Noise source distribution and mean-flow structure of coaxial jets

This study examines the noise and flow field characteristics of convergent coaxial jets with different diameter and velocity ratios. Experimental investigation of coaxial jet using two separate testing facilities enables us to seek connections among the various noise metrics, including sound pressure level (SPL), overall sound pressure level (OASPL), and noise source maps, and the flow field properties of the jet, including mean velocity, inflectional layers, centerline velocity, and primary and secondary core lengths. An anechoic acoustic testing facility is used to perform noise measurement tests and a Pitot rake velocity testing facility is used to obtain the mean velocity flow field of the jet plume.;The results indicate that for zero velocity ratio (single-stream jet), the region near the nozzle emits strong high-frequency noise. Addition of a secondary flow reduces the convective velocity of the eddies in the primary shear layer. This is consistent with elimination of Mach waves in the region near the nozzle. The secondary flow also reduces the growth rate of the jet and elongates the primary potential core. Increasing the velocity ratio suppresses the near-nozzle noise and extends downstream the location of the peak noise. The location of peak noise tracks well the end of the primary core. The suppression of high-frequency noise is explained by the creation and elongation of the secondary core as the velocity ratio increases. The acoustic trends with velocity ratio are similar for polar angles ranging from 20+/- to 120+/-. This range of polar angles includes both large-scale and fine-scale turbulence noise which have been shown to radiate in downstream (small polar angles) and upstream (large polar angles) directions, respectively. The results indicate that the coaxial jet has a noise source distribution fundamentally different from that of a single-stream jet.