PARALLEL BLADE–VORTEX INTERACTION

Abstract

Laser–Doppler velocimetry was employed to experimentally investigate two-dimensional airfoil–vortex interaction. Vortices were generated by sinusoidally oscillating a NACA 0012 airfoil about its quarter-chord at a reduced frequency ofk=2·05 and an amplitude of ±10° in angle of attack. The target airfoil, a NACA 632A015 was immersed in the wake, two chord lengths downstream of the vortex generator's trailing edge. Phase-averaged velocity measurements of the flow around the target airfoil were made with the latter at angles of attack ofα=0° (unloaded blade) andα=10° (loaded blade). A close encounter with a counterclockwise rotating vortex was studied for both angles of attack and a head-on collision which splits the counterclockwise rotating vortex in two was investigated for collision which splits the counterclockwise rotating vortex in two was investigated forα=10°. Vorticity fields were constructed and surface pressure fluctuations on the airfoil were determinded. It was found that the most violent interaction occurs when the disturbing vortex passes very near over the loaded airfoil. On the other hand, with head-on collision, the disturbing vortex is split, the pressure spikes are drastically reduced and the vortex disintegrates. The opportunity arises therefore to unload a blade during the short interval of vortex interaction, in order to reduce the severity of blade–vortex interaction.