Etudes d'interactions acoustiques non-linéaires dans les jets confinés, par la fonction de bicohérence

Abstract
Self-sustained tones are encountered when a flow passes two orifice plates, sufficiently close together, inside a duct. The upstream orifice plate creates a confined jet which impinges on the edge of the downstream orifice plate. The shear layer oscillations (coherent structures) of the confined jet in the presence of a resonant acoustic field (pipe mode), constitute the main features of cavity tones. The tones are synchronized by the pipe modes. When the flow velocity is varied for a fixed geometry, frequency jumps from one mode to another are observed. Pressure spectra showing intense acoustic energy localised in narrow bands near pipe mode frequencies are measured. We emphasize the existence of transition flow regimes during which quadratic non-linear coupling between spontaneously excited modes can take place. A digital bispectral analysis (bicoherence function) is used to quantify the degree of non-linear interactions of acoustic waves of finite amplitude in the so-called transition region.