Numerical Investigation of Deep Dynamic Stall for a Helicopter Blade Section

Abstract

For helicopters in forward flight at high advance ratios, because of the cyclic variation of the angle of attack, the flow over the retreating blade can stall. This phenomenon, which is known as dynamic stall, limits the maximum airspeed of the helicopter. Physical understanding of the highly complex unsteady flow can serve as a catalyst for the development of novel flow control strategies that counter the detrimental effects of dynamic stall. For the present implicit large-eddy simulations, the complex flow over a rotor blade section with a Sikorsky SCC-A09 airfoil is decomposed into a pitching, surging, and yawing flow (dynamic crossflow). The heaving/plunging motion resulting from the unsteady blade bending under load is disregarded. Instead, the centrifugal and Coriolis accelerations are modeled. Three different combinations of the elementary flows are considered: 1) pitching and surging (baseline); 2) pitching, surging, and rotational accelerations; and 3) pitching, surging, and yawing. Simulations for an outboard blade section at a chord-based Reynolds number and a reduced frequency exhibit all relevant stages of dynamic stall reported in the literature: The bursting of a laminar leading-edge bubble is followed by the emergence of a strong dynamic stall vortex, which is eventually shed. When the harmonic yawing motion is added, the laminar bubble bursts earlier and the dynamic stall vortex is shed sooner. The addition of the rotational accelerations reduces the coherence of the dynamic stall vortex and, as a result, diminishes the cycle-to-cycle variations of the lift coefficient.