An Investigation of CFD Predictions for Rotor Hover Performance Near Stall Conditions

Accurate rotor stall prediction is critical to determine rotor’s maximum performance and stall boundary. While Computational Fluid Dynamics (CFD) has shown significant improvement in accuracy over the past several decades, accurate prediction of separation and stall remains a challenge. Often, suspicious abrupt stall is observed from the CFD simulations at high thrust conditions. In this study, the issue is demonstrated with two publicly available rotors: Hover Validation and Acoustic Baseline (HVAB) rotor and XV-15 rotor. Furthermore, fictitious Simplified Blade (SB) models were used to examine tip design (anhedral and different airfoils) impact on stall behavior. Also, a static VR-12 airfoil model was used to examine various numerical schemes for stall prediction. The blade tip anhedral design delayed stall by altering blade-vortex interaction strength. The rotor stall prediction followed trend of the tip airfoil stall prediction. For the XV-15 rotor tested outdoor, it was found that headwind situation could have favorable impact on stall. Among tested turbulence models, Menter’s Shear Stress Transport (SST) model and Spalart-Allmaras (SA) Delayed Detached Eddy Simulation (DDES) model showed premature stall. The standard Menter baseline two-equation model (BSL) showed delayed stall from the VR-12 airfoil study, and this behavior was maintained for rotor simulations, making improved correlation with test data where previous models failed with sudden stall. Current study indicated further attention to the turbulence model improvement is necessary for accurate stall prediction.