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_λ = 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. By using a simple low-order closure model, we construct a spatially-extended description of the turbulence propagation and decay, and compare its predictions of energy profile and non-diffusive dynamics with data.