Decay and propagation of an isolated turbulent blob

We create and sustain an isolated blob of turbulence by repeatedly firing together vortex loops. In the steady state, our PIV and 3D PTV measurements reveal that the blob consists of a turbulent core ($Re_{lambda}=50-300$) surrounded by comparatively quiescent fluid. The properties of the vortex loops determine the turbulent intensity and the scales of motion within the blob. When the injection of vortex rings stops, a spherical front that separates the turbulent core from the quiescent surroundings, begins to propagate within the chamber, and the turbulence decays. This turbulence endures throughout the decay process, lasting more than fifteen minutes, as evidenced by the energy spectrum. Through experimental comparison of turbulence induced by different methods within the same chamber, we demonstrate that the large-scale turbulence motion dictates the decay law of energy. Using a simple low-order closure model, we create a spatially-extended description of turbulence propagation and decay. We then compare its energy profile predictions and decay law with the data. Crucially, the observed turbulent front shows qualitative features of the non-diffusive transport captured by this mean field theory, as opposed to simple diffusion. This model also provides a consistent explanation for the different decay laws observed in the experiments.