Low Reynolds Number Flow through a Sudden Enlargement

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A flow in a sudden enlargement of a circular pipe is affected directly by the sudden change in the pipe cross section. Behavior of the flow after a sudden enlargement, with Reynolds number belonging to the laminar flow region, has not yet been clarified. Since sudden enlargement in pipes are a significant component of a piping system, clarification of the above noted flow characteristics will offer possibilities for a pipeline design based on realistic conditions. The following results were obtained by the investigation of flow through sudden enlargement visualized by the hydrogen bubble method and tracer technique. (1) In the downstream of a sudden enlargement, the flow separates at a region where the flow goes reversely along the wall surface. (2) The reattachment point in a cross section of separated flow changes periodically within a certain range along the axial direction. (3) The static pressure on the wall surface in the enlarged region becomes lower than that in the reattachment region and, as a result, the pressure changes periodically when the reattachment point is oscillated.
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