Aeroacoustics of diffusers: An experimental study of typical industrial diffusers at Reynolds numbers of O(10(5))

Diffusers as used in gas transport systems have an optimal pressure recover y but are unstable due to marginal flow separation. Coupling of diffuser fl ow oscillation with acoustic modes in a pipe has been demonstrated in a rec ent work by Kwong and Dowling [J. Fluids Eng. 116, 842 (1994)] to drive flo w unsteadiness. Considered here in addition to the diffuser at a pipe termi nation is the aeroacoustic response of a diffuser in a long pipe. in both c ases reflection coefficient measurements show that at moderate and low ampl itudes of the acoustical particle velocity compared to the main flow veloci ty, diffusers are aeroacoustic sources similar to the whistler nozzle and t he hum. This confirms the observations of Kwong and Dowling. At higher acou stical velocity amplitudes diffusers become strong absorbers, which can be explained in terms of a quasistationary flow model. Finally, an indication is provided for possible remedial measures when a stable how is needed. (C) 2001 Acoustical Society of America. [DOI: 10.1121/1.1329618].