ACOUSTIC STUDY OF B-HELICAL MODE FOR CHOKED AXISYMMETRICAL NOZZLE

An acoustic near-field study was performed for an axisymmetric conical convergent nozzle operating at a pressure ratio corresponding to a fu lly expanded Mach number of 1.3. The acoustic measurements were perfor med in the nozzle exit plane using multiple sensors arranged around th e periphery of the nozzle. The acquired data were simultaneously digit ized. The acoustic spatial characteristics, describing the large-stale structure associated with the preferred shear layer instability mode, were determined for the dominant B screech mode. The nozzle was fitte d with a lip thickening device to determine the effect of this geometr ic variable on the spatial structure of the jet. For the thin-lipped c onfiguration, the flapping structure of the B screech mode was found t o precess in a time-dependent manner about the jet axis. Increasing of the nozzle exit lip thickness altered the spatial characteristics of this mode from a napping to a time-dependent napping or spinning. Usin g the autobicoherence spectrum, frequency dependencies were found to e xist in the acoustic data of the thick-lipped configuration. These dep endencies may be related to the nonlinearity of the jet and/or the tim e-dependent nature of the B mode spatial structure.