In this investigation the complex unsteady flow through and around a channel in the presence of an obstruction at the entry is studied by solving directly the unsteady Navier-Stokes equations. Depending on the location of the obstruction, the flow through the channel either reverses its direction, stagnates or continues in the same direction as that of the flow outside but with reduced magnitude. The Reynolds number of 4000 is considered, as experimental results are available for comparison. The computed results are in close agreement with experiments. The computations help with better understanding of the phenomenon of reverse flow and fluid pumping. © 2005 Published by The Japan Society of Fluid Mechanics and Elsevier B.V. All rights reserved.