Wing-tip vortex wandering: comparison of rapid scanning and static hot wire measurements

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 NACA 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 streamwise distance up to 116% of the vortex core radius, whereas it is fairly invariant with increasing angle of attack. Furthermore, wandering amplitude slightly decreases 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 vortex core radius and, consequently, the measured vortex appears more diffuse and weaker than in reality. The shape of the axial velocity profiles corrected for wandering 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