lnfluence of a back-flow flap on the dynamic stall flow topology

Abstract

Dynamic stall is a major concern for highly loaded helicopter rotors in fast forward flight. The potential of a backflow flap for dynamic stall reduction is investigated. The flap assembly is mounted on the suction side of a helicopter airfoil undergoing deep-stall pitch oscillations. Wind-tu nnel experiments using high-speed particle image velocimetry were conducted to identify the flow topology and to imiestigate the flap's method of operation. A phaseaveraged proper orthogonal decomposition (POD) is used to identify relevant flow events and to compare test cases with and without flap. The evolution of the la rge-scale dynamic stall vortex in the in itial phases of flow separation is analyzed in detail. The back-flow flap splits the vortex into two smaller vortices and thereby reduces the pitching moment peak. This effect can be described through the eigenmode coefficients of the POD. The study closes with an analysis of different pitching frequencies, which do not affect the flap's method of operation.