This paper confirms the finding of discrete quasi-periodic streamwise vortical structures in the radial flow behind the separation line over a rotor blade undergoing dynamic stall in forward flight. It follows our finding of such structures over a blade undergoing transient stall induced by an inflow obstructer in a hover facility. Co-rotating structures suggest that the free shear layer formed at stall rolls up into discrete cells. The spanwise spacing of structures is approximately the height of the separated flow region. The presence of these structures helps explain the nature of the radial flow variation from root to tip in the separated flow region. This, as far as we know, is the first capture and measurement of the quasi-periodic radial velocity field on a rotating blade, and in particular of the profile of the strong radial velocity above a rotating blade in stall. Nomenclature ABS = Advancing blade side RBS = Retreating blade side P IV = Particle Image Velocimetry P LSI = Pulsed Laser Sheet Imaging u, v, w = Velocity in the x, y, z directions x, y, z = Cartesian coordinate system in the stream, normal and span directions U = Velocity Y AG = Ytrrium Alumnium Garnet crystal for a solid state laser µ = Advance ratio. The ratio of freestream speed to blade tip speed.

The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information.Send comments regarding this burden estimate or any other aspect of this collection of information, including suggesstions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA, 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any oenalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS. a. REPORT DIAGNOSTICS OF 3D DYNAMIC STALL ON ROTOR BLADES ABSTRACT SECURITY CLASSIFICATION OF:This project investigated the nature of the velocity field behind the separation line in dynamic stall on rotor blades, particularly the centrifugal effects and radial velocity field. Initial experiments were performed using a single-bladed rotor in hover with transient stall induced by an inflow obstructor. A 2-bladed teetering rotor setup was then constructed in the Georgia Tech 7' x 9' tunnel. PIV measurements confirmed the finding of discrete quasi-periodic streamwise vortical structures in the radial flow behind the separation line in both experiments. Co-rotating structures suggest that the free shear layer formed at stall rolls up into discrete cells. The spanwise spacing of structures is approximately the height of the separated flow region. The presence U