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

Abstract

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...

Stall-Induced Vibrations of the AVATAR Rotor Blade

“…Addition of stochastic inflow velocities from this model had little impact on the direction of motion at the bifurcation (flutter) point though. Skrzypiński et al [11] investigated vortex shedding from leading and trailing edge of a typical wind turbine airfoil at 90° angle of attack as a function of non-dimensional edgewise displacement amplitude, A*, and time period, T*, using RANS and LES simulations. "Locking in" of vortex shedding into edgewise airfoil motion occurred near a Strouhal number of 0.13, with the lock-in frequency range widening with growing ratio of airfoil displacement velocity to inflow velocity (as represented by the ratio, A*/T* [11]).…”

“…Skrzypiński et al [11] investigated vortex shedding from leading and trailing edge of a typical wind turbine airfoil at 90° angle of attack as a function of non-dimensional edgewise displacement amplitude, A*, and time period, T*, using RANS and LES simulations. "Locking in" of vortex shedding into edgewise airfoil motion occurred near a Strouhal number of 0.13, with the lock-in frequency range widening with growing ratio of airfoil displacement velocity to inflow velocity (as represented by the ratio, A*/T* [11]). At the same time, the peak non-dimensional power extracted from the flow at lock-in was reduced with increasing A*/T*, highlighting the non-linearity of the phenomenon.…”

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

“…The difference against measurements ranges from 10% to 20%. The overall quality of the predictions is considered acceptable taking into account that CFD computations reported in Skrzypiński et al . predict at 90° angle of attack, a mean C D of 3.1 for 2D analysis and 2.5 for 3D analysis (extruded profile one chord length in the spanwise direction).…”

“…Two‐dimensional RANS CFD predictions presented in Skrzypiński et al . for the same airfoil indicate that for high Reynolds, relevant to wind turbine blades applications, the Strouhal number at α = 90° is ~0.13.…”

Aeroelastic stability of idling wind turbines

“…It was concluded that the differences in the predicted post-stall characteristics and the stability limits between 2-D and 3-D CFD analysis are significant. Skrzypiński et al (2014b) also investigated vortex shedding phenomena occurring at very high AOA in the vicinity of 90 • using CFD models. Also in this work, a typical elastically mounted blade section was considered.…”