VORTICAL FLOW OVER A 3-D BACKWARD-FACING STEP

Numerical simulations of Navier-Stokes equations were performed for in compressible Navier-Stokes flow inside a channel. The flow field under investigation was characterized as having a backward-facing step, wit h an expansion ratio gamma = H/h = 1.9423, over which a fully develope d channel flow is suddenly expanded into the channel with a channel wi dth, B, to upstream channel height, h, ratio B/h = 2, 4, 6, and 10. Nu merical solutions for this backward-facing step problem were obtained on the basis of span ratios and Reynolds numbers. For the Reynolds num bers considered, Re = 100, 389, 800, we elaborate on the changes in th e flow topology according to solutions computed at Re = 389. We use to pology theory as a guide to studying flows that are kinematically poss ible. This theory is mathematically rigorous and helps find critical p oints, from which we can sketch complicated flow patterns by clarifyin g the three-dimensional flow separation just behind the step and the f low reattachment on the downstream roof. Clearly visible on the roof o f the channel is the separation-reattachment flow feature. Notably add ressed is that separated flow on the roof is only confined to regions near the two side walls and does not extend over the whole span. The o nset of a near-wad pair of counterrotating vortices is associated with the presence of closed separation-reattachment bubbles. They are visi ble at transverse planes, just upstream of termination of the roof rec irculation bubble, near the two end walls.