ASA 128th Meeting - Austin, Texas - 1994 Nov 28 .. Dec 02

2pPAb8. Sound transmission in a pipe developing laminar flow: Upstream/downstream phase speed differences.

Christopher L. Morfey

Inst. of Sound and Vib. Res., Univ. of Southampton, Southampton SO17 1BJ, UK

Malcolm G. Smith

Univ. of Southampton, Southampton SO17 1BJ, UK

In a uniform rigid-walled duct, plane waves propagate with axial phase speed (1(plus or minus)M) times the sound speed, according to the inviscid plug-flow model. The difference between the downstream and upstream phase speeds provides a measure of the flow rate; the same concept can be extended to realistic profiles of M (Mach number) across the duct section, although the lowest-order mode is not a plane wave any more. Calculations were carried out to provide a means of converting phase speed differences to flow rates, assuming that propagation is confined to a single low-order mode (i.e., near-axial propagation). The results were then compared with phase speed measurements made in a cylindrical steel tube, through which air was pumped at a controlled steady flow rate. Close but not perfect agreement was found, which raises the question of whether our neglect of thermoviscous phenomena is justified, particularly near the duct walls. [Work supported by British Gas.]