MS10.8 - Dynamics of Wind Energy Systems

Propeller aircraft engines, and more generally engines with a large rotating part (turboprops, high bypass ratio turbojets, etc.) are widely used in the industry and are subject to numerous developments in order to reduce their fuel consumption. In this context, unconventional architectures such as open rotors or distributed propulsion appear, and it is necessary to consider the influence of these systems on the aircraft's stability in flight. Indeed, the tendency to lengthen the blades and wings on which these propulsion devices are fixed increases their flexibility and promotes the whirl flutter risk. This phenomenon of aeroelastic instability is due to the precession movement of of the propeller rotation axis, which changes the attack angle of the flow on the blades and creates unsteady aerodynamic forces and moments that can amplify the motion and make it unstable. The whirl flutter instability can ultimately lead to the engine destruction. We note the existence of a critical speed of the incident flow. If the flow velocity is lower than this value, the motion is damped and the system is stable, whereas beyond this value, the flow provides energy to the system (negative damping) and the motion becomes unstable. A reference model of whirl flutter is based on the work of Houbolt & Reed who proposed an analytical expression of the aerodynamic load on a rigid blade propeller whose axis orientation is subject to small perturbations. Their work considered a propeller having four degrees of freedom (forward translation and roll neglected), a flow undisturbed by the blades and a propeller not generating any thrust in the absence of precession. The unsteady aerodynamic forces were then obtained using the thin airfoil theory and the strip theory. In the present study, a general movement of the propeller is considered (six degrees of freedom). The acceleration and rotation of the flow by the propeller are modeled using a Blade Element Momentum Theory (BEMT) approach, and the thrust is considered by the choice of an arbitrary blade pitch angle. The aerodynamic load is obtained using Theodorsen’s theory, a more complete method than thin airfoil theory, which models the wake vorticity and takes into account the phase delay of the aerodynamic load with respect to the propeller motion. Due to the frequency dependency of the lift and moment, a reduced order model of the aerodynamic load is constructed in order to perform linear stability analysis. This step, which was not necessary in the work of Houbolt & Reed, leads to the apparition of new “hidden” variables modeling the dynamic of the flow. This model of the aerodynamic load on the propeller is then coupled with two structural models to study the stability of a full propeller/wing system - a first one modeling a propeller in pitch and yaw and a second more complex one using beams to represent the wing and the pylon.

The ability of vortex generators (VGs) to improve aerodynamic performance has led to their widespread application in wind turbines. The complicated inflow that a rotor perceives and the propensity for flow separation cause VGs on wind turbines to typically suffer high flow unsteadiness. At such conditions, dynamic stall, an event leads to a dynamic delay of stall of an airfoil undergoing motions when the airfoil exceeds the static stall angle, which is prone to occur. The periodic loading resulting from dynamic stall leads to increased fatigue load on wind turbine blades and thus reduces their lifespan. Vortex generators are designed using experimental tests but also numerical modeling. Due to the cost of the former in the iterative design process, we need capable modeling techniques to provide a design direction or to optimise the VGs configurations. Though CFD is able to model the features of VGs, lower-fidelity and efficient engineering-type models are needed for a fast iterative evaluation. Although there are models at this fidelity level to replicate the steady effects of VGs, we don't have a fast and efficient tool to simulate the unsteady behavior of airfoils with VGs. This work develops an unsteady double-wake panel model (DWM) with viscous-inviscid interaction for the dynamic aerodynamic performance prediction of VG-equipped airfoils in dynamic stall. The baseline model couples the inviscid and viscous flow effects by solving the governing equations of unsteady potential flow together with the integral boundary layer equations on the surface panels using a semi-inverse iterative algorithm. The wake is modeled by the "double wake" concept, vortex sheets are shed at both the trailing edge and the separation location. A source-term approach proposed in the literature to mimic VGs effects through an artificial increase in mixing at the VG location has been applied to DWM. The validation of DWM shows sufficient accurate results compared to experimental data to claim the model's validity. For steady cases, DWM can predict the effects of VG correctly that the linear polar got extended and the stall got delayed by VGs. And it can also reflect the effect of different VG sizes and VG chordwise locations. For unsteady pitching cases, DWM can capture the difference between upstroke and downstroke, the difference between the free and forced transition, and the different effects of the different VG sizes generally well. However, DWM predicts a later and sharp stall and a later reattachment in some cases. As such, DWM tends to overpredict the VG effect in certain configurations. Overall, DWM gives significantly accurate results to claim sufficient validity of the model in a preliminary evaluation of an airfoil's capability to prevent stall with VGs. The relative changes in the results from the different VG configurations to the VG module also allow for a preliminary analysis of desired VGs location and initial sizing. A few limitations are identified to improve the model's accuracy in predicting the transition location, separation and reattachment, and drag forces in future development.

As the demand for renewable energy increases, wind turbine rotors will become larger with slender blades. Vortex Generators (VGs) are used for passive flow control to avoid flow separation and reduce unsteady loading on the thick root section of slender blades due to their simplicity, inexpensiveness, and the ability to retrofit them to blades. Aerodynamic load calculations for VGs involve long experimental campaigns or resource intensive CFD calculations. Due to their inherent time-intensive nature aeroelastic optimisation and design tools prefer to use simplified but accurate analysis tools for aerodynamic load calculations of clean airfoils. One such class of tools are viscous-inviscid interaction solvers that use Integral Boundary Layer (IBL) methods for viscous calculations in the boundary layer coupled with an inviscid solver for the rest of the domain. The most popular example of this tool is XFOIL. An engineering model for VGs in IBL methods has previously been developed and implemented in XFOIL. In this research, the model parameters are tuned for implementation in another tool RFOIL based on XFOIL. RFOIL has been developed for accurate and robust analysis of wind turbine airfoils with improvements for thick airfoils and rotational corrections. The aerodynamic performance predicted by both the implementations in XFOIL and RFOIL is then validated with an extensive database of airfoil data consisting of airfoils between 21% to 60% thickness, as well as Reynolds numbers between 1 million to 14 million. Further, the computational robustness of both implementations is tested to provide recommendations of the maximum values of VG geometry parameters (height, length, and inflow angle) that can be analysed using the VG model. Finally, the computation time for the VG model is compared with that for a clean airfoil analysis in the same viscous-inviscid interaction solvers RFOIL and XFOIL. The investigation provides overall recommendations about the use-cases and viability of the engineering model in blade-section design methodologies in the industry.

The prediction of vortex-induced vibrations is an essential topic for slender and lightweight structures like chimneys. In recent years, the available prediction models have been questioned in the context of their application for towers of wind turbines. One reason is that the significant tower height has simply not been covered in former model calibrations (based on measurements on chimneys). Another aspect is the different dynamic and aerodynamic behavior of wind turbines due to the structural features and the influence of the rotor on the aerodynamic damping. Measurements in the BLWT have been performed using a force-vibration test set-up to determine the pressure correlation over height, which is a piece of essential information for the modeling and prediction of VIV. Measurements have been performed for different ratios of slenderness, different reduced velocities, and different excitation amplitudes. Based on the quite broad set of parameters, it is possible to update a present widely distributed prediction model, which is the "correlation length method." The paper will implement such a recommendation, especially for very tall and slender supporting structures of WTG. The specific behavior of wind turbines is discussed using a specific turbine model for the same set-up of forced-induced wind tunnel test. In doing so, the aerodynamic influence due to the presence of the rotor can be studied for different positions. Additionally, aeroelastic tests with the same wind turbine have been performed, focussing on the aerodynamic damping and the influence of different blade positions. The findings have been compared to analytical models, and suitable recommendations are made.