This work concerns high-fidelity numerical simulations of a rotor wake, with focus on the tip vortices and their stability. Blade-resolved and actuator line lattice-Boltzmann simulations are performed on a symmetric baseline rotor, as well as on a rotor with asymmetries. The asymmetry has the purpose of destabilizing the tip vortices to enhance wake recovery and hence the performance of potential downstream turbines. Limitations in the actuator line method are highlighted, and we show the potential of addressing these limitations with a so-called “preset” actuator line, where the forces are extracted from blade-resolved simulations, or an analytical load model, which as input only requires the thrust and power coefficients. Simulations agree well with experimental results and leapfrogging is captured, even with a coarse actuator line simulation. The asymmetric rotor is shown to improve power in the far-wake by 12%.

@en

This study presents results from a wind tunnel experiment on a three-bladed horizontal axis wind turbine. The model turbine is a scaled-down version of the IEA 15 MW reference wind turbine, preserving the non-dimensional thrust distribution along the blade.

Flow fields were captured around the blade at multiple radial locations using particle image velocimetry. In addition to these flow fields, this comprehensive dataset contains spanwise distributions of bound circulation, inflow conditions and blade forces derived from the velocity field. As such, the three blades' aerodynamics are fully characterised. It is demonstrated that the lift coefficient measured along the span agrees well with the lift polar of the airfoil used in the blade design, thereby validating the experimental approach.

This research provides a valuable public experimental dataset for validating low- to high-fidelity numerical models simulating state-of-the-art wind turbines. Furthermore, this article establishes the aerodynamic properties of the newly developed model wind turbine, creating a baseline for future wind tunnel experiments using this model.@en

Vertical axis wind turbines (VAWTs) have been identified as a technology that, in association with wake steering, can increase power density of wind farms. In this study, we validate a free wake method for VAWT wake prediction, which leads to satisfactory results. We then use this method to simulate wake steering by means of fixed pitched blades and struts. We demonstrate that combining pitched wakes and struts can lead to very advantageous wake behavior, but only when the interactions between the tip vortices are taken into account. The possibility to inject more high momentum flow into the wake while moving the vortex system away from the next turbine could make pitched blades and struts a powerful tool for future wind farms.

@en

With distributed propulsion and electric vertical take-off and landing aircraft on the rise, fast and accurate methods to simulate propeller slipstreams and their interaction with aircraft components are needed. In this work, we compare results obtained with a filament-based free wake panel method to experimental and previously validated numerical data. In particular, we study a propeller-wing configuration at zero angle of attack and the aerodynamics of the blade-resolved slipstream interaction with the wing. We use a prescribed wake on the wing and a free wake on the propeller, which greatly accelerate the computations. Results indicate that, while forces are overpredicted due to the inviscid nature of the panel method, the free wake is able to capture the slipstream deformation and shearing with remarkable success. We find that a filament-based free wake panel method can be a useful tool for propeller-wing interaction in preliminary aircraft design.

@en

Numerical simulations of a wind turbine blade with and without trailing-edge serrations are validated with full-scale field test of a 130 m diameter onshore wind turbine. Simulations focus on trailing-edge noise and are conducted on extruded airfoil sections of the blade using the lattice-Boltzmann method and very large eddy simulations, which are then propagated to the far-field using the Ffowcs Williams-Hawkings approach, simulating the rotation of the sections and the noise of the entire rotor. Far-field noise spectra at two mean wind speeds are used for validation, with the sound power level of the simulations being within 2.5 dB of field test and the total noise reductions attributed to the serrations being captured within 0.6 dB.

@en

We investigate the aerodynamics of a surging, heaving, and yawing wind turbine with numerical simulations based on a free-wake panel method. We focus on the UNAFLOW (UNsteady Aerodynamics of FLOating Wind turbines) case: a surging wind turbine which was modeled experimentally and with various numerical methods. Good agreement with experimental data is observed for amplitude and phase of the thrust with surge motion. We achieve numerical results of a wind turbine wake that accurately reproduce experimentally verified effects of surging motion. We then extend our simulations beyond the frequency range of the UNAFLOW experiments and reach results that do not follow a quasi-steady response for surge. Finally, simulations are done with the turbine in yaw and heave motion, and the impact of the wake motion on the blade thrust is examined. Our work seeks to contribute a different method to the pool of results for the UNAFLOW case while extending the analysis to conditions that have not been simulated before and providing insights into nonlinear aerodynamic effects of wind turbine motion.@en

The recent increased interest in distributed propulsion and electric vertical take-off and landing vehicles have made propeller wake interactions with the aircraft more relevant. The interaction between high-lift wings and propeller slipstreams are still not fully understood and several research efforts are being carried out to improve that knowledge. Lattice-Boltzmann, very large eddy simulations of a propeller-wing-flap configuration are conducted in this work. The simulations are validated with experimental data, with very good agreement of surface static pressure, surface shearlines, and wake total pressure. The complex separation patterns on the flap and their interaction with the slipstream of the propeller are well captured. The effects of grid resolution and laminar-to-turbulent transition are demonstrated. With the simulations validated, they are used to better understand the flow field of this configuration. We find that the angle of attack has a strong effect on how the slipstream is split over the wing, that the tip vortices wrap around the wing leading-edge instead of being cut by it, and that increased circulation stabilizes the tip vortices on the suction side, while making the tip vortices on the pressure side more unstable.@en

Simulating entire wind farms with an actuator line model requires significant computational effort, especially if one is interested in wake dynamics and wants to resolve the tip vortices. A need to explore unconventional approaches for this kind of simulation emerges. In this work, the actuator line method is implemented within a lattice-Boltzmann flow solver, combined with a sliding mesh approach. Lattice-Boltzmann solvers have advantages in terms of performance and low dissipation, while the sliding mesh allows for local refinement of the blade and tip vortices. This methodology is validated on a well-documented case, the NREL Phase VI rotor, and the local refinement is demonstrated on the NREL 5 MW rotor. Results show good agreement with reference Navier–Stokes simulations. Advantages and limitations of the sliding mesh approach are identified.

@en

In this work, the interaction of grid-generated turbulence with airfoils of different thicknesses, namely, a National Advisory Committee for Aeronautics (NACA) 0008 and a NACA 0018, is investigated, leading to a deeper understanding of the influence of the airfoil geometry on the near-field flow and on the far-field pressure fluctuations. Experimentally validated lattice-Boltzmann simulations are used to analyze the flow properties in the leading-edge (LE) vicinity. The analysis of the velocity fluctuations near the LE shows that momentum is transferred from the streamwise to the transverse velocity for the NACA 0008 airfoil interacting with a large turbulence length scale. This mechanism changes with the increase in the airfoil thickness because the inflow turbulence length scale becomes comparable to the airfoil thickness in the LE region, resulting in a higher concentration of vortices near the LE oriented in the transverse direction, creating high-velocity fluctuations in the spanwise direction. The near- and far-field pressure fluctuations are analyzed to understand the impact of the inflow turbulence distortion on these parameters and the limitations of analytical methods for real airfoils. Results show that the wall-pressure fluctuations are affected by the turbulence distortion in the LE region. Thick airfoils have noise directivity patterns significantly different compared to the Amiet predictions for higher frequencies, radiating higher noise levels upstream of the LE than the thin airfoil. This is likely associated with a drastic change in the pressure fluctuation distribution near the airfoil LE region, attributed to the change in the distortion of the vortical structures in the LE area.

@en

This paper presents low speed fluid structure interaction simulations of a highly flexible wing at various flow conditions, including flutter and excitation from sinusoidal gusts. Such wings are becoming more relevant in recent years, due to their potential for improving aerodynamics and reducing weight, while their flutter characteristics are particularly challenging to address, as the modal properties of the wings change as deflections increase. Calculations are based on time domain coupling of a geometrically exact beam structural model and a 3D free wake panel method, modeling the outer surface of the wing, which allow for nonlinear effects in terms of geometrical deformations and the flow at low computational cost. Static and aeroelastic wing deflections are in line with experimental data of the Pazy wing, which is a benchmark for highly flexible wings from Technion. Two flutter mechanisms are predicted within 1 to 3 m/s of the experimental range. An analysis of the flutter modes is performed, showing that the second torsion mode plays a role in flutter, something that had not been published before. Limit cycle oscillations are achieved and are shown to compare well with reference data, with the frequency being within 1% of the experimental value. Finally, results of gust simulations of the Pazy wing are compared to data from experiments and corrections for the wind tunnel measurements are proposed, which should facilitate future validation efforts. This work serves as a contribution to the Pazy wing dataset and is a step towards mid-fidelity simulations for more complex configurations.

@en

We investigate the aerodynamics of a surging wind turbine with numerical simulations based on a free wake panel method. We start by demonstrating the method's capability to simulate a plunging airfoil, which provides some insights that are later used to interpret results of a surging rotor. We then validate the method on a non-surging wind turbine and discuss the strengths and weaknesses of our approach. Next, we focus on the UNAFLOW case: a surging wind turbine which was modelled experimentally and with various numerical methods. Good agreement with experimental data is observed for amplitude and phase of the thrust with surge motion. For the first time, we achieve numerical results of a wind turbine wake that accurately reproduce experimentally verified effects of surging motion. Finally, we extend our simulations beyond the frequency range of the UNAFLOW experiments and reach results that do not follow a quasi-steady response. Using the plunging airfoil data, we justify the behavior observed in the non-linear range. Our work seeks to contribute a different method to the pool of results for the UNAFLOW case, while extending the analysis to conditions that have not been simulated before.

@en

This paper shows fluid structure interaction simulations of a highly flexible wing at various flow conditions, including flutter regime. This is achieved with two-way time domain coupling of a geometrically exact beam structural model and a 3D free wake panel method, modelling the outer surface of the wing, which allow for non-linear effects of the geometry deformation and the flow to be taken into account. Static and aeroelastic wing deflections are compared to experimental data of the Pazy wing with good accuracy. Two regions of flutter onset are predicted within the experimental range. An analysis of the flutter modes is performed. This serves as a step towards mid-fidelity simulations for more complex configurations, including fuselage effects and tail interactions.@en

Transonic buffet simulations are carried out for the OAT15A airfoil and the NASA Common Research Model. Simulations are performed with a Lattice-Boltzmann based solver, which has been extended to transonic flow capabilities. The results are compared to data from the literature, including recent wind tunnel investigations, and further flow analysis is performed, focusing on the unsteady flow-field. Overall, good agreement is observed for the two-and three-dimensional simulations.@en

Unsteady simulations using PowerFLOW, a Lattice-Boltzmann-method-based solver, of a rectangular deep (L/D= 4.5), open-flow cavity at subsonic and supersonic speeds (M= 0.6 to 1.5) are presented. Acoustic sound pressure levels and cavity modes are compared to experiments and analytical models. The simulations show a good quantitative and qualitative agreement with the experiments for the spectra at various probes located in and around the cavity. The shear layer mode is characterized by Rossiter modes and the frequencies are well predicted. The three-dimensionality in the flow is also identified by the presence of lateral pressure gradients, resulting in the interaction of lateral and longitudinal modes.@en

A Lattice-Boltzmann flow solver is used to simulate the unsteady flow around swept and unswept wings in the presence of ice shapes on the leading edge. The stall mechanism is studied for roughness ice, ice horns, and scallop ice. These types of flows are particularly challenging, due to the extremely complex geometries involved and the flow separations, which are inherently unsteady, and hence troublesome for traditional steady-state solvers. @en