Low-Frequency Dynamics of Flow over a Backward-Facing Step

The low-frequency (Sr<0.016) dynamics of turbulent flow (Re-h=5100) downstream of a backward-facing step are examined by low-pass filtering the velocity fields from a long run-time large-eddy simulation. The low-frequency behaviors were found to set the location of streamwise reattachment. Examination of the instantaneous three-component velocity fields showed regions of flow where the streamwise velocity was momentarily driven all the way back to the step face. After the flow reaches the step, it is then directed along the step face in the spanwise direction, resulting in large regions of wall-normal vorticity omega(y). These large regions of vorticity are opposed by smaller regions of counter-rotating flow offset in the spanwise direction and were responsible for the spanwise variation of the reattattachment. The low-pass filtered streamwise velocity fields just above the lower wall showed significant spanwise oscillations with a mean wavelength of the streamwise low-pass filtered reattachment position of lambda(z)approximate to 2h, consistent with previous studies at different Reynolds and Mach numbers. As the step geometry is homogeneous in the spanwise direction, this shows that this behavior is not the result of wall or edge effects, which is a conclusion that cannot easily be drawn from experimental studies with limited step-height-to-width ratios.