J. zyxwvutsrqpo Fluid Mech. zyxwvutsr (1982), zyxwvutsr wd. zyxwvutsrqpon 116, pp. 157-186 Printed in Great Britain 157 Self-sustained low-frequency components in an impinging shear layer By C. KNISELYf AND D. ROCKWELL Department of Mechanical Engineering and Mechanics, Lehigh University, Bethlehem, PA 18015 (Received 6 October 1980 and in revised form 8 June 1981) zyx Oscillations of a cavity shear layer, involving a downstream-travelling wave and associated vortex formation, its impingement upon the cavity corner, and upstream influence of this vortex-corner interaction are the subject of this experimental investigation. Spectral analysis of the downstream-travelling wave reveals low-frequency com- ponents having substantial amplitudes relative to that of the fundamental (instability) frequency component; using bicoherence analysis it is shown that the lowest-frequency component can interact with the fundamental either to reinforce itself or to produce an additional (weaker) low-frequency component. In both cases, all frequency com- ponents exhibit an overall phase difference of almost 2kn (k zyxw = 1,2, ...) between separation and impingement. Furthermore, the low-frequency and fundamental components have approximately the same amplitude growth rates and phase speeds; this suggests that the instability wave is amplitude-modulated at the low frequency, as confirmed by the form of instantaneous velocity traces. At the downstream corner of the cavity, successive vortices, arising from the ampli- fied instability wave, undergo organized variations in (transverse) impingement location, producing a low-frequency component(s) of corner pressure. The spectral content and instantaneous trace of this impingement pressure are consistent with those of velocity fluctuations near the (upstream)shear-layer separation edge, giving evidence of the strong upstream influence of the corner region. 1. Introduction In recent years, self-sustained oscillations of impinging shear layers such as the jet-edge, mixing-layer-edge, and cavity-shear-layer-corner configurations have been extensively investigated with the objectives of attenuating noise generation and flow- induced vibrations of structural components. The review of Rockwell & Naudascher (1979), for example, summarizes characteristics of these oscillations. The general features of such oscillations are illustrated in figure 1 for the case of a cavity flow. Studies of this class of flows have focused on the primary (or predominant) frequency of oscillation arising from hydrodynamic instability of the separated shear layer, its variation with flow velocity and impingement length, occurrence of jumps in frequency, and possible hysteresis effects. Relatively little attention has been given t Present address: Stromungslabor 1504, Gebriider Sulzer A.G., CH-8401 Winterthur, Switzerland. 6-2