Stereoscopic Particle Image Velocimetry data from a 2-bladed rigid NACA0013 rotor undergoing retreating blade dynamic stall in a low-speed wind tunnel, are analyzed to understand the phenomenon of 3-dimensional reattachment at the end of the dynamic stall cycle. Continuing from prior studies on the inception and progression of 3-D rotating dynamic stall for this test case, phase-resolved, ensemble-averaged results are presented for two values of rotor advance ratio at two spanwise stations along the blade. The results show the nominal reattachment getting delayed in rotor azimuth with higher advance ratio. At low advance ratio reattachment starts at the leading-edge and progresses towards the trailing-edge with a vortex shedding transporting excess vorticity sheds from the leading-edge and convects away, with the flow reattaching behind it. At higher advance ratio, the vortical structure shrinks in size while the flow close to the trailing-edge appears to reattach. Spanwise vorticity transport appears to be the mechanism. The difference could be attributed to the lower chordwise velocity of the blade at higher advance ratio, bringing in a rotation effect.

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