A contribution to the study of unsteady behaviour of separated and reattaching flows over a backward-facing is reported.Unsteady wall pressure and velocity field measurements are done in order to clarify the main separate Reynolds number dependencies (based on the step height and the free stream velocity) with regards to downstream external parameters (e.g. expending ratio and boundary layer thickness). The recirculation length and the secondary separation point, as well as the statistics of the velocity filelds and the surface pressure, are analysed and compared to previous studies.

The experiments are performed in an optically accessible closed-loop wind tunnel with a 2mx2mx10m test section. The step height of the backward-facing step is 83mm and corresponds to an expansion ratio of 1.04, i.e. negligible inference of the upper wall. An effectively nominally two-dimensional ow is provided by the large span of 2000mm yielding an aspect ratio of 24. A set of 25 static pressure taps in parallel with 25 sub-miniature piezo-resistive Kulite XCQ-062 sensors were distributed in the middle plane downstream of the back-facing step. The unforced flows were studied using a standard two-component TSI particle image velocimetry (PIV) system with two cameras. For every flow configuration, 2000 double-frame images were recorded with a repetition rate of 7Hz. Synchronized pressure and PIV measurements were used to analysed the statistical properties as well as the streamwise time-space characteristics of separated flows.

Main flow characteristics were first investigated for seven Reynolds numbers ranged from 31500 to 182600. A comparison with previous studies was done in order to highlight the expanding ratio influences on the main separation parameters. The emphasis of this work was also placed on the convective motion of the vortical flow structures. Unsteady pressure spectra indicated that in the region close to the separation point, even for high Reynolds number, the low frequency flapping motion is dominant over the high frequency mode of the large-scale vortical structure. The ability to understand the flow field unsteadiness can lead to the development of active and/or passive flow control techniques. Current works are being done searching for active control laws strategies to control these instabilities.