Noise and Spreading of Subsonic Coannular Jets—Comparison with Single Equivalent Jet

摘要

The issue of scaling of noise as well as spreading of subsonic coannular jets is revisited. Far-field noise and centerline pitot-static pressure surveys are conducted with concentric, circular nozzles having an outer-to-inner diameter ratio of 1.42. Both the inner nozzle and the outer annular passage are convergent. Outer-to-inner Mach number ratio R is varied over a large range from 0 to about 10. Results are examined on the basis of single equivalent jet parameters calculated by satisfying continuity, momentum, and energy equations. The results confirm that coannular jets with "normal" velocity profiles are noisier than the equivalent jet. Jets with "inverted" velocity profiles are also found to be noisier except in the R range of 1-1.5. There, a few cases exhibit lower intensities (by a fraction of a dB) relative to the intensity at R = 1. However, other run conditions exhibit the lowest intensity at R = 1. Thus, the present data with unheated flow and the given nozzle geometry do not clearly exhibit noise reduction with the inverted velocity profile jets. For the flowfield, the asymptotic Mach number decay rates as well as the potential core lengths are found to be comparable to those of the equivalent jet.