SUMMARYThe prediction of the aerodynamic performance of pitching airfoils in stall conditions is considered in the context of strong viscous-inviscid interaction modelling. The aim of the work is to demonstrate the capabilities of a low-cost dynamic stall model well suited for engineering applications. The model is formulated on the basis of a standard panel method combined with a vortex blob approximation of the wake. The development of the boundary layer over the airfoil and the evolution of the shear layer in the wake are taken into account by means of strong viscous-inviscid interaction coupling. To this end a transpiration layer is added to the inviscid formulation which represents the displacement effect viscosity results in the flow while the non-linear coupled equations are solved simultaneously. Separation is modelled by introducing a second wake originating from the separation point ('double-wake' concept) which is provided as part of the boundary layer solution. The theoretical presentation of the model is supported with favourable comparisons to four sets of wind tunnel measurements.
Aeroelastic stability is a key issue in the design process of wind turbines towards both enchanced stability and increased fatigue life. The theory and models behind the state-ofthe-art aeroelastic stability tools developed for the analysis of the complete wind turbine at the Centre for Renewable Energy Sources and the National Technical University of Athens are presented in this article. Application examples of stability calculations for a pitch, variable speed and a stall-regulated wind turbine are also presented.for the edgewise blade mode shapes. On the other hand, Rasmussen et al. 3 developed a dynamic stall model taking variations in both angle of attack and flow velocity into account, while Chaviaropoulos 4 used eigenvalue analysis combined with unsteady aerodynamics, in the form of the engineering-type ONERA model, 5 to investigate in a linearized sense the flap/lag stability properties of an isolated blade section. The conclusions drawn from the latter work, which are still valid, were the following.'It is seen that in the absence of structural damping the system is neutrally stable when viscous effects are neglected. As soon as viscous effects are taken into account, the system response becomes unstable, especially at the lower reduced frequency regime corresponding to the blade near-tip area. Parametric studies have shown that instability is amplified when (i) the density ratio R f (the air density divided by the normalized linear density of the blade) takes higher values, (ii) the blade natural frequencies become lower and (iii) the lift loss and/or the lift loss and drag derivatives of the aerofoil polar curves take higher values. Performing polar curve computations for five NACA 63-2XX profiles of different thickness, it appears that thicker profiles have more stable behaviour. This indicates once again that instabilities, when present, are triggered from the outer part of the blade. Structural damping has a dramatic stabilizing effect. Even a small amount of structural damping in the edge direction can drastically suppress the range and strength of the unstable region. Low ambient temperature favours aeroelastic instabilities, reducing the structural damping and increasing the density factor. Disregarding Reynolds number effects on the aerodynamic performance, it appears that the flap/lead-lag instability is independent of the blade size for aerodynamically (tip speed is maintained) and structurally similar blades.'The solution of the typical section stability problem with an engineering-type aerodynamic model provided important knowledge at the qualitative level but also significant uncertainty at the quantitative level. Along these lines the problem of the typical section was revisited in the framework of the VISCEL European project, using this time an unsteady Navier-Stokes treatment of the aerodynamics and working, inevitably, in the time domain (see References 6 and 7 for details). The exercise confirmed an earlier finding that linear models appear more conservative in evaluating instabilities...
As the size of commercial wind turbines increases, new blade designs become more flexible in order to comply with the requirement for reduced weights. In normal operation conditions, flexible blades undergo large bending deflections, which exceed 10% of their radius, while significant torsion angles toward the tip of the blade are obtained, which potentially affect performance and stability. In the present paper, the effects on the loads of a wind turbine from structural nonlinearities induced by large deflections of the blades are assessed, based on simulations carried out for the NREL 5 MW wind turbine. Two nonlinear beam models, a second order (2nd order) model and a multibody model that both account for geometric nonlinear structural effects, are compared to a first order beam (1st order) model. Deflections and loads produced by finite element method based aero-elastic simulations using these three models show that the bending–torsion coupling is the main nonlinear effect that drives differences on loads. The main effect on fatigue loads is the over 100% increase of the torsion moment, having obvious implications on the design of the pitch bearings. In addition, nonlinearity leads to a clear shift in the frequencies of the second edgewise modes.
There seems to be a significant uncertainty in aerodynamic and aeroelastic simulations on megawatt turbines operating in inflow with considerable shear, in particular with the engineering blade element momentum (BEM) model, commonly implemented in the aeroelastic design codes used by industry. Computations with advanced vortex and computational fluid dynamics models are used to provide improved insight into the complex flow phenomena and rotor aerodynamics caused by the sheared inflow. One consistent result from the advanced models is the variation of induced velocity as a function of azimuth when shear is present in the inflow. This gives guidance to how the BEM modeling of shear should be implemented. Another result from the advanced vortex model computations is a clear indication of influence of the ground, and the general tendency is a speed up effect of the flow through the rotor giving a higher power than in uniform flow. On the basis of the consistent azimuthal induction variations seen in the advanced model results, three different BEM implementation methods are discussed and tested in the same aeroelastic code. A full local BEM implementation on an elemental stream tube in both azimuth and radial direction seems to be closest to the advanced model results. Copyright © 2011 John Wiley & Sons, Ltd.
Abstract. This paper presents the integration of a near-wake model for trailing vorticity, which is based on a prescribed-wake lifting-line model proposed by Beddoes (1987), with a blade element momentum (BEM)-based far-wake model and a 2-D shed vorticity model. The resulting coupled aerodynamics model is validated against lifting-surface computations performed using a free-wake panel code. The focus of the description of the aerodynamics model is on the numerical stability, the computation speed and the accuracy of unsteady simulations. To stabilize the near-wake model, it has to be iterated to convergence, using a relaxation factor that has to be updated during the computation. Further, the effect of simplifying the exponential function approximation of the near-wake model to increase the computation speed is investigated in this work. A modification of the dynamic inflow weighting factors of the far-wake model is presented that ensures good induction modeling at slow timescales. Finally, the unsteady airfoil aerodynamics model is extended to provide the unsteady bound circulation for the near-wake model and to improve the modeling of the unsteady behavior of cambered airfoils. The model comparison with results from a free-wake panel code and a BEM model is centered around the NREL 5 MW reference turbine. The response to pitch steps at different pitching speeds is compared. By means of prescribed vibration cases, the effect of the aerodynamic model on the predictions of the aerodynamic work is investigated. The validation shows that a BEM model can be improved by adding near-wake trailed vorticity computation. For all prescribed vibration cases with high aerodynamic damping, results similar to those obtained by the free-wake model can be achieved in a small fraction of computation time with the proposed model. In the cases with low aerodynamic damping, the addition of trailed vorticity modeling shifts the results closer to those obtained by using the free-wake code, but differences remain.
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