A study on the improvement of noise suppression in a low-noise valve of the multihole type. 2nd report, Noise reduction by modifying the exit shape of each hole.

Abstract

The noise characteristics and flow behavior of supersonic jet flow discharged through a hole with a modified-shape exit have been investigated experimentally over a range of ratios up to 7 in order to reduce noise from each jet of multiple supersonic jets. Two types of holes are used: one is a step hole with an abrupt enlarged exit, the other is a tapered hole with a conical diffuser. Substantial reductions in both peak and broadband sound pressure are obtained over the range of operating pressure ratios in both types of holes. Schlieren photographs recorded at these operating conditions reveal the appearance of a characteristic pattern of shock structure which effectively eliminates the usual repetitive shock cells associated with supersonic jet flows. The dependence of the noise reduction on the observed flow changes is discussed form the point of view that the flow velocity at the exit is reduced through the so-called λ-type strong shock wave heading into the jet flow. For multiple supersonic jets discharged through the multiple-step holes, the same noise reductions are also achieved.