Fluid–structure interaction computations for geometrically resolved rotor simulations using CFD

This paper presents a newly developed high‐fidelity fluid–structure interaction simulation tool for geometrically resolved rotor simulations of wind turbines. The tool consists of a partitioned coupling between the structural part of the aero‐elastic solver HAWC2 and the finite volume computational fluid dynamics (CFD) solver EllipSys3D. The paper shows that the implemented loose coupling scheme, despite a non‐conservative force transfer, maintains a sufficient numerical stability and a second‐order time accuracy. The use of a strong coupling is found to be redundant. In a first test case, the newly developed coupling between HAWC2 and EllipSys3D (HAWC2CFD) is utilized to compute the aero‐elastic response of the NREL 5‐MW reference wind turbine (RWT) under normal operational conditions. A comparison with the low‐fidelity but state‐of‐the‐art aero‐elastic solver HAWC2 reveals a very good agreement between the two approaches. In a second test case, the response of the NREL 5‐MW RWT is computed during a yawed and thus asymmetric inflow. The continuous good agreement confirms the qualities of HAWC2CFD but also illustrates the strengths of a computationally cheaper blade element momentum theory (BEM) based solver, as long as the solver is applied within the boundaries of the employed engineering models. Two further test cases encompass flow situations, which are expected to exceed the limits of the BEM model. However, the simulation of the NREL 5‐MW RWT during an emergency shut down situation still shows good agreements in the predicted structural responses of HAWC2 and HAWC2CFD since the differences in the computed force signals only persist for an insignificantly short time span. The considerable new capabilities of HAWC2CFD are finally demonstrated by simulating vortex‐induced vibrations on the DTU 10‐MW wind turbine blade in standstill. Copyright © 2016 John Wiley & Sons, Ltd.     

Analysis of vortex‐induced and stall‐induced vibrations at standstill conditions using a free wake aerodynamic code

Vortex‐induced and stall‐induced vibrations of a 2D elastically mounted airfoil at high angles of attack in the vicinity of 90° are investigated using a vortex type model. Such conditions are encountered in parked or idling operation at extreme yaw angles provoked by control system failures. At very high angles of attack, massive flow separation takes place over the entire blade span, and vortex shedding evolves downstream of the blade giving rise to periodically varying loads at frequencies corresponding to the Strouhal number of the vortices shed in the wake. As a result, vortex‐induced vibrations may occur when the shedding frequency matches the natural frequency of the blade. A vortex type model formulated on the basis of the ‘double wake’ concept is employed for the modelling of the stalled flow past a 2D airfoil. By tuning the core size of the vortex particles in the wake, the model predictions are successfully validated against averaged 2D measurements on a DU‐96‐W‐180 airfoil at high angles of attack. In order to assess the energy fed to the airfoil by the aerodynamic loads, the behaviour under imposed sinusoidal edgewise motions is analysed for various oscillation frequencies and amplitudes. Moreover, stall‐induced and vortex‐induced vibrations of an elastically mounted airfoil section are assessed. The vortex model predicts higher aeroelastic damping as compared with that obtained using steady‐state aerodynamics. Excessive combined vortex‐induced and stall‐induced edgewise vibrations are obtained beyond the wind speed of 30 m s^?1. Copyright © 2014 John Wiley & Sons, Ltd.     

Damping of edgewise vibration in wind turbine blades by means of circular liquid dampers

This paper proposes a new type of passive vibration control damper for controlling edgewise vibrations of wind turbine blades. The damper is a variant of the liquid column damper and is termed as a circular liquid column damper (CLCD). Rotating wind turbine blades generally experience a large centrifugal acceleration. This centrifugal acceleration makes the use of this kind of oscillatory liquid damper feasible with a small mass ratio to effectively suppress edgewise vibrations. A reduced 2‐DOF non‐linear model is used for tuning the CLCD attached to a rotating wind turbine blade, ignoring the coupling between the blade and the tower. The performance of the damper is evaluated under various rotational speeds of the rotor. A special case in which the rotational speed is so small that the gravity dominates the motion of the liquid is also investigated. Further, the legitimacy of the decoupled optimization is verified by incorporating the optimized damper into a more sophisticated 13‐DOF aeroelastic wind turbine model with due consideration to the coupled blade‐tower‐drivetrain vibrations of the wind turbine as well as a pitch controller. The numerical results from the illustrations on a 5 and a 10MW wind turbine machine indicate that the CLCD at an optimal tuning can effectively suppress the dynamic response of wind turbine blades. Copyright © 2015 John Wiley & Sons, Ltd.     

Computational fluid dynamics simulation of the aerodynamics of a high solidity,small‐scale vertical axis wind turbine

A computational fluid dynamics simulation was performed for a small‐scale, high solidity (σ = 0.48) H‐type Darrieus vertical axis wind turbine. Two‐dimensional unsteady Reynolds‐averaged Navier–Stokes equations were solved for the turbine numerical model, which has a large stationary domain and smaller rotating subdomain connected by a sliding mesh interface. The simulation results were first validated against steady‐state airfoil data. The model was then used to solve for three rotating blades with constant ambient flow velocity (Re = 360,000) over numerous blade speed ratios. The high solidity and the associated low blade speed ratio and rotational speed of the turbine result in complex flow–blade interaction mechanisms. These include dynamic stall resulting in vortex shedding, vortex impingement on the source blade and significant flow momentum extraction causing reduced power production from the downstream blade pass. Copyright © 2011 John Wiley & Sons, Ltd.     

Self‐induced vibrations of a DU96‐W‐180 airfoil in stall

This work presents an analysis of two‐dimensional (2D) and three‐dimensional (3D) non‐moving, prescribed motion and elastically mounted airfoil computational fluid dynamics (CFD) computations. The elastically mounted airfoil computations were performed by means of a 2D structural model with two degrees of freedom. The computations aimed at investigating the mechanisms of both vortex‐induced and stall‐induced vibrations related to a wind turbine blade at standstill conditions. In this work, a DU96‐W‐180 airfoil was used in the angle‐of‐attack region potentially corresponding to stall‐induced vibrations. The analysis showed significant differences between the aerodynamic stability limits predicted by 2D and 3D CFD computations. A general agreement was reached between the prescribed motion and elastically mounted airfoil computations. 3D computations indicated that vortex‐induced vibrations are likely to occur at modern wind turbine blades at standstill. In contrast, the predicted cut‐in wind speed necessary for the onset of stall‐induced vibrations appeared high enough for such vibrations to be unlikely. Copyright © 2013 John Wiley & Sons, Ltd.     

Characterization of the unsteady flow in the nacelle region of a modern wind turbine

A three‐dimensional Navier–Stokes solver has been used to investigate the flow in the nacelle region of a wind turbine where anemometers are typically placed to measure the flow speed and the turbine yaw angle. A 500 kW turbine was modelled with rotor and nacelle geometry in order to capture the complex separated flow in the blade root region of the rotor. A number of steady state and unsteady simulations were carried out for wind speeds ranging from 6 m s^?1 to 16 m s^?1 as well as two yaw and tilt angles. The flow in the nacelle region was found to be highly unsteady, dominated by unsteady vortex shedding from the cylindrical part of the blades, which interacted with the root vortices from each blade, generating high‐velocity gradients. As a consequence, the nacelle wind speed and the nacelle flow angle were found to vary significantly with the height above the nacelle surface. The nacelle anemometry showed significant dependence on both yaw and tilt angles with yaw errors of up to 10 degrees when operating in a tilted inflow. Copyright © 2010 John Wiley & Sons, Ltd.     

Improved Modal Dynamics of Wind Turbines to Avoid Stall‐induced Vibrations

Stall‐induced edgewise blade vibrations have occasionally been observed on three‐bladed wind turbines over the last decade. Experiments and numerical simulations have shown that these blade vibrations are related to certain vibration modes of the turbines. A recent experiment with a 600 kW turbine has shown that a backward whirling mode associated with edgewise blade vibrations is less aerodynamically damped than the corresponding forward whirling mode. In this article the mode shapes of the particular turbine are analysed, based on a simplified turbine model described in a multi‐blade formulation. It is shown that the vibrations of the blades for the backward and forward edgewise whirling modes are different, which can explain the measured difference in aerodynamic damping. The modal dynamics of the entire turbine is important for stability assessments; blade‐only analysis can be misleading. In some cases the modal dynamics may even be improved to avoid stall‐induced vibrations. Copyright © 2003 John Wiley & Sons, Ltd.     

Influence of inflow conditions on turbine loading and wake structures predicted by large eddy simulations using exact geometry

A large eddy simulation was performed on an National Renewable Energy Laboratory (NREL) phase VI wind turbine (10 m diameter), using the exact blade geometry, to determine the influence of different inflow conditions on the aerodynamic loadings and the near wake characteristics. The effects of the three inflow conditions, uniform inflow, linear wind shear and linear wind shear with turbulence, are investigated. Wind shear causes periodic variations in power and aerodynamic loading with an additional force component exerted along the lateral direction. Significant separation occurs in the high wind region on the suction side of the blades, resulting in unstable loading in off‐design inflow conditions. Because of the shear effect between the near‐blade tip vortex and ambient flow, the strong vortex core in the helical structure dissipates and transforms into a continuous vorticity sheet when x/D > 1.5. The combination of inflow turbulence and wind shear enhances the turbulence generation mechanism in the near wake, where energy is withdrawn from large wake structures and converted into energy of small‐scale structures. Copyright © 2015 John Wiley & Sons, Ltd.     

基于升力线理论的大型风力机气弹响应研究

针对风力机不断向大型化发展的趋势,导致结构柔度增加,气弹耦合特性和振动增强,研究了大型风力机高效精确的气弹响应分析方法。为了更准确模拟大型风力机气流沿叶片展向的三维流动现象,采用螺旋尾涡升力线模型代替传统叶素动量理论,建立了叶片气动载荷分析模型,进而结合风力机多体系统动力学模型,构建了机组的气弹耦合动力学方程和数值求解方法。以某10 MW风力机叶片为例,研究了稳态风况下不同风速的叶片气动性能,以及有效攻角、切向力等沿叶展方向的分布特点,并与采用修正叶素动量理论的气弹分析程序（HAWC）对比,结果表明,升力线理论无需引入经验修正模型即能获得叶素动量理论经修正后的分析精度。最后,通过非稳态风况下风力机的气弹响应分析,证明本文方法对大型风力机气弹耦合分析的有效性和准确性。     

Vortex‐induced vibrations of a DU96‐W‐180 airfoil at 90° angle of attack

This work presents an analysis of vortex‐induced vibrations of a DU96‐W‐180 airfoil in deep stall at a 90° angle of attack, based on 2D and 3D Reynolds Averaged Navier Stokes and 3D Detached Eddy Simulation unsteady Computational Fluid Dynamics computations with non‐moving, prescribed motion and elastically mounted airfoil suspensions. Stationary vortex‐shedding frequencies computed in 2D and 3D Computational Fluid Dynamics differed. In the prescribed motion computations, the airfoil oscillated in the direction of the chord line. Negative aerodynamic damping, found in both 2D and 3D Computational Fluid Dynamics computations with moving airfoil, showed in the vicinity of the stationary vortex‐shedding frequency computed by 2D Computational Fluid Dynamics. A shorter time series was sufficient to verify the sign of the aerodynamic damping in the case of the elastic computations than the prescribed motion. Even though the 2D computations seemed to be capable of indicating the presence of vortex‐induced vibrations, the 3D computations seemed to reflect the involved physics more accurately. Copyright © 2013 John Wiley & Sons, Ltd.     

An investigation on wind turbine resonant vibrations

Wind turbine resonant vibrations are investigated based on aeroelastic simulations both in frequency and time domain. The investigation focuses on three different aspects: the need of a precise modeling when a wind turbine is operating close to resonant conditions; the importance of estimating wind turbine loads also at low turbulence intensity wind conditions to identify the presence of resonances; and the wind turbine response because of external excitations. In the first analysis, three different wind turbine models are analysed with respect to the frequency and damping of the aeroelastic modes. Fatigue loads on the same models are then investigated with two different turbulence intensities to analyse the wind turbine response. In the second analysis, a wind turbine model is excited with an external force. This analysis helps in identifying the modes that might be excited, and therefore, the frequencies at which minimal excitation should be present during operations. The study shows that significant edgewise blade vibrations can occur on modern wind turbines even if the aeroelastic damping of the edgewise modes is positive. When operating close to resonant conditions, small differences in the modeling can have a large influence on the vibration level. The edgewise vibrations are less visible in high turbulent conditions. Using simulations with low‐level turbulence intensity will ease this identification and could avoid a redesign. Furthermore, depending on the external excitation, different aeroelastic modes can be excited. The investigation is performed using aeroelastic models corresponding to a 1.5 MW class wind turbine with slight variations in blade properties. Copyright © 2015 John Wiley & Sons, Ltd.     

The application of passive air jet vortex‐generators to stall suppression on wind turbine blades

An experimental study was performed to assess the feasibility of passive air jet vortex‐generators to the performance enhancement of a domestic scale wind turbine. It has been demonstrated that these simple devices, properly designed and implemented, can provide worthwhile performance benefits for domestic wind turbines of the type investigated in this study. In particular, this study shows that they can increase the maximum output power coefficient, reduce the cut‐in wind speed and improve power output at lower wind speeds while reducing the sensitivity to wind speed unsteadiness. A theoretical performance analysis of a 500 kW stall‐regulated wind turbine, based on blade element momentum theory, indicates that passive air jet vortex‐generators would be capable of recovering some of the power loss because of blade stall, thereby allowing attainment of rated power output at slightly lower average wind speeds. Copyright © 2016 John Wiley & Sons, Ltd.     

Rotor‐tower interactions of DTU 10MW reference wind turbine with a non‐linear harmonic method

In this paper, a computational study of the DTU 10MW reference wind turbine unsteady aerodynamics is presented. The whole wind turbine assembly was considered, including the complete rotor and the tower. The FINE/Turbo flow solver developed by NUMECA International was employed for the simulations. In particular, the Non‐Linear Harmonic (NLH) method was applied in order to accurately model flow unsteadiness at reduced computational cost. Important vortex shedding structures were identified at low blade span range and all along the tower height. A strong interaction between rotor and tower flows was also observed. Lastly, the performance of the NLH approach was compared against a standard Unsteady Reynolds‐Averaged Navier Stokes simulation. The same complex unsteady flow phenomena were captured by both technologies. Nevertheless, the NLH approach was found to be 10 times faster than the Unsteady Reynolds‐Averaged Navier Stokes method for this particular application. Copyright © 2016 John Wiley & Sons, Ltd.     

Aerodynamic characteristics of wind turbine blades with a sinusoidal leading edge

The aerodynamic characteristics of a kind of bionic wind turbine blades with a sinusoidal leading edge have been investigated in this paper based on a three‐dimensional Reynolds‐averaged Navier–Stokes simulation. The calculated results show that compared with a straight leading‐edge blade, the new‐type blade has a great improvement in shaft torque at high wind speeds. The localized vortices shedding from the leading‐edge tubercles, which can generate a much greater peak of the leading‐edge suction pressure than that from the straight leading‐edge case, are the physical essentials to enhance the wavy blade's aerodynamic performances as the blade goes into stall. In particular, the outboard segment from the 60%R station to the blade tip is the key region for wavy leading‐edge blades to improve the aerodynamic characteristics at high‐speed inflows. In this key region, a wavy blade can obtain a greater power output as the wavelength l and the waveheight δ increase. The present numerical results also show that the wavy leading‐edge shape is unfavorable for a wind turbine blade under the design conditions (e.g., at the rated wind speed). At these conditions, an early boundary‐layer separation as a result of the geometric disturbances of the leading‐edge tubercles will inevitably result in a visible shaft‐torque reduction in the wavy‐blade cases. Anyway, the wavy blades still tend to generate a more robust power output as a whole from 10 to 20 m s ^?1 than the original NREL phase‐VI blade. Copyright © 2011 John Wiley & Sons, Ltd.     

Investigation of the theoretical load alleviation potential using trailing edge flaps controlled by inflow data

A novel control concept for fatigue load reduction with trailing edge flaps based on the measurement of the inflow locally on the blade was presented. The investigation was conducted with the aeroelastic code HAWC2. The aerodynamic modelling in the code is based on blade element momentum theory. The simulations were carried out for the NREL 5MW reference wind turbine, and the mean wind speed at hub height was 8 m s^?1. The turbine was operated with fixed rotational speed. The energy at the blade is concentrated in spectral bands centred at multiples of the rotational frequency up to three times the rotational frequency. The highest fatigue load reduction was achieved when the inflow sensor was placed at the outer parts of the blade. In the best case, the reduction of the local fatigue loads induced by the blade sectional normal force was 60%. The control method gave the highest fatigue load reductions in conditions with strong wind shear. The demands for the flap actuator in terms of deflection angles was ±10°. The requirements in terms of the flap deflection velocity depend mainly on the inflow turbulence intensity. The maximum value was ±40°s^?1 for 20% inflow turbulence intensity. Unsteady aerodynamic effects seem to be negligible. Copyright © 2015 John Wiley & Sons, Ltd.     

Blade load mitigation control design for a wind turbine operating in the path of vortices

As wind turbine rotor size continues to increase, load mitigation becomes an important control objective. Turbines with hub heights of nearly 100m operate in the stable, nocturnal boundary layer where coherent turbulence can be generated by atmospheric phenomena outside the surface layer. These coherent turbulent structures may contribute to blade fatigue loads that can be mitigated with advanced control algorithms. Disturbance accommodating control (DAC) methods were implemented in a wind turbine structural dynamics simulation code to mitigate transient blade load response induced by a simple, Rankine vortex in the inflow. As a best‐case scenario, a full‐state feedback controller (which included a very detailed disturbance model) showed that blade flap damage equivalent load caused by the vortex passing through the rotor could be reduced by 30% compared to one that resulted from simulation of a typical proportional‐integral (PI) controller. A realizable DAC controller that incorporates only the vertical shear component of the vortex reduced loads by 9% compared to that resulting from simulation of a PI controller. The load reduction was even greater when the vortex was superimposed over full‐field, homogeneous turbulence. DAC methods have the flexibility to incorporate properties of coherent turbulent inflow structures in the controller design to mitigate blade fatigue loads. Further work must be done to develop disturbance models as more details about the turbulent structures are identified. Copyright © 2007 John Wiley & Sons, Ltd.     

Gaussian vs non‐Gaussian turbulence: impact on wind turbine loads

From large‐eddy simulations of atmospheric turbulence, a representation of Gaussian turbulence is constructed by randomizing the phases of the individual modes of variability. Time series of Gaussian turbulence are constructed and compared with its non‐Gaussian counterpart. Time series from the two types of turbulence are then used as input to wind turbine load simulations under normal operations with the HAWC2 software package. A slight increase in the extreme loads of the tower base fore‐aft moment is observed for high wind speeds when using non‐Gaussian turbulence but is insignificant when taking into account the safety factor for extreme moments. Other extreme load moments as well as the fatigue loads are not affected because of the use of non‐Gaussian turbulent inflow. It is suggested that the turbine thus acts like a low‐pass filter that averages out the non‐Gaussian behaviour, which is mainly associated with the fastest and smallest scales. Copyright © 2016 John Wiley & Sons, Ltd.     

Computational fluid dynamics analysis of the wake behind the MEXICO rotor in axial flow conditions

This paper presents a computational investigation of the wake of the MEXICO rotor. The compressible multi‐block solver of Liverpool University was employed, using a low‐Mach scheme to account for the low‐speed flow near the blade and in the wake. In this study, computations at wind speeds of 10, 15 and 24 m s ^{? 1} were performed, and the three components of the velocity were compared against experimental data around the rotor blade up to one and a half rotor diameters downstream. Overall, fair agreement was obtained with the computational fluid dynamics showing good vortex conservation near the blade. Vorticity values revealed discontinuities in the wake at approximately 70%R, where two different aerofoils with different zero‐lift angles are blended. The results suggest that all‐Mach schemes for compressible computational fluid dynamics methods can deliver good performance and accuracy over all wind speeds for flows around wind turbines, without the need to switch between incompressible and compressible flow methods. Copyright © 2014 John Wiley & Sons, Ltd.