Crackle Noise in Heated Supersonic Jets

Abstract

Crackle Noise in Heated Supersonic JetsCrackle noise from heated supersonic jets is characterized by the presence of strong positive pressure impulses resulting in a strongly skewed far-field pressure signal. These strong positive pressure impulses are associated with N-shaped waveforms involving a shocklike compression and, thus, is very annoying to observers when it occurs. Unlike broadband shock-associated noise which dominates at upstream angles, crackle reaches a maximum at downstream angles associated with the peak jet noise directivity. Recent experiments (Martens et ai, 2011, "The Effect of Chevrons on Crackle-Engine and Scale Model Results," Proceedings of the ASME Turbo Expo, Paper No. GT2011-46417) have shown that the addition of chevrons to the nozzle lip can significantly reduce crackle, especially in full-scale high-power tests. Because of these ohseiyations, it was conjectured that crackle is associated with coherent large scale fiow structures produced by the baseline nozzle and that the formation of these structures are interrupted by the presence of the chevrons, which leads to noise reduction. In particular, shocklets attached to large eddies are postulated as a possible aerodynamic mechanism for the formation of crackle. In this paper, we test this hypothesis through a high-fidelity large-eddy simulation (LES) of a hot supersonic jet of Mach number 1.56 and a total temperature ratio of 3.65. We use the LES solver CHARLES developed by Cascade Technologies, Inc., to capture the turbulent jet plume on fullyunstructured meshes.

Near-field shock formation in noise propagation from a high-power jet aircraft

“…Currently, the jet aeroacoustics community predominantly views crackle as a source phenomenon. If defined as waveform asymmetry, this is true, as both this study and recent numerical simulations 18 indicate that pressure skewness originates relatively close to the nozzle. However, if perception is linked to the steepness of the acoustic shocks, this study uniquely indicates that the crackle is more pronounced farther from the jet.…”

“…This qualitatively agrees with previous findings of laboratory and numerical studies. [17][18][19] Further, there is greater congruity between the pressure skewness in Fig. 2(a) and the spatial map of the 2000 Hz data in Fig.…”

“…Ffowcs Williams et al 16 indicated that a skewness greater than 0.4 distinctly crackled and subsequent experimental 17 and numerical 18,19 studies have focused on quantifying pressure skewness. The conclusion has been that positive skewness (i.e., an asymmetric waveform with relatively infrequent large positive values, and many smaller negative values) is generated at the source, and therefore crackle is also a source phenomenon.…”

Coalescence in the Sound Field of a Laboratory-Scale Supersonic Jet

“…With the advent of more sophisticated numerical modeling tools, closer inspections of the near-field activity has been realized. For example, large-eddy simulations [14] have shown sawtooth pressure waveforms emerging directly from the jet turbulence, thereby concluding how further steepening, due to cumulative nonlinear distortion, is unnecessary for steep crackle waves to be observed away from the jet shear layer. Similar features were observed by Anderson and Freund [15] by using a direct numerical simulation to illustrate the clumping together of shorter waves in the very near field of supersonic mixing layers.…”

A second golden age of aeroacoustics?

“…Another important trend is the use of simulations to discover important unsteady flow phenomena associated with strong sound emissions. Leakage of shock waves trapped in the supersonic core flow of pressure-mismatched jets [71][72][73][74] enabled by strong vortices and the recent discovery that crackle, which is a particularly annoying noise, originates as a shocklet embedded within energetic eddy motions [75,76] are just two examples.…”