High spatial resolution experiments in the near field of a trailing vortex using a Stereoscopic Particle Image Velocimetry technique have been carried out. A NACA 0015 model with flat tip has been tested for several Reynolds numbers and angles of attack.
An axisymmetric meandering of the vortex is observed and a discussion on the aperiodicity correction method has identified the helicity peak as the most convenient
indicator of the vortex centre. The axial velocity in the centre of the vortex has been recorded always as an excess except for low angle of attack cases where intermittent
peaks of excess and deficit are superimposed on a large patch of deficit velocity.
The double vortex structure and the consequent double inflection in the tangential velocity profiles is also studied with reference to a vortex age parameter. At already 2 chords of distance from the trailing edge the profiles exhibit axisymmetric behaviour. A spiral structure of the vortex core has been reported as effect of the early stage of the rolling up and considerations on the rotation confirmed the high dependency of the initial phase of the rolling up with the tip shape. The square tip produces a strong asymmetry of the vortex core and an intense secondary vortex.
Good agreement of the tangential velocity and the circulation profiles between the experiments and analytical vortex expressions has been observed. The results confirm
the existence of a three-part vortex structure, namely an inner, a logarithmic and an outer region of the vortex where the former is affected by the initial vortex structure
and the latter is not universal but shows a dependence on the angle of attack
