Simulation of Dynamic Stall on an Elastic Rotor in High-Speed Turn Flight

摘要

In the present work, a highly loaded, high-speed turn flight—which was classified as dynamic-stall relevant—of Airbus Helicopters' Blue-copter demonstrator helicopter was numerically investigated with a loose coupling of the CFD code FLOWer and the rotorcraft comprehensive code CAMRAD II, performing a three-degree-of-freedom trim of the isolated rotor. On the CFD side, a high-fidelity DDES was carried out and differences between the SA-DDES and SST-DDES turbulence model as well as the influence of elastic twist (elastic versus torsionally rigid rotor), of additionally modeled helicopter components (hub, fuselage, and tail boom) and of a variation of the target rotor thrust (low, mid, and high) were investigated. Computed rotor control angles and pitch-link loads were then compared to flight-test measurements. The main findings are as follows: 1) The rotor flow field in the present flight condition is highly unsteady and complex, as the flow phenomena of trailing-edge separation, BVI, several different dynamic-stall events and shock-induced separation occur throughout a large portion of the azimuth. 2) The first dynamic-stall event happens in the third quadrant of the rotor disk and might be triggered by BVI. The dynamic-stall vortex quickly spreads in- and outboards, inducing overshoots of sectional thrusts and pitching moments. The kink in the planform of the rotor blade seems to delay the outboard spreading of dynamic stall and is the origin of a tip-vortex like apex vortex. Several other typical dynamic-stall events and streamwise vortices, which sustain overshoots of sectional thrust and pitching moments, appear in the outboard region of the blade in the fourth quadrant.