Wing-tip vortex wandering: comparison between pressure probe rapid scanning and static hot-film measurements

Wandering is a typical feature of wind-tunnel generated vortices and it consists in random fluctuations of the vortex core. Vortices measured by static measuring techniques appear to be more di®use than in reality. Rapid scanning of a tip vortex generated from a tapered NACA 0012 half-wing was performed with a five hole pressure probe. The aim of this measuring technique is to obtain velocity signals theoretically not affected by wandering by means of sufficiently fast scans performed through the vortex core in order to consider the vortex itself fixed during each scan. The vortex centre position distributions evaluated from each rapid scanning test at a specific downstream location were found to be accurately represented through bi-variate normal probability density functions. The comparison of the rapid scan- ning data, not affected by wandering, with static measurements carried out through a three-component hot-film probe allowed the smoothing effects of wandering on static measurements to be evaluated. Furthermore, the rapid-scanning data high- lighted flow features otherwise hidden from the static measurements. For instance, it was found that a switch from a wake flow to jet flow by increasing the wing angle of attack, proposed by several authors, does not seem to be a proper representa- tion of the axial velocity field of a wing-tip vortex, but rather to be the result of wandering smoothing effects on the actual velocity field. During the roll-up of a vortex an excess of the axial velocity should always be present in correspondence of the vortex centre due to a negative axial pressure gradient. Proceeding down- stream a decay due to viscosity can occur, so that the axial velocity excess can be reversed in a defect surrounded by axial velocity overshoots in correspondence of the core radius. More downstream, a predominant axial velocity defect is singled out without any other surrounding velocity excesses. Consequently, the wing angle of attack only influences the downstream distance where the axial velocity excess is reversed in a deficit; indeed, a delayed switch occurs by increasing the vortex strength, and thus the wing angle of attack. Tests were performed to investigate on the behaviour of wandering by varying the streamwise distance, the wing angle of attack or the Reynolds number. The largest wandering amplitude was generally found along an outboard-upwards to inboard-downwards direction. Furthermore, wandering was found to be not a self-induced phenomenon, as its amplitude is in- creased for more diffuse vortices or with reduced strength. The vortex strength seems to be the principal vortex parameter controlling the wandering; indeed, nei- ther the downstream distance, the wing angle of attack or the free-stream velocity have an absolute influence on wandering. In other words, the wandering amplitude can be reduced by increasing the wing angle of attack, the free-stream velocity or by reducing the streamwise distance from the wing, but if the vortex is sufficiently strong or concentrated it may be completely insensitive to the variation of these parameters.