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Abstract
Turbulence is hard to control. A plethora of experimental methods have been developed to generate this elusive state of matter, leading to fundamental insights into its statistical and structural features as well as its onset. In all cases, however, the material boundaries of the experimental apparatus pose a challenge for understanding what the turbulence has been fed, and how it would freely evolve. Here, we build and control a confined state of turbulence using elemental building blocks – vortex rings. We create a stationary and isolated blob of turbulence in a quiescent environment, initiated and sustained solely by vortex rings. We assemble a full picture of its three-dimensional structure, onset, energy budget and tunability. The incoming vortex rings can be endowed with conserved quantities, such as helicity, which can then be controllably transferred to the turbulent state. When the injection of vortex rings stops, a spherical front that separates the turbulent region from the quiescent surroundings propagates in the chamber, and the turbulence decays. By using a simple low-order closure model, we construct a spatially-extended description of the turbulence propagation, and compare its predictions of energy profile and non-diffusive dynamics with data. The turbulent blob, which can be measured in its entirety and is free to evolve in isolation, offers a playground for fundamental studies on inhomogeneous turbulence, response of turbulence to a periodic drive, and the role of conserved quantities in turbulence decay and cascading process.