Time evolution of laminar flow over a three-dimensional backward-facing step

A numerical investigation of laminar flow over a backward-facing step is pr esented within the transient context. The analysis is concerned with the st ep geometry and flow conditions reported by Armaly et al. Results show that there is generally good agreement between the three-dimensional results an d the experimental data for Re < 400, and excellent agreement with the two- dimensional results. Insight is also provided into the rich character of th e end-wall-induced three-dimensional vortices in the channel downstream of the step. To this end, the topological theory is adopted to draw the partic le oil streaklines on the roof, floor, step plane and the end-wall, from wh ich lines of separation and reattachment are theoretically determined. Toge ther with the Lagrangian particle track plot in the flow interior, light is shed on the formation of a secondary flow pattern, and thus the developmen t of longitudinal vortices. The present transient analysis increases the un derstanding of the complex interaction of the end-wall-induced vortices and the mainstream flow. This helps to reveal the mechanism responsible for th e increasing penetration of the three-dimensional flow structure into a reg ion near the mid-plane of the channel, at which the flow is essentially two -dimensional. Copyright (C) 1999 John Wiley & Sons, Ltd.