The laminar and turbulent flow over a backward-facing step placed in a square duct was investigated numerically.
The present investigation is mainly concerned with the flow having the aspect ratio (step width/step height) of 3 and the area expansion ratio of 2:3. Three-dimensional effects were significant due to the small aspect ratio. To simulate turbulent flows, both a standard k-ε model and a non-linear k-ε model were employed and the results were compared. The non-linear model was found to yield better results. An adequate simulation of transition from the pressure-induced secondary motion to turbulence-driven one could be described with non-linear model in three-dimensional flows. From the numerical results, the existence of the corner vortex and the flow field associated with it were clarified. The evolution and decay of corner vortices seems to be essential in describing three-dimensional sidewall effect. The reattachment length of the three-dimensional flow was found to be considerably shorter than the corresponding two-dimensional flow. The evolution of longitudinal vortices was visualized. Surface flow patterns which clearly demonstrate three-dimensional aspects of the flow were presented. Based on various data available, topological flow pattern was also sketched. To support the findings explored in the present work, experimental data were compared with the numerical data where applicable. For this purpose, additional calculations for the flows with aspect ratios of 1 and 3.3 were carried out. For both cases, the expansion ratio was 1:2.
