This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications. U.S. Department of Energy (DOE) reports produced after 1991 and a growing number of pre-1991 documents are available free via www.OSTI.gov.NREL prints on paper that contains recycled content.

No abstract

This article investigates the aero‐elastic response of the DTU 10‐MW RWT blade in deep stall conditions with angles of attack in the vicinity of 90 degrees. The simulations were conducted with the high‐fidelity fluid–structure interaction simulation tool HAWC2CFD employing the multi‐body‐based structural model of HAWC2 and the incompressible computational fluid dynamics solver EllipSys3D. The study utilizes detached eddy simulation computations and considers the three‐dimensional blade geometry including blade twist and taper. A preliminary frequency analysis of the load variations on a stiff blade showed that an inclined inflow with a velocity component along the blade axis can trigger a spanwise correlated vortex shedding over large parts of the blade. Moderate wind speeds were sufficient to generate vortex shedding with frequencies close to the first edgewise eigenfrequency of the blade. Aero‐elastic computations of the elastic blade confirmed the findings of the frequency analysis. Inflow conditions with inclination angles between Ψ = 20° and Ψ = 55° and relatively low to moderate wind speeds between V = 16 and V = 26ms−1 were sufficient to trigger severe edgewise blade vibrations with blade tip amplitudes of several metres. The investigated inflow conditions are considered realistic and might occur when the wind turbine is idling or standing still and the yaw system is unable to align the wind turbine with the incoming wind. Copyright © 2016 John Wiley & Sons, Ltd.

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. . reaches the value at which the vortex-shedding frequency approaches the natural frequency of the cylinder, the vortexshedding frequency locks into the natural frequency of the cylinder. In other words, the vortex-shedding frequency is then controlled by the vibration of the cylinder. The flow visualization work of Williamson and Roshko 12 shows how the vortices are forced to interact with the vibration of the cylinder. This phenomenon is known as lock-in. The vortexshedding frequency unlocks and jumps back to the value corresponding to the Strouhal number as the dimensionless flow speed increases further. The width of the lock-in range in terms of the flow speed may increase with the vibration amplitude.Locked-in VIVs is a potential threat to large wind turbine blades at standstill. In the present work-which was a study of VIVs of the DU96-W-180 airfoil-similarities between the response of the cylinder in the aforementioned experiment and the response of the airfoil model in deep stall were investigated. The present study included 2D and 3D unsteady CFD computations. These included computations on non-moving, prescribed motion and elastically mounted airfoil suspensions. Stationary vortex-shedding frequencies corresponding to the 2D and 3D computations were obtained by performing frequency analysis of the loading on the non-moving airfoils. In the prescribed motion computations, the airfoil was forced to oscillate in the direction of the chord line. The elastically mounted airfoil computations were made with both one (1) and three (3) DOFs for the movement. The motivation for including both the prescribed motion and elastically mounted airfoil computations was that, on one hand, elastically mounted airfoil computations are the best reflection of real-life blade vibration. On the other hand, prescribed motion computations allow us to learn about the basic mechanisms of the forcing from the fluid at the structure in a more easily con...

The present study investigated physical phenomena related to stall-induced vibrations potentially existing on wind turbine blades at standstill conditions. The study considered two-dimensional airfoil sections while it omitted three-dimensional effects. In the study, a new engineering-type computational model for the aeroelastic response of an elastically mounted airfoil was used to investigate the influence of temporal lag in the aerodynamic response on the aeroelastic stability in deep stall. The study indicated that even a relatively low lag significantly increases the damping of the model. A comparison between the results from a model with lag imposed on all force components with the results from a model with lag imposed exclusively on the lift showed only marginal difference between the damping in the two cases. A parameter study involving positions of the elastic hinge point and the center of gravity indicated that the stability is relatively independent of these parameters. Another parameter study involving spring constants showed that the stability of each mode is dependent only on the spring constant acting in the direction of the leading motion of the mode. An investigation of the influence of the added mass terms showed that only the pitch-rate and flapwise-acceleration terms have any influence on the stability. An investigation of three different profiles showed that the stability is heavily dependent on the aerodynamic characteristics of the profiles-mainly on the lift. It was also shown that only the edgewise mode is unstable in deep stall. Moreover, independent of the amount of temporal lag in the aerodynamic response of the model, the inflow-angle region in the vicinity of 180 ı remains unstable in the edgewise mode. Therefore, this inflow-angle region may create stability problems in real life. The other type of vibrations potentially present at standstill conditions is vortex-induced, being outside the scope of the present study. NOMENCLATUREA i D parameters used for defining the indicial response function AA D aerodynamic axis b i D parameters used for defining the indicial response function c D chord line C D chord length c 0 D line through EA and CG C lin D D dynamic drag coefficient linearized around the system's equilibrium state C 0 D D drag coefficient value at the equilibrium state C Dyn D D dynamic drag coefficient C St D D static drag coefficient C lin L D dynamic lift coefficient linearized around the system's equilibrium state C 0 L D lift coefficient value at the equilibrium state