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Title:Crackle noise from high-speed free-shear-flow turbulence
Author(s):Buchta, David A
Director of Research:Freund, Jonathan B.
Doctoral Committee Chair(s):Freund, Jonathan B.
Doctoral Committee Member(s):Bodony, Daniel J.; Pantano, Carlos A; Hilgenfeldt, Sascha
Department / Program:Mechanical Sci & Engineering
Discipline:Theoretical & Applied Mechans
Degree Granting Institution:University of Illinois at Urbana-Champaign
turbulence simulation
Abstract:The sound from high-speed jets, such as on military aircraft, is distinctly different than that from lower-speed jets, such as on commercial airliners. Atop the already loud noise, a higher speed adds an intense, fricative, and intermittent character. The corresponding pressure fluctuations also have a peculiar shape with strong, steep compressions and weaker, rounded expansions, which is thought to be responsible, at least in some part, for their distinct perception. A complete explanation of this distinct and important aspect of jet noise is lacking, and in particular, the root turbulence mechanisms that give rise to the skewed acoustic signal are unknown. Direct numerical simulations (DNS) of high-speed free-shear-flow turbulence are used to assess the underlying mechanisms and quantify near-field nonlinearity as potential sources of the radiated wave field, especially the pressure skewness. Though the DNS is restricted to a modest Reynolds-number range, the simulated turbulence is shown to have a broad range of scales and reproduce the energy spectra of realistic turbulence. This configuration is presented as a near-nozzle `piece' of a relatively high-Reynolds-number jet. At high speeds, with Mach number $M\gtrsim2.5$, the simulated near-field pressure signals reproduce the distinct crackle-like features observed with relatively flat shock-like waves, with sharp, steep compressions followed by weaker, rounded expansions. Their corresponding pressure skewness for $M\gtrsim2.5$ exceeds the level $S_k(p')\gtrsim 0.4$ associated with perception of jet crackle. Detailed assessments of the factors leading to $S_k(p')$ show that the skewed pressure waves occur immediately adjacent to the turbulence source, at the edge of the rotational region. Also, direct observation of the near-acoustic field indicates that the pressure waves have complex three-dimensional structures and nonlinearly merge as they propagate. Where these waves intersect above the mixing layers, the pressure compressions are stronger than their corresponding expansions. We investigate the near-field wave development and provide a complete statistical assessment of the factors transporting the pressure skewness. For $M\gtrsim2.5$, nonlinear interactions above the mixing layers, which add to $S_k$, are balanced by damping molecular effects, which subtract from it. Thus, the `footprint' of crackle with $S_k\gtrsim0.4$ is generated essentially at the turbulence source. Invoking the stability characteristics of high-speed free-shear flows, which are known to change character at high speed, we assess the sensitivity of skewness to changes in the turbulent structure, which we adjust using a novel forcing approach. Pressure skewness is shown insensitive to the three-dimensional structure and to the strength of the structures in its source. Using a larger, higher-Reynolds-number simulation, we likewise show that $S_k$ is Reynolds number insensitive. Finally, we develop a reduced gas dynamics description that neglects the turbulence dynamics \textit{per se} and present a description of a mechanism that leads to $S_k(p') > 0.4$. The model flows reproduce the essential skewness characteristics observed in the DNS. At its core, this mechanism shows simply that nonlinear compressive effects lead directly to stronger compressions than expansions and thus $S_k(p')>0$.
Issue Date:2016-09-12
Rights Information:Copyright 2016 David A. Buchta
Date Available in IDEALS:2017-03-01
Date Deposited:2016-12

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