## Abstract

Jets with Mach numbers are well known to emit an intense, fricative, so-called crackle sound, having steep compressions interspersed with weaker expansions that together yield a positive pressure skewness 0]]>. Its shock-like features are obvious hallmarks of nonlinearity, although a full explanation of the skewness is lacking, and wave steepening alone is understood to be insufficient to describe its genesis. Direct numerical simulations of high-speed free-shear flows for Mach numbers , , and in the Reynolds number range are used to examine the mechanisms leading to such pressure signals, especially the pressure skewness. For and , the pressure immediately adjacent the turbulence already has the large associated with jet crackle. It also has a surprisingly complex three-dimensional structure, with locally high pressures at compression-wave intersections. This structure is transient, and it simplifies as radiating waves subsequently merge through nonlinear mechanisms to form the relatively distinct and approximately two-dimensional Mach-like waves deduced from laboratory visualizations. A transport equation for is analysed to quantify factors affecting its development. The viscous dissipation that decreases is balanced by a particular nonlinear flux, which is (of course) absent in linear acoustic propagation and confirmed to be independent of the simulated Reynolds numbers. Together these effects maintain an approximately constant in the near acoustic field.

Original language | English (US) |
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Pages (from-to) | 383-408 |

Number of pages | 26 |

Journal | Journal of Fluid Mechanics |

Volume | 832 |

DOIs | |

State | Published - Dec 10 2017 |

## Keywords

- aeroacoustics
- jet noise
- shear layer turbulence

## ASJC Scopus subject areas

- Condensed Matter Physics
- Mechanics of Materials
- Mechanical Engineering
- Applied Mathematics