Shear layer oscillation along a perforated surface: A self-excited large-scale instability

Abstract

Self-sustained oscillations of a free-shear flow, i.e., a jet, mixing layer, or wake, can arise from its impingement upon a downstream boundary; these oscillations are well documented. The present experiments demonstrate the existence of analogous oscillations for the case of a shear layer along a perforated plate. The spectrum of the pressure fluctuation at impingement shows sharply defined peaks. Moreover, the predominant frequency of oscillation exhibits a variation with either impingement length or inflow velocity that is remarkably similar to the case of the corresponding free-shear layer in absence of the perforated plate. Quantitative imaging yields patterns of large-scale vorticity concentrations and streamline topology, which have similar features for the shear layer along the perforated plate and the free-shear layer in absence of the plate. All of these observations indicate existence of a feedback mechanism for the shear layer along the perforated plate of finite length.