Abstract
Expanding pipes with orifice plates are often utilized as silencers to reduce the noise in fluid machineries. However, intense aerodynamic tonal sound can be generated from flows through such expanding pipes. To clarify the generation mechanism and conditions for intense tonal sound, sound measurements and flow visualization were performed for a flow through an expanding pipe with two orifice plates, where three circular cavities were formed in the expanding pipe. The effects of the freestream Mach number and orifice radius on the flow and sound were analyzed. The variation in acoustic radiation with freestream Mach number demonstrated that the Strouhal number of the most intense tonal sound based on the cavity length changes discretely at particular freestream Mach numbers, where a shear layer mode number corresponding to the number of vortices in the cavity is varied. The tonal sound becomes intense owing to the coupling of the shear layer and circumferential acoustic modes. The rotational and stational acoustic modes were identified based on the circumferential phase distributions of pressure fluctuations in the expanding pipe, whereas in-phase pressure fluctuations occurred at a relative higher Mach number. The visualized vortical structures corresponded to these phase distributions of the pressure fluctuations. The sound measurements with different orifice radii showed that the tonal sound occurred at a lower Strouhal number with a larger orifice radius, which indicates weaker effects of the orifice plate on the vortex shedding in the expanding pipe. At a smaller orifice radius, acoustic radiation occurred at a higher Strouhal number owing to the flow acceleration through the orifices, which resulted in intensified sound propagation into the outside of the expanding pipe along the effects of the acoustic mode particularly at a high Mach number.
