Analysis and prediction of shock-induced near-field acoustics from an exhaust pipe

Abstract

In this study an acoustic problem of a single shock wave moving through a circular pipe instead of a real pulsating flow with blast waves is considered. Due to the failure of a linear acoustic theory for the problem of interest, the method of computational fluid dynamics is employed to analyze the present problem. For this purpose, the axisymmetric Euler equations are solved by a high-resolution method which consists of a fifth-order weighted essential non-oscillation scheme for spatial discretization and a fourth-order Runge–Kutta method for time integration. In order to reduce the large computational time on a single computer, parallel computation with seven personal computers has been conducted. The detailed flow and sound pressure fields inside and downstream of the pipe are studied. The near-field sound pressure level downstream of the pipe was computed. The sound lobe and its orientation are investigated. In particular, an interesting phenomenon of spatial variation of velocity components associated with the sound lobe is reported. It is found that the spatial variations of the streamwise velocity component and pressure are in phase. However, the spatial variations of the radial velocity component and pressure are out of phase. Moreover, the generation mechanism of acoustics is attributed to the fact of the back-and-forth reflections of upstream-moving expansion waves generated at the pipe wall corner when the shock wave diffracts around the corner. The back-and-forth wave reflections result in the formation of interlacing high- and low-pressure regions that change with time.