Does a Model Scale Nozzle Emit the Same Jet Noise as a Jet Engine?

Abstract

A careful experimental study has been carried out to verify whether a model scale nozzle produces the same jet noise as a jet engine, with the twin goals of (a) validating the practice of carrying out jet noise research at model scale for full-scale applications, and (b) aiding the identification of jet noise in measured total spectra from jet engines, which have contributions from several noise sources. A special test was carried out to measure pure jet noise from a low bypass ratio jet engine at all the angles. Aeroacoustic measurements were made from model scale nozzles at the Boeing Low Speed Aeroacoustic Facility at the same operating conditions as the jet engine. Many issues that are pertinent for the comparison of scaled model data with engine data have been investigated thoroughly. These include the requirements of the instrumentation system for model and engine tests, suitable methodology for the calculation of the atmospheric attenuation coefficients, propagation effects, data repeatability, scaling of jet noise spectra, effects of the disparate Reynolds numbers and mixing perimeters between model nozzles and engines, etc. It is established that the weather corrections are accurate even at very high frequencies of interest in model tests. A new phenomenon of nonlinear propagation effects is identified for subsonic heated jets when the convective Mach number exceeds unity. It is proposed that the convective Mach number is a better indicator of nonlinear processes than high sound amplitudes, as has been believed in the past. It is demonstrated that with the proper scaling factors, both narrowband and one-third octave jet spectra can be collapsed. The concept of increasing the shear perimeter for achieving better mixing and hence reducing noise has no merit, even at super-critical pressure ratios. Excellent agreement between scaled model and engine spectra is demonstrated at all angles and frequencies for a variety of power levels. It is shown that it is possible to acquire high-quality spectra at high frequencies, provided proper care is taken. Finally, it is concluded from the above results that jet noise research carried out at model scale is applicable to jet engines.