Flow structure produced by the interaction and merger of a pair of co-rotating wing-tip vortices

Abstract

Experiments have been performed to study the co-rotating wing-tip vortex pair produced by a pair of rectangular wings in a split-wing configuration. Detailed measurements made in cross-sections upstream and downstream of merger reveal, for the first time, the complex turbulence structure of this flow. The vortices spiral around each other and merge some 20 chordlengths downstream of the wings. As merger is approached the vortices lose their axisymmetry – their cores develop lopsided tangential velocity fields and the mean vorticity field is convected into filaments. The cores also become part of a single turbulence structure dominated by a braid of high turbulence levels that links them together. The braid, which quite closely resembles the structure formed between adjacent spanwise eddies of transitional mixing layers, grows in intensity with downstream distance and extends into the vortex cores. Unlike a single tip vortex, the unmerged cores appear turbulent.The merging of the vortices wraps the cores and the flow structure that surrounds them into a large turbulent region with an intricate double spiral structure. This structure then relaxes to a closely axisymmetric state. The merged core appears stable and develops a structure similar to the laminar core of a vortex shed from a single wing. However, the turbulent region formed around the vortex core during the merger process is much larger and more axisymmetric than that found around a single wing-tip vortex.

Multisensor Hot Wire Vorticity Probe Measurements of the Formation Field of Two Corotating Vortices

“…The pyramidal shape of the sensors' array minimizes thermal radiation between sensors and ensures that no sensor is in the wake of another sensor for flow angles less than almost 45 • . The agreement of measurements obtained with a 12 wire, a 4 wire and an X wire probe support the findings of Devenport et al [17], Hoffmann and Joubert [24] that probe interference with the flow is negligible.…”

“…In their work, Devenport et al [15][16][17] and Hoffmann and Joubert [24], attempted to quantify the wandering effects evaluating the possible existing error and its influence in the distortion of "real" flow evolution. They concluded that in flow cases involving well established distinguished axisymmetric flow structures wandering effects can be reasonably estimated.…”

“…The rotational velocity field around each core induces a rotational velocity to the other vortex and thus both vortex cores are spiraling around each other, developing a braid of two vortices and deforming the external flow field in the downstream direction. Gradually the interaction flow field links both vortices together until the final merging and the formation of a new stable linear vortex (Devenport et al [17], Chen et al [11]). …”

Experiments on the elliptic instability in vortex pairs with axial core flow

“…Similar observations were reported by Chen, Jacob & Savaş (1999) and Ortega, Bristol & Savaş (2003), who studied a four-vortex system generated by a flapped wing in a towing tank. The only known quantitative data were presented by Devenport et al (1997) and Devenport, Vogel & Zsoldos (1999). They investigated counter-and co-rotating vortices generated by two symmetrical wings in a wind tunnel.…”

The merging of two co-rotating vortices: a numerical study

“…In two-dimensional flows, vortex merging [2][3][4][5][6][7][8] is the principal ingredient of the inverse cascade [9][10][11] although three vortex interactions may be also significant. It also explains the thickening of the mixing layer width [12] and is considered in the context of aircraft wakes [13,14]. In meteorology or geophysics, dispersion of passive scalars is partly governed by the merging of large scale vortices.…”