Abstract Summary
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.
