Reverse flow in a circular duct with an obstruction at the entry

In this paper the reverse flow in a duct (diameter DD) with an obstruction at the front (which is a disc), is investigated using PIV. The gap gg between the obstruction and the entry to the duct is systematically varied and it is found that maximum reverse flow occurs at a g/Dg/D value of 0.5. The flow is stagnant around g/Dg/D of 1.25 and forward flow occurs for g/Dg/D values of 1.5 and above. The vortex formation length behind the disc is smaller than that of the plate and the variation of the magnitude of reverse flow with the gap gg between the axisymmetrical circular duct and two-dimensional channel differs considerably. The reverse flow phenomena in the circular duct with an obstruction at the entry can be explained by the vortex formation length and low pressure behind the obstruction.