In this paper, an aerodynamic model consisting of a lifting line-based trailed vorticity model and a blade element momentum (BEM) model is described. The focus is on the trailed vorticity model, which is based on the near wake model (NWM) by Beddoes and has been extended to include the effects of downwind convection and to enable a faster and more accurate computation of the induction, especially close to the blade root and tip. The NWM is introduced to model the detailed steady and unsteady induction from the first part of the trailed vorticity behind the individual rotor blades. The model adds a radial coupling between the blade sections and provides a computation of tip loss effects that depends on the actual blade geometry and the respective operating point. Moreover, the coupling of the NWM with a BEM theory-based far wake model is presented. To avoid accounting for the near wake induction twice, the induction from the BEM model is reduced by a coupling factor, which is continuously updated during the computation to ensure a good behavior of the model in varying operating conditions. The coupled near and far wake model is compared with a simple prescribed wake lifting line model, a BEM model and full rotor computational fluid dynamics (CFD) to evaluate the steady-state results in different cases. The model is shown to deliver good results across the whole operation range of the NREL 5-MW reference wind turbine.

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.

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...

In this work, a 2D aero-servo-elastic model of an airfoil section with 3 degrees of freedom (DOF) based on the 2D CFD solver EllipSys2D to calculate the aerodynamic forces is utilized to calculate the load reduction potential of an airfoil equipped with an adaptive trailing edge fl ap (ATEF) and subjected to a turbulent infl ow signal.The employed airfoil model corresponds to a successfully tested prototype airfoil where piezoelectric actuators were used for the fl apping. In the present investigation two possible control methods for the fl ap are compared in their ability to reduce the fl uctuating normal forces on the airfoil due to a 4 s turbulent infl ow signal and the best location of the measurement point for the respective control input is determined. While Control 1 uses the measurements of a Pitot tube mounted in front of the leading edge (LE) as input, Control 2 uses the pressure difference between the pressure and suction side of the airfoil measured at a certain chord position.Control 1 achieves its maximum load reduction of R Std(F y ) = 76.7% for the shortest Pitot tube of the test, i.e. a Pitot tube with a length of 0.05% of the chord length. Control 2 shows the highest load reduction of R Std(F y ) = 77.7% when the pressure difference is measured at a chord position of approximately 15%.