A Study on the Improvement of Noise Suppression in a Low-noise Valve of the Multihole Type: 3rd Report, Noise Reduction Mechanism and Induced Supersonic Jet Flow in the Step Hole

Supersonic jet flows near the abrupt enlarged exit of a step hole on the occurrence of noise reduction have been studied experimentally and theoretically to clarify the principle of the noise reduction and to estimate the operating condition. The so-called λ-type or X-type shock wave progression is observed in the supersonic jet flow by schliren photography. A theoretical model of the shock wave patterns is developed and analyses on the flow field around the shock wave are performed. From the fact that the theoretical flow velocity just downstream of the shock wave shows strong correlation to the measured noise reduction, it is clearly concluded that the noise reductions are due to the reduction of jet exhaust velocity through the strong shock wave. Also, the theoretical model can estimate the operating condition where the noise reduction occurs, and facilitate the design of an acoustically improved step hole.