MEASUREMENTS OF THE NEAR WAKE OF A ROTOR IN FORWARD FLIGHT

This paper describes initial measurements of the near wake of a 2-bl ded teetering, untwisted, square-tipped rotor in forward flight. Issues of periodicity and repeatibility of the core are studied using laser velocimetry and flow-visualization on the front of the rotor wake where cycle to cycle variations are expected. Core passage uncertainty from cycle to cycle is less than 1 of rotor azimuth. Velocity measurements on the advancing blade side show wake-like core-axial velocities higher than the core circumferential velocity for the first 180 of vortex age. Both the axial and circumferential velocities approach 50 % of blade tip speed. The inboard vortex sheet has substantial wake-like velocities and rolls up into a concentrated circulatory region of high wake-like axial velocity rotating opposite to the tip-vortex, within 30 from the blade. The fully-developed values of the vortex core circulation, radius and axial velocity remain constant over 180 of age as current measurements indicate. Measurements on the front, rear and retreating blade side of the rotor are in progress