On the Nonlinearities in the Noise Radiated from High-Speed Model Jets

Open Access
Petitjean, Benoit Philippe
Graduate Program:
Aerospace Engineering
Doctor of Philosophy
Document Type:
Date of Defense:
October 11, 2006
Committee Members:
  • Dennis K Mc Laughlin, Committee Chair
  • Philip John Morris, Committee Member
  • Kenneth Steven Brentner, Committee Member
  • Victor Ward Sparrow, Committee Member
  • Anthony A Atchley, Committee Member
  • George A Lesieutre, Committee Member
  • aeroacoustics
  • jet noise
  • nonlinear acoustics
The role of nonlinearity in the generation and propagation of the noise radiated from high-performance jet aircraft is not a well understood phenomenon. To address this problem, an extensive experimental program has been carried out on high-speed model jets, including both flow-field and acoustic measurements from pure air and helium/air mixture jets over a large range of operating conditions. Of particular interest is the study of the relationships between such complex mechanisms as nonlinear propagation, Mach wave emission and jet crackling. The latter refers to an annoying component of supersonic turbulent jet mixing noise that appears to be due to random sets of sharp compressions followed by weak expansions. Frequency-domain analyses indicate that there is agglomeration of energy at the higher frequencies as the propagation distance increases for both subsonic (highly-heated) and supersonic jets. The computation of an indicator of nonlinearity confirms that energy is transferred from mid-frequency range to the higher-end of the spectra, as a consequence of long-range propagation. Evidence of a nonlinear energy transfer towards lower frequencies is also found at supersonic jet exhaust velocities. Further comparison of the current results with measurements performed in a different facility (Boeing Low Speed Aeroacoustic Facility) shows good overall agreement. A very important result is the identification of the jet convective Mach number as a critical parameter that could possibly be used to predict the onset of nonlinear effects (more so than OASPL values, for example). The need for accurate values of convective Mach number constitutes the main motivation for the implementation of nonintrusive optical instruments in this work. An optical deflectometry system is used to provide both qualitative and quantitative analyses in the jet shear layers exhausted from circular nozzles. Jet convection velocities and convective Mach numbers are determined from two-point, space-time, cross-correlations for pure air jets and show excellent agreement with data from the literature at lower speeds. However, helium/air mixture jets display considerably lower levels of correlation and notably reduced convection velocity values, in contrast to what schlieren photographs would indicate. It is very likely that visualization of the large-scale turbulent patterns in helium/air mixture jets is inhibited by the thick visual shear layer dominated by smaller scale structures. Finally, the time-domain data collected from both acoustic and flow-field measurements are carefully examined with the aim of clarifying a number of features associated with the nonlinear distortion of acoustic waves. More specifically, it is clearly established that the jet convective Mach number is a critical parameter that may be used to predict the onset of crackle in the pressure signals (and not directly nonlinear propagation effects). Overall, nonlinear propagation effects as such are estimated to be too weak to generate the typical shock-like patterns and waveform asymmetry associated with crackling signals. Considerable experimental evidence is gathered to suggest that the phenomenon known as ‘crackle’ may in fact represent the acoustic signature of the strong Mach waves radiated from the jet shear layer. Simple mechanisms are proposed to qualitatively explain how jet crackling can in turn induce significant nonlinear effects in the course of propagation.