Unraveling the interplay of two counter-rotating helical vortices by INM’s Alessandro Capone and Francisco Alves Pereira is one of the seven entries designated as 2019 APS/DFD Gallery of Fluid Motion Award Winners.
APS (American Physical Society ) is one of the world’s largest organization of physicists (over 55 000 members), working to advance the knowledge of physics through its research journals (including the prestigious Physical Review and Physical Review Letters). The APS Division of Fluid Dynamics’ Gallery of Fluid Motion is meant to be a visual record of the aesthetic and science of contemporary fluid mechanics. Each year, the submissions are judged based on both striking visual qualities and scientific interest. Top-ranked videos are designated as Milton Van Dyke Awardees or Gallery Winners. The 72nd Annual Meeting of the APS Division of Fluid Dynamics was held in Seattle, WA from November 23, 2019 to November 26, 2019.
The video examines the interaction between two coaxially aligned helical vortices induced by two counter-rotating propellers of slightly different diameters operating in a water tunnel. The video brings to light the interaction between the two coherent structures through high-speed visualizations performed in low pressure conditions that trigger the onset of cavitation in the vortex cores, thus enabling their imaging. Complex phenomena may be observed; two helical systems merge and breakup in the region of closest distance and strongest vorticity. This event is followed by a reconnection process that bridges the broken parts, resulting in the formation of isolated vortex rings advected by the accelerated flow from the propeller system. These periodically generated vortex rings, spatially organized in an alternating pattern, have a sawtooth appearance and show evidence of entrainment, stretching and roll-up due to the velocity gradients along the radial direction. Residual vorticity threads are also observed that intermittently bridge consecutive rings, as a result of momentum transfer.