Understanding the near and post-stall behavior of wind turbine blade airfoils through multi-fidelity CFD simulations: the case of S809 airfoil

For many years, wind turbine blade experts have based their designs on airfoil polars obtained with panel methods. These are quick to be set and sufficiently robust. On the other hand, their accuracy intrinsically decreases after stall, where simple post-stall extrapolation functions are introduced. Also, the complex shapes of wind turbine blade airfoils, especially the thick ones for use near the hub, introduce challenging simulation requirements at low Reynolds (Re) numbers. Both these issues are now coming at hand in many modern wind energy applications, like for example in stall-controlled small turbines or in utility-scale rotors experiencing extreme loading in parked conditions. Overall, it is acknowledged that higher-accuracy airfoil polars are needed to keep the reliability of engineering design methods high. The present study aims at numerically investigating the potential impact of high-fidelity, Computational Fluid Dynamics (CFD) techniques in predicting the near and post-stall behaviour of a typical wind turbine blade airfoil in comparison to both state-of-the-art panel methods and RANS CFD approaches. To this end, the ubiquitous S809 test case has been selected, as it is known to be particularly challenging and for which experimental data have been available from the literature. More specifically, an extended analysis has been first carried out using a RANS approach (both steady and unsteady) to assess the sensitivity on the main simulation settings, with particular focus on turbulence closure. Then, a few high-fidelity simulations based on both the traditional DDES, and the innovative LBM approaches have been performed to investigate the rate of improvement in modeling the underlying physics when the airfoil operates in the post-stall regime. Finally, a critical analysis of the different approaches is carried out and the prospects for wind energy applications are discussed.