Multitube turbojet noise‐suppression studies using cross‐correlation techniques

The noise‐producing region of a suppressed turbojet exhaust is studied by cross‐correlating static pressure fluctuations within the exhaust with farfield sound for Mach numbers up to M_j=0.99, using a 31‐tube nozzle having an area ratio A_r=3.1. Measurements made with an unsuppressed turbojet exhaust having an equivalent exit area and operating under effectively equal thrust levels serve as the experimental control for this work. 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. Static pressure fluctuation spectrums in the suppressor flow are peaked at the eddy passage frequency and a double spectral peak is found in the region where the suppressor jets coalesce. Estimates of the eddy‐scale‐length to jet‐diameter ratio made from measurements of local jet velocity and eddy passage frequency indicate that a reduction in eddy size occurs when neighboring suppressor tube flows are brought into close proximity to one another. Maximum normalized cross‐correlation coefficients in the unsuppressed turbojet exhaust of approximately c_PF?0.2 are found for M_j=0.99 at an angle of 30° from the jet axis. Normalized cross‐correlation coefficients are significantly lower in the interior of the multitube suppressor flow. Measurements of the partially normalized cross‐correlation function show that sound radiated from within the hot, high‐speed suppressor flow is refracted by the flow from neighboring jets. This work demonstrates that the high‐frequency sound radiated directly to the farfield by a multitube suppressor nozzle is primarily from the exterior tubes and that the overall suppression of noise can be attributed to a reduction in turbulence intensity.