Abstract
The formation and evolution of a front between turbulent and nonturbulent fluid in a two-dimensional flow are studied using a combined experimental-numerical technique. Turbulence was created by towing a grid of vertical bars partway through a tank that was filled with a two-layer stratified fluid. The velocity field at several times during the evolution of the turbulence was obtained by photographing neutrally buoyant particles suspended in the sharp density interface. Each realization was used to compute the advance of lines of particles released in the flow near the initial location of the grid. The computed location and shape of the turbulent/nonturbulent front, representing the distribution of a passive scalar in the flow, correlate well with the ‘‘zero contour’’ of the absolute-vorticity field. It is shown that the turbulent/nonturbulent front, in the most distorted cases, may be described by a fractal dimension over a restricted range of length scales. Certain geometric characteristics of the frontal region, in particular the probability distribution of a passive scalar, the frontal width, and length are also examined; the latter appears to grow with a power of time related to the fractal dimension of the interface.
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