(A) study on the acoustic characterization of perforated elements in the presence of flow

Abstract

Perforated elements are widely used in the silencers of many fluid machines to suppress noise and flow instability. Because the duct system in fluid machine, for example, automotive mufflers and combustion system flow ducts, always conveys the flow of medium, the flow can be forced through the perforation or be tangential to the perforation plane, or a mixture of both flows exists. It is well known that the mean flow affects the acoustic impedance of an aperture, but the detailed nature and understanding of the physics are not perfectly understood yet. In this research, the acoustic impedance of perforated plates with mean flow was investigated both theoretically and experimentally. The theoretical impedance model under the grazing flow condition was compared with the experimental results. Two theoretical models were investigated: particle displacement match (PDM) and particle velocity match (PVM). By validating the theory using experimental results, it was found that the theory predicts the resistance well, but not the reactance. The theory can predict the negative resistance, which was found in the measured resistance within a specific Strouhal number range. The theoretical acoustic impedance model under the bias flow condition was derived considering the interaction effect between orifices. To consider the interaction effect between orifices, the acoustic impedance of an orifice in a partition across a tube was analyzed by solving the incompressible Euler equation. The measured acoustic impedance under bias flow condition showed nonlinear trends at high Mach number conditions. Predicted and measured acoustic impedance are nondimensionalized using the following parameters: the acoustic impedance of an orifice divided by the Mach number, the thickness-to-radius ratio, and the Strouhal number. The simplified impedance model which can consider a combined flow condition was also suggested. Although the strength and position of the shear layer cannot be obtained b...