Passive control of backstep flow

The passive control of turbulent flow downstream of a backward-facing step using a permeable reattachment surface is described. An impermeable reattachment surface leads to the upstream convection of rotational fluid and pressure disturbances which is absent when a permeable surface is used. In the case of one permeable configuration; "flapping" (a low frequency unsteadiness) is completely removed as indicated by wall-pressure spectra and the analysis of velocity data obtained from triple-split film probes. The convective mode of feedback found with an impermeable reattachment surface is removed by a 12.5h length of permeable surface which has the effect of damping the vortical structures that impinge on it and provides steady upstream convection through a sealed plenum beneath the reattachment surface. The question of the stability of such flows is addressed, and it is suggested that, while the How is likely to be convectively unstable over a large region, the appearance of a sufficiently large region of absolute instability is not a necessary condition for the appearance of flapping because the global unsteadiness is driven by the impingement of large eddies at reattachment. An estimate of the ratio of the instability wavelength to a growth-rate length scale is of order 10, which is much larger than is perhaps generally accepted. (C) 1998 by Elsevier Science Inc. All rights reserved.