Noise and Spreading of a Subsonic Coannular Jet - Comparison with Single Equivalent Jet

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 approximately 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 single equivalent jet. Jets with "inverted" velocity profiles are also found to be noisier except in a narrow R-range of 1-1.5. In the latter range, contrasting the inference in previous studies of IVP jets, the present data do not exhibit a clear noise reduction. When normalized with equivalent jet parameters the asymptotic Mach number decay rate, as well as potential core length, are found to be comparable to those of a single jet. However, an abrupt shift in the virtual origin is noted across R=1.