Xue D.   Pan C.   Wang J.   Borodulin V.   Kachanov Y.  

Determining time-scale of laminar wing-tip vortex instability by visualization

Reporter: Xue D.

The counter-rotating wing-tip vortex pair in the aircraft wake has hazardous effect on ensuing flight safety and airport capacity. In the past few decades, a number of investigations were taken with the aim of diminishing the strength of wing-tip vortex by means of facilitating the corresponding instability [1 2]. Previous studies [3 4] have shown that the wing-tip vortices do not decay quickly by viscous diffusion, but undergo a sinusoidal instability instead, which eventually lead to the formation of a train of vortex rings and finally transit to turbulence. Triggering cooperative wing-tip vortex instabilities can be a promising strategy to control wing-tip vortices. In this sense, a full understanding on the physics of the instability characteristics and mechanisms is essentially needed for practical application of such strategy. On considering the difficulty in measuring the instability dynamics in laboratory, stability calculations within a linear (or at most weak nonlinear) framework were the first choice in the past. However, the limitation of this analytical method is also distinct, such as in the scenario of strong nonlinearity or mode interaction. In the present study, we propose a visualization-based method to extract the time-scale of naturally-excited wing-tip vortex instability. This effort is the first step towards studying the cooperative instability of vortex pair experimentally. Visualization of the wing-tip vortices was conducted in a high-quality water channel, where naturally excited instability of a pair of wing-tip vortex was clearly observed. By tracking the vortex core center’s variation along time, the characteristic of the vortex instability was quantified. Moreover, we used Dynamic Mode Decomposition method (DMD) [5 6] to extract the typical time-scale from visualization image series, whose results were comparable to that obtained from vortex center tracking.

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