Multitube turbojet noise-suppression studies using cross-correlation techniques

The noise-generating region of a suppressed turbojet exhaust is studied by cross-correlating static pressure fluctuations within the exhaust with far-field sound for Mach numbers up to 0.99, using a 31-tube nozzle having an area ratio of 3.1. Measurements made with an unsuppressed turbojet exhaust having an equivalent area ratio and operating under effectively equal thrust loads serve as the experimental control. Static pressure-level measurements, made with a calibrated high-temperature acoustically damped probe tube, show that noise suppression by multitube nozzles results from reduced turbulence levels. The maximum fluctuating static-pressure level in the unsuppressed turbojet exhaust is typically 5-6 dB higher than static-pressure levels in the suppressed exhaust under conditions of effectively equal static thrust. This suggests that the turbulence intensity in the multitube suppressor flow is reduced in excess of 20% compared with the unsuppressed jet exhaust.