Abstract
Wandering is a universal feature of wind-tunnel generated vortices and it consists of abrupt displacements of the vortex core location. Rapid scanning measurements on the tip vortex generated from a N ACA 0012 half-wing model were performed in order to achieve data not affected by wandering effects. This measurement technique allows to evaluate the instantaneous vortex centre locations. Wandering was characterized by a bi-variate gaussian function fitting the probability density function evaluated from experimental vortex centre locations. Wandering is surely attenuated with increasing vortex strength, consequently, this phenomenon can not be considered a self-induced one. For vortices with a moderate strength, wandering might be strongly dependent on flow conditions and measurement locations. It is found that wandering amplitude increases roughly linearly with increasing stream- wise distance up to 116% of the vortex core radius, whereas it is fairly invariant with increasing angle of attack. Furthermore, wandering amplitude slightly de- creases with increasing free-stream velocity. The wandering smoothing effects on mean velocity profiles were estimated comparing rapid scanning data with static hot wire measurements. In extreme circumstances wandering is responsible to 30%
underestimate of the peak tangential velocity and 85% overestimate of the vor- tex core radius and, consequently, the measured vortex appears more diffuse and weaker than in reality. The shape of the axial velocity profiles corrected for wander- ing effects exhibit a significant velocity defect in the vortex core up to 20% of the free-stream velocity, and an overshoot at the core border. At high angles of attack a shift between wake flow and jet flow was observed at the vortex core. Wandering leaded to 70% underestimate of the axial velocity deficit and to 30% underestimate of the velocity excess. Finally, secondary vorticity structures were singled out from rapid scanning data in proximity to the main vortex. Furthermore, from a spectral analysis performed on the hot wire measurements, it is found a general increase of the fluctuating energy at low frequencies with approaching the vortex core.
