Direct aeroacoustic simulation of a flow through an expanding pipe with orifice plates

Abstract

Expanding pipes with orifice plates are utilized as silencers for fluid machinery. However, tonal sounds can be generated from flows through the silencers with such a configuration. To clarify the mechanism of tonal sound generation from a flow over a circular expanding pipe with orifice plates, flow and acoustic fields were directly solved on the basis of the compressible Navier-Stokes equations. Prediction results demonstrated that tonal sounds occurred at multiple frequencies. The phase-averaged flow fields for each frequency indicated that vortex rings or spiral vortices were periodically shed in the flow around the orifice plates. Acoustic radiation occurred owing to the collision of these vortices with the orifice plates, which led to the occurrence of acoustic resonance in the expanding pipe. The predicted circumferential phase distributions of pressure fluctuations demonstrated the coexistence of different acoustic modes in the expanding pipe. The relationship between acoustic modes and vortical behaviors was presented. The effects of inflow Mach number on the vortex structures and acoustic modes for the resonance were also discussed.