The direct numerical simulation methodology was employed to analyze the unsteady features of a three-dimensional separated flow and heat transfer around a downward step in a rectangular channel. Numerical calculations were carried out using the finite difference method. The Reynolds number Re based on the mean velocity at inlet and the step height was varied from 300 to 1000. The channel expansion ratio ER is 2.0 under a step aspect ratio of 36.0. It is found that the flow is steady upto Re = 500, but becomes sensibly unsteady at Re = 600 as accompanying a remarkable increase of the three-dimensionality of the flow and temperature fields. Nusselt number reaches its maximum in the reattachment flow region and also in the neighborhood of the side wall, and their locations depend greatly upon Re.

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