Experimental investigation of the output duct role in the transient bleed valve noise

Abstract

This work presents the second step of an experimental study of the noise radiated by a complete flow discharge/control system allowing to expand or evacuate a flow under pressure by passing though diaphragms or perforated plates. The first step of the study, focused on the study of the noise radiated by the passage of the flow through the perforated plates/diaphragms, allowed to identify the presence of three distinct radiation sources: a broadband noise associated with the mixing of the flow at the exit of the perforations and which is strongly linked to the geometry, a shock noise (screech and broadband shock associated noise) associated with the presence of shock cells in the flow for supersonic regimes and a tonal noise associated with a feedback loop and appearing for low subsonic operating points. By adding a duct downstream to the discharge zone to be closer to real geometries found in industry, the broadband noise is strongly modify by the appearance of strong acoustic resonances in the outlet duct. These resonances are moreover strongly affected by the operating point which drives the flow intensity in the duct. A simple analytical model is proposed in order to quickly predict the different acoustic modifications induced by the outlet duct in case of simple geometries. Finally, the shock noise, as observed without duct, is totally suppressed but is replaced by "base-pressure oscillation" responsible for strong low frequency tones for diaphragms and perforated plates with large cross-sections.