Flow noise and self-excitation of pipes

Abstract

Measurement of flow noise in smooth pipes indicates that it behaves as a monopole at very low flow rates. As the flow velocity is increased, the sound pressure level varies as the sixth power of velocity and then as the eighth power at higher velocities. Eventually, choking of acoustic energy occurs in the upstream direction and saturation due to nonlinearity occurs in the downstream direction. The excitation of acoustic modes in the pipe in the presence of obstacles that shed vorticity leads to intense sound radiation. Some experiments conducted at high subsonic flow in circular pipes with crosswise cylindrical obstructions are described. The coupling of acoustic modes of a side branch resonator with the axial modes of the pipe is explored. Observations on how to make a nonresonating side branch cavity are presented. [This work was supported by the U. S. Navy (Office of Naval Research) under Contract N00014-67-A-0204-0019.]