This study considers a case of uniform flow into the bottom of a rectangular vertical tube array, with outflow from a point sink in one bottom corner. Experimental results show that, upon a step change to a higher density, a clearly defined density interface forms, and buoyancy forces cause channeling of the dense fluid. The tube array geometry permits an analytic solution, which agrees well with the experiments. Standard control-volume numerical methods, however, predict a diffuse, thick, and viscous density interface, in contrast to the thin interface seen in the experiment. This viscous effect can be attributed to numerical diffusion, and a Lagrangian interface tracking procedure is shown to reduce the diffusion effect greatly.

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