Low-Speed Static Stall Suppression Using Steady and Pulsed Air-Jet Vortex Generators

Abstract

An experimental investigation was undertaken to assess the ability of low-momentum steady and unsteady air injection, using pitched and skewed air-jet vortex generators on a Royal Aircraft Establishment 9645 rotor section in a nominally two-dimensional flow, to suppress quasi-steady flow separation at low speeds (0.07 < M < 0.12). Results from this study show that unsteady blowing using air-jet vortex generators at ajet-to-freestream velocity ratio of 2.0 and pulsing frequencies equal to or above 50 Hz led to an ∼20% increase in C N max , while steady blowing with an equal amount of total mass flow as the unsteady case led only to an ∼5% increase. Similar results were confirmed with respect to the delay in drag and pitching-moment divergence. A correlation was found between aerofoil trailing-edge shedding frequency and maximum normal force enhancement, where a maximum normal force coefficient was obtained at a reduced pulsing frequency, F + ∼0.6, corresponding with the natural shedding frequency of the aerofoil in stall. Tufts applied to the upper surface of the aerofoil highlighted interesting periodic wavelike motions of the separated area downstream of the air-jet orifices at low frequencies below ∼10 Hz, and they showed that the advance and retreat of the separated region was in synchronization with the activation and deactivation of the air jets during each pulse. The oscillations increased with increasing jet pulsing frequency until no wavelike motion could be detected.