Creation and control of an isolated turbulent blob fed by vortex rings*

We experimentally study a stationary, isolated blob of turbulence, initiated and sustained by the collisions of multiple vortex rings. Our PIV and 3D PTV measurements reveal that the blob consists of a turbulent core (Taylor Reynolds number: 50-300) surrounded by comparatively quiescent fluid. By examining the mass and enstrophy flux, distribution of energy and enstrophy, and turbulent statistics, we assemble a complete picture of its three-dimensional structure, onset, energy budget, and tunability. Crucially, the injected vortex rings can be endowed with conserved quantities such as helicity. We demonstrate that helicity can be controllably transferred to a turbulent state. This system provides an ideal playground to investigate the generation and decay of turbulence with controlled inputs of energy, enstrophy, and helicity.