The interaction of a pair of unequal strength counter-rotating vortices is examined using a variety of visualization methods, including volumetric particle image velocimetry. Developed vortex cavitation in the cores of the vortices is also used to characterize the interaction of the initially parallel vortices. A pair of hydrofoils was used to generate two nearly parallel vortices with varying attack angle combinations conditions over a modest range of Reynolds numbers. The vortex pairs that are produced undergo an instability that was first analysed by Crow (1970 AIAA J ., vol. 8 (12), pp. 2172–2179), where the vortices interact through mutual induction, eventually leading to large deformations. Velocimetry is used to determine the characteristics for three regimes of the flow: the upstream region, effectively the initial condition of the parallel vortex pair; a midstream region where the vortices are interacting during the linear regime of the instability; a downstream region where the vortical flow is strongly three-dimensional resulting from the nonlinear vortex interactions. Properties of the vortices were measured in all three regions, including the local circulation, core size, eccentricity and velocity along the vortex axis. The rate of vortex stretching for the secondary (weaker vortex) was characterized as it undergoes strong deformation. The observed development of the instability was compared with the predictions of the theory by Crow.
Knister et al. (Mon,) studied this question.