based Reynolds number was . The experiments were conducted in a closed-loop wind tunnel with four different inflow turbulence intensities ( , , and ) generated using an active turbulence grid. Force balance measurements revealed that increasing the scale of the FST increased the maximum lift and delayed stall. Digital image correlation (DIC) measured deflections of the wing’s structure. Spanwise bending was found to be the dominant deformation. While the wing vibrated at its natural frequency in all conditions, FST increased the amplitude of the vibrations. A similar spectral signature was observed in the lift force fluctuations as well. Stereoscopic particle image velocimetry measurements were obtained two chord lengths downstream of the trailing edge simultaneously with DIC. FST decreased the vortex strength, and marginally increased vortex diffusion and size. It also increased the vortex meandering amplitude, while reducing the meandering frequency band. For the cases with a turbulence intensity of and , the frequency of meandering and the wing’s vibration were similar and a phase relation between the two motions was observed. Proper orthogonal decomposition of the vortex (after removing meandering) and the subsequent velocity field reconstruction revealed temporal fluctuations in the vortex strength at the same frequency as the wing’s vibration. This was linked to the lift force fluctuations induced by the wing’s deformations.

The wake merging of two side-by-side porous discs with varying disc spacing is investigated experimentally in a wind tunnel. Two disc designs used in the literature are employed: a non-uniform disc and a mesh disc. Hot-wire anemometry is utilised to acquire two spanwise profiles at 8 and 30 disc diameters downstream and along the centreline between the dual-disc configuration up to 40 diameters downstream. The spanwise Castaing parameter profiles confirm the appearance of rings of internal intermittency at the outermost parts of the wakes. These rings are the first feature to interact between the discs. After this point, the turbulence develops to a state whereby an inertial range is observable in the spectra. Farther downstream, the internal intermittency shows the classical features of homogeneous, isotropic turbulence. These events are repeatable and occur in the same order for both types of porous discs. This robustness allows us to develop a general map of the merging of the two wakes.

In this article, a recently developed adaptive version of super-twisting is applied to the control of the aerodynamic lift on a wind turbine blade section, while considering local disturbances in the airflow. The proposed control law serves as a model-free control strategy, relying on only two parameters. This strategy reduces the need for tuning and modeling efforts: the first parameter governs the speed of gain variation, while the second parameter is associated with the desired accuracy, enabling control of unknown dynamics. We discuss the capability of the proposed model to track the lift reference of a wind turbine blade in the presence of external perturbations and actuator saturation. Experimental results demonstrate the feasibility of such control.

The potential for flow manipulation through the use of an active grid is vast. In this study, we investigate the effects of six distinct flapping excitation protocols on turbulent flows, employing both hot-wire anemometry and particle image velocimetry (PIV) with a large field of view, measuring close to  in the streamwise direction. Our primary focus is on comparing the results, particularly examining spectra and the integral length scale L. While the spectra derived from hot-wire measurements, transformed into wave number space through Taylor’s hypothesis, exhibit strong agreement with spatial spectra from PIV data, we observe a significant dependency of L on the length of the data set.

Wind energy systems, such as horizontal-axis wind turbines and vertical-axis wind turbines, operate within the turbulent atmospheric boundary layer, where turbulence significantly impacts their efficiency. Therefore, it is crucial to investigate the impact of turbulent inflow on the aerodynamic performance at the rotor blade scale. As field investigations are challenging, in this work, we present a framework where we combine wind tunnel measurements in turbulent flow with a digital twin of the experimental set-up. For this, first, the decay of the turbulent inflow needs to be described and simulated correctly. Here, we use Reynolds-averaged Navier–Stokes (RANS) simulations with k−ω turbulence models, where a suitable turbulence length scale is required as an inlet boundary condition. While the integral length scale is often chosen without a theoretical basis, this study derives that the Taylor micro-scale is the correct choice for simulating turbulence generated by a regular grid: the temporal decay of turbulent kinetic energy (TKE) is shown to depend on the initial value of the Taylor micro-scale by solving the differential equations given by Speziale and Bernard (1992). Further, the spatial decay of TKE and its dependence on the Taylor micro-scale at the inlet boundary are derived. With this theoretical understanding, RANS simulations with k−ω turbulence models are conducted using the Taylor micro-scale and the TKE obtained from grid experiments as the inlet boundary condition. Second, the results are validated with excellent agreement with the TKE evolution downstream of a grid obtained through hot-wire measurements in the wind tunnel. Third, the study further introduces an airfoil in both the experimental and the numerical setting where 3D simulations are performed. A very good match between force coefficients obtained from experiments and the digital twin is found. In conclusion, this study demonstrates that the Taylor micro-scale is the appropriate turbulence length scale to be used as the boundary condition and initial condition to simulate the evolution of TKE for regular-grid-generated turbulent flows. Additionally, the digital twin of the wind tunnel can accurately replicate the force coefficients obtained in the physical wind tunnel.

As wind energy expands worldwide, the demand of reliable, fast, cost-efficient wind turbine wake models is growing. This is a significant challenge as wind turbines face various inflow conditions that include turbulence, inhomogeneities/instationarities and upstream wakes. In consequence, an enormous number of engineering models, each one based on different physical concepts, has been proposed. The majority focuses on the far wake where the mean velocity recovers and turbulence decays after it built up. We argue that the most important, or the leading, parameter for wake modeling is the length scale of a virtual origin. Testing different models from the literature for data sets from laboratory wind turbines and multi-megawatt turbines obtained by LiDAR, we find that all models perform significantly better when such a virtual origin is added. Our results can therefore be used for a yet missing definition of a near wake zone.