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The desire for low cost of energy is driving the development of modern wind turbines toward more and more flexible designs. With respect to aerodynamic evaluation of these designs based on CFD this pose a requirement for the capability to compute the flow on deforming grids around heavily deforming geometries. The present paper describes an efficient explicit method suited for deforming geometries in a structured grid approach, here illustrated for airfoil and a wind turbine rotor simulations. Based on the present investigation, it is illustrated that the solution on the deformed grids deviates less than a fraction of a percent from the solution on the original grids both for the airfoil and the rotor case.

Design optimization and aeroelastic load evaluation for wind turbines are still infeasible for CFD due to computational cost. These tasks are carried out using 3D engineering aerodynamic methods, where the local aerodynamic loads are obtained from tabulated 2D aerodynamic polars, which makes the models fast. With recent developments such as highly flexible blades, coned/prebent/swept blades, the relative inflow direction to the “inner” 2D airfoil section systems can have a significant component in the spanwise direction. The Crossflow Principle (CP) is used to treat the effects of this in a physically consistent manner. Cases dominated by pressure forces are described well with the CP method, but it is shown in the paper that CP fails to describe the part of the forces stemming from the friction forces correctly. It is shown in the paper that the power loss due to friction forces will be underestimated with the crossflow principle for rotors with significantly swept blades or other designs with a significant amount of local crossflow on the blades. The present work presents a simple model to correct the baseline crossflow principle method to take into account also the friction force part in a consistent manner. The model is validated with 3D CFD results on a 2D airfoil section. It is shown that the new model successfully corrects for the addition of the viscous forces due to spanwise flow component. The paper includes examples of the effect of using the model on rotor designs with different amounts of blade sweep simulated using a blade element momentum (BEM) and blade element vortex cylinder (BEVC) methods.

In this work we present multi-fidelity steady-state aeroelastic framework that leverages the state-of-the-art simulation tool HAWC2 for the structural model, and a variety of aerodynamic models, comprising of the low fidelity blade element momentum (BEM) method, the medium fidelity blade element vortex cylinder (BEVC) method and the coupled near wake and vortex cylinder method, and finally the high-fidelity CFD solver EllipSys3D. The aeroelastic framework is part of AESOpt, an aerostructural framework for design of wind turbine blades. The different aerodynamic models are applied to compute the aeroelastic steady state of the newly designed IEA 22 MW Reference Wind Turbine. The results show a very good agreement between the medium- and high-fidelity aerodynamic models with a maximum of 2.7% difference between the high-fidelity aeroelastic response and that of the lower fidelities.

Among a few field experiments on wind turbines for analyzing laminar-Turbulent boundary layer transition, the results obtained from the DAN-AERO and aerodynamic glove projects provide significant findings. The effect of inflow turbulence on boundary layer transition and the possible transition mechanisms on wind turbine blades are discussed and compared to CFD (computational fluid dynamics) simulations of increasing fidelity (Reynolds-Averaged Navier-Stokes, RANS; unsteady Reynolds-Averaged Navier-Stokes, URANS; and large-eddy simulations, LESs). From the experiments, it is found that the transition scenario changes even over a single revolution with bypass transition taking place under the influence of enhanced upstream turbulence, for example, such as that from wakes, while natural transition is observed in other instances under relatively low inflow turbulence conditions. This change from bypass to natural transition takes place at azimuthal angles directly outside the influence of the wake indicating a quick boundary layer recovery. The importance of a suitable choice of the amplification factor to be used within the eN method of transition detection is evident from both the RANS and URANS simulations. The URANS simulations which simultaneously check for natural and bypass transition match very well with the experiment. The LES predictions with anisotropic inflow turbulence show the shear-sheltering effect and a good agreement between the power spectral density plots from the experiment and simulation is found in case of bypass transition. A condition to easily distinguish the region of transition to turbulence based on the Reynolds shear stress is also observed. Overall, useful insights into the flow phenomena are obtained and a remarkably consistent set of conclusions can be drawn.

Iterative steady Reynolds-averaged Navier-Stokes (RANS) solvers are widely used to obtain polars representing the aerodynamic characteristics of airfoils. Unsteadiness is en11 countered in the flow for many of the simulated angles of attack due to phenomenon such as vortex shedding and separation bubbles. In such cases, the steady RANS solver may become numerically unstable. As a consequence it is unable to find a converged solution, with the solver residuals often entering limit cycle oscillations. The BoostConv method is an alternative to the classical Newton’s method to compute unstable steady-state solutions of dynamical systems. It has been previously applied for obtaining steady laminar solutions of the Navier-Stokes equations. However, it demonstrated a lack of robustness by failing to always find the steady-state turbulent solutions of the RANS equations. To overcome this problem, we propose a modification in the application of the BoostConv method to compute steady-state turbulent solutions of the RANS equations. The modified BoostConv method is coupled with an iterative steady RANS solver for computing converged two-dimensional steady-state flows over airfoils. The proposed modification in the application procedure introduces a relaxation parameter that improves the convergence and robustness of the BoostConv method. We show this by finding lift and drag curves with angles of attack spanning 360° for airfoils representative of those used in modern wind turbines. The flow solutions are converged to machine precision at all angles of attack using the novel application procedure of the BoostConv method. Additionally, for flow cases convergent with the original BoostConv method, the modified BoostConv method showed faster convergence rates. The proposed modification provides an enhancement of the BoostConv method that can be easily integrated into existing implementations. By improving the method’s robustness and convergence rates in finding steady-state turbu

A parked wind turbine can be subjected to large yaw angles, which causes vortex shedding. Under some particular inflow conditions, the interaction between the shed vorticity and the blade structure could lead to Vortex-Induced Vibrations (VIV). VIV are an instability where the response grows until it reaches a limit cycle oscillation. In this paper, we will focus on this last part of the response. We will see that the shedding makes the aeroelastic system linear time-periodic, and study its stability with Floquet theory. We will use high-fidelity simulations of different inflow conditions, and quantify the stability of the system using a model-free approach.

This work explores the use of a trailing edge flap for the mitigation of the Vortex Induced Vibrations (VIV) of a wind turbine blade in standstill. Fluid structure interaction simulations are performed, coupling a computational fluid dynamics code with a multi-body structural solver. A publicly available 96 m long wind turbine blade is selected for the study, and labeled as IEA 10MW. The effects of a flap actuation in the range [−90°, 90°] are analyzed, for a given set of inflow conditions, and compared to the aeroelastic response of the original IEA 10MW geometry. While the consideration of the flap does lead to reductions on the amplitude of the edgewise vibrations, these are only noticeable when operating outside of the design limits of the aerodynamic device. A simple non-dimensional analysis seems to indicate that the potential of the flap installation with regards to VIV mitigation is very limited, as the instability is only shifted towards different inflow conditions and the expected effects on the maximum amplitude of vibrations are minor.

The actuator line method is a widely used technique to model wind turbines in computational fluid dynamics, as it significantly reduces the required computational expense in comparison to simulations using geometrically resolved blades. Actuator line coupled to an aeroelastic solver enables not only the study of detailed wake dynamics but also aeroelastic loads, flexible blade deformation and how this interacts with the flow. Validating aeroelastic actuator line predictions of blade loading, deflection and turbine wakes in complex inflow scenarios is particularly relevant for modern turbine designs and wind farm studies involving realistic inflows, wind shear or yaw misalignment. This work first implements a vortex-based smearing correction in an aeroelastic coupled actuator line, and performs a grid resolution and smearing parameter study which demonstrates significant improvement in the blade loading and in the numerical dependencies of predicted thrust and power output. A validation is then performed using a 2.3 MW turbine with R = 40 m radius, comparing against blade resolved fluid-structure interaction simulations and full-scale measurement data, in both laminar and turbulent inflows including both high shear and high yaw misalignment. For an axisymmetric laminar inflow case, the agreement between blade resolved and actuator line simulations is excellent, with prediction of integrated quantities within 0.2%. In more complex flow cases, good agreement is seen in overall trends but the actuator line predicts lower blade loading and flapwise deflection, leading to underpredictions of thrust by between 5.3% and 8.4%. The discrepancies seen can be attributed to differences in wake flow, induction, the reliance of the actuator line on the provided airfoil data and the force application into the computational domain. Comparing the wake between coupled actuator line and blade resolved simulations for turbulent flow cases also shows good agreement in wake deficit

This work presents a high-fidelity shape optimization framework based on computational fluid dynamics (CFD). The presented work is the first comprehensive curved tip shape study of a wind turbine rotor to date using a direct CFD-based approach. Preceding the study is a thorough literature survey particularly focused on wind turbine blade tips in order to place the present work in its context. Then follows a comprehensive analysis to quantify mesh dependency and to present needed mesh modifications ensuring a deep convergence of the flow field at each design iteration. The presented modifications allow the framework to produce up to six-digit-accurate finite difference gradients which are verified using the machine-accurate Complex-Step method. The accurate gradients result in a tightly converged design optimization problem in which the studied problem is to maximize power using 12 design variables while satisfying constraints on geometry, as well as on the bending moment at 90 % blade length. The optimized shape has about 1 % r/R blade extension, 2 % r/R flapwise displacement, and slightly below 2 % r/R edgewise displacement resulting in a 1.12 % increase in power. Importantly, the inboard part of the tip is de-loaded using twist and chord design variables as the blade is extended, ensuring that the baseline steady-state loads are not exceeded. For both analysis and optimization an industrial-scale mesh resolution of above 14×106 cells is used, which underlines the maturity of the framework.

We present numerical predictions and experimental observations of the drag force on the major branches of a model tree as well as the wind field in its wake. The tree model was mounted on a force sensor in a wind tunnel. The numerical simulations solved both the Reynolds Averaged Navier Stokes and detached eddy simulations, in combination with an immersed boundary model representation of the tree, while the measurements were obtained in the WindEEE Dome. Contrary to previous studies on trees or fractal structures, we find that the simulated forces on the tree skeleton were strongly dependent on the roughness of its surface. Upon setting a proper surface roughness, the simulations were capable of predicting both forces and wake fields, which were in good agreement with the observations both in terms of mean wind speed, mean turbulence and spectra. Furthermore, both simulations and observations showed that turbulence and mean wind speed were reduced for up to a distance of two tree heights downwind of the tree skeleton, underlining the efficiency of trees to remove momentum from a flow.

The aerodynamic performance of a NACA 633‐418 airfoil has been analyzed with disturbances in approximately 1000 different configurations focused on the frontal 10% of the airfoil. The configuration parameters are based on field test samples and rain erosion test specimens. The most important trends are presented by 500 configurations each simulated for 6°, 8°, and 10° angle of attack. The simulations are performed with the DTU Wind Energy in‐house 2D CFD Reynolds‐averaged Navier–Stokes solver, EllipSys2D, combined with the eN transition model for the laminar‐turbulent boundary layer transition. The configurations are modeled by a direct geometrical modification of the airfoil shape. The results show that the most important parameters are the position and the depth/height of the disturbance, with up to 35% lift reduction and 90% lift/drag reduction within the specified angle of attacks and disturbance parameter ranges.

Spectral analysis was performed on the time series data computed from pressure measurements on the New MEXICO (Model Rotor Experiments under Controlled Conditions) rotor in standstill conditions. As a priori, 3D airfoil polars were recreated from standstill measurements and compared against 2D airfoil polars and flat plate theory results to verify the measurements. The spectral analysis revealed the presence of dominant shedding frequencies for certain ranges of the geometric angle of attack. Two dominant shedding modes were identified: One was associated with bluff body vortex shedding, and the other was associated with low Strouhal number shedding. No dominant shedding frequencies were observed for angles of attack beyond 50°. The research improves on our current understanding of the unsteady nature of the stall regime, along with providing insight into the existence of vortex‐induced vibrations on a wind turbine in standstill conditions.

Spectral analysis was performed on the time series data computed from pressure measurements on the New MEXICO (Model Rotor Experiments under Controlled Conditions) rotor in standstill conditions. As a priori, 3D airfoil polars were recreated from standstill measurements and compared against 2D airfoil polars and flat plate theory results to verify the measurements. The spectral analysis revealed the presence of dominant shedding frequencies for certain ranges of the geometric angle of attack. Two dominant shedding modes were identified: One was associated with bluff body vortex shedding, and the other was associated with low Strouhal number shedding. No dominant shedding frequencies were observed for angles of attack beyond 50°. The research improves on our current understanding of the unsteady nature of the stall regime, along with providing insight into the existence of vortex-induced vibrations on a wind turbine in standstill conditions., Wind Energy

The present work describes an exploratory work aiming to analyze the impact of trailing edge flaps activation on Vortex Induced Vibrations (VIV) suppression. A computational study of the VIV of the AVATAR rotor blade, a 10 MW design suitable for offshore locations, was performed. A Fluid Structure Interaction (FSI) approach was adopted for the simulations, coupling an Improved Delayed Detached Eddy Simulations (IDDES) flow solver with a beam-based structural model. Initial simulations based on the clean geometry identified significant edgewise VIV for certain free stream velocity and flow inclination angles. Inflow conditions showing the maximum amplitude of blade vibrations were used in order to test several trailing edge flap geometries and operating angles. The best flap configuration found in this parametric study managed to suppress the VIV phenomenon. However, when assessing a wider range of inflow conditions, the amplitudes of vibration of the blade equipped with flaps were found to be equivalent to the ones obtained for its clean counterpart. It is therefore concluded that a re-calibration of the flap operating angle should be required in order to adapt it to the considered wind speed and wind direction.

The wind energy industry relies heavily on computational fluid dynamics (CFD) to analyze new turbine designs. To utilize CFD earlier in the design process, where lower-fidelity methods such as blade element momentum (BEM) are more common, requires the development of new tools. Tools that utilize numerical optimization are particularly valuable because they reduce the reliance on design by trial and error. We present the first comprehensive 3-D CFD adjoint-based shape optimization of a modern 10 MW offshore wind turbine. The optimization problem is aligned with a case study from International Energy Agency (IEA) Wind Task 37, making it possible to compare our findings with the BEM results from this case study and therefore allowing us to determine the value of design optimization based on high-fidelity models. The comparison shows that the overall design trends suggested by the two models do agree, and that it is particularly valuable to consult the high-fidelity model in areas such as root and tip where BEM is inaccurate. In addition, we compare two different CFD solvers to quantify the effect of modeling compressibility and to estimate the accuracy of the chosen grid resolution and order of convergence of the solver. Meshes up to 14 × 106 cells are used in the optimization whereby flow details are resolved. The present work shows that it is now possible to successfully optimize modern wind turbines aerodynamically under normal operating conditions using Reynolds-averaged Navier–Stokes (RANS) models. The key benefit of a 3-D RANS approach is that it is possible to optimize the blade planform and cross-sectional shape simultaneously, thus tailoring the shape to the actual 3-D flow over the rotor. This work does not address evaluation of extreme loads used for structural sizing, where BEM-based methods have proven very accurate, and therefore will likely remain the method of choice.

One of the main uncertainties in airfoil and rotor design is the determination of the position of transition on the blade as this can have considerable influence on the aerodynamic load sand in particular rotor power. Even for simulations on a rotor in uniform, steady inflow the determination of the position of transition is challenging. The atmospheric inflow on a real MW rotor adds a new level of complexity and uncertainty to the transition modelling and prediction.What is the impact of the atmospheric turbulence on transition compared with transition on a blade section in wind tunnel flow ? The present work is aiming at improving our insight into this subject by analysing transition data measured in the DanAero experiment in 2009 on an industrial 2MW turbine with an 80m rotor with LM38.8 m blades. Transition position is determined from analysing spectra of high frequency (50kHz) surface pressure fluctuations from 56 flush mounted microphones at a radius of 37 m. The position of transition is correlated with measured inflow angle close to the section with transition data. A comparison of the rotor transition with a similar set-up in the LM Glasfiber wind tunnel for transition detection on a 2D blade section copy of the rotor blade, shows a much earlier transition on the pressure side of the rotor blade. On the suction side the difference is much smaller due to an already early transition on the 2D blade section in the wind tunnel, probably caused by small surface irregularities or bumps. Analysing the pressure fluctuations in the laminar boundary layer close to the leading edge which are closely correlated with the turbulent inflow it can be seen that below 300 Hz the energy level on the rotor is much higher than in the tunnel. However,above 300 Hz they are comparable. It is finally shown that the transition on the rotor on the pressure side is moving forward when the spectral energy from 100-300 Hz in the laminar boundary layer increases.

The wind energy industry relies heavily on CFD to analyze new turbine designs. To utilize CFD further upstream the design process where lower fidelity methods such as BEM are more common, requires the development of new tools. Tools that utilize numerical optimization are particularly valuable because they reduce the reliance on design by trial and error. We present the first comprehensive 3D CFD adjoint-based shape optimization of a modern 10&thisp;MW offshore wind turbine. The optimization problem is aligned with a case study from IEA Wind Task 37, making it possible to compare our findings with the BEM results from this case study, allowing us to determine the value of design optimization based on high-fidelity models. The comparison shows, that the overall design trends suggested by the two models do agree, and that it is particularly valuable to consult the high-fidelity model in areas such as root and tip where BEM is inaccurate. In addition, we compare two different CFD solvers to quantify the effect of modeling compressibility and to estimate the accuracy of the chosen grid resolution and order of convergence of the solver. Meshes up to 14 · 106 cells are used in the optimization whereby flow details are resolved. The present work shows that it is now possible to successfully optimize modern wind turbines aerodynamically under normal operating conditions using RANS models. The key benefit of a 3D RANS approach is that it is possible to optimize the blade planform and cross-sectional shape simultaneously, thus tailoring the shape to the actual 3D flow over the rotor, which is particularly important near the root and tip of the blade. This work does not address evaluation of extreme loads used for structural sizing, where BEM-based methods have proven very accurate, and therefore will likely remain the method of choice.

The present paper describes an exploratory work aiming to analyze the impact oftrailing edge flaps activation on Vortex Induced Vibrations (VIV) suppression. A computationalstudy of the VIV of the AVATAR rotor blade, a 10MW design suitable for offshore locations,was performed. A Fluid Structure Interaction (FSI) approach was adopted for the simulations,coupling an Improved Delayed Detached Eddy Simulations (IDDES) flow solver with a beam-based structural model. Initial simulations based on the clean geometry identified significantedgewise VIV for certain free stream velocity and flow inclination angles. The inflow conditionsshowing the maximum amplitude of blade vibrations were used in order to test several trailingedge flap geometries and operating angles. The best flap configuration found in that parametricstudy managed to suppress the VIV phenomenon. However, when assessing a wider range ofinflow conditions, the amplitudes of vibration of the blade equipped with flaps were found tobe equivalent to the ones obtained for its clean counterpart. It is therefore concluded that are-calibration of the flap operating angle should be required in order to adapt it to the consideredwind speed and wind direction.

This paper presents tests carried out in the Virginia Tech Stability Wind Tunnel with Kevlar walls to allow for aeroacoustic measurements. Two-dimensional Computational Fluid Dynamics (CFD) calculations using the EllipSys2D solver were carried out to investigate the impact of no-slip condition, porosity, and flexibility of the simulated walls on the aerodynamics of a 0.9m-chord DU91-W250 airfoil at a Reynolds number of 3M. The accuracy of the airfoil simulation was first benchmarked against the experimental data, including data obtained in the Stability Tunnel hard-wall test section, and then the impact of boundary conditions on the simulation of the Kevlar test-section was investigated. The CFD was able to reproduce most of the airfoil characteristics (its lift, pressure distributions, and velocity profiles measured through the airfoil wake) as well as the growth of the wind tunnel wall boundary layers downstream of the model. All three boundary condition parameters (porosity, deflection, and no-slip condition) were found to have a high impact on the accuracy of the simulation. In particular, the no-slip condition was found to influence the pressure difference across the Kevlar walls and thus the transpiration through them, making it important in the aerodynamic correction process. The comparisons also highlighted some issues that need to be investigated further. The CFD consistently under-predicted drag while predicting transition locations upstream of those measured. Calculations also showed slightly larger blockage effects on the airfoil pressure distributions than those measured. This may be the result of an over-estimate of the blockage associated with transpiration through the wall.