Laser-Doppler and numerical studies of backward-facing step flows

A laser Doppler anemometer is used to measure the velocity distribution and reattachment length downstream of a single backward-facing step mounted in a two-dimensional channel. Results are presented for laminar, transitional, and turbulent flow of air for Reynolds numbers between 70 and 8000. The measurements not only give the expected primary zone of recirculating flow attached to the backward-facing step but also show additional regions of flow separation downstream of the step and on both sides of the channel test section. Experimentally, however, it is shown that the two-dimensionality of the flow downstream of the step can only be maintained for Reynolds numbers less than or approximately equal to 420 or greater than or approximately equal to 6000. The flow in the interval between is three-dimensional, remaining symmetric to the center plane of the channel test section. The study also includes numerical predictions of experiments for those Reynolds numbers for which the flow maintains its two-dimensionality on the experiments. Good agreement between experimental and numerical results is obtained under these circumstances.