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Title:Very near-nozzle shear-layer turbulence and jet noise
Author(s):Fontaine, Ryan
Director of Research:Freund, Jonathan B.; Austin, Joanna M.
Doctoral Committee Chair(s):Freund, Jonathan B.
Doctoral Committee Member(s):Austin, Joanna M.; Elliott, Gregory S.; Christensen, Kenneth T.
Department / Program:Mechanical Sci & Engineering
Discipline:Theoretical & Applied Mechans
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Jet Noise
near nozzle
boundary layer
shear layer
particle image velocimetry (PIV)
linear stablity
Abstract:Jet noise is sensitive to nozzle configurations and most current aircraft operate close to Federal Aviation Administration (FAA) and other noise restriction levels, with more stringent requirements expected in the future. At the current time, however, prediction capabilities lack the fidelity to accurately model the noise production from a high-Reynolds-number turbulent jet, which impedes progress toward quieter designs. A small-scale anechoic facility has been designed, constructed, and validated which allows for nozzle testing. Both axisymmetric and complex nozzle geometries have been investigated utilizing traditional machining and rapid prototyping capabilities. In addition to noise measurements, a number of flow diagnostic techniques have been implemented allowing the very-near-nozzle region to be studied in detail. The facility construction, validation, and capabilities are described in detail. Flow and noise studies are performed on two separate passive nozzle control methods. The first is a parametric study of how nozzle lip perturbations affect far-field noise through the use of chevron-style nozzles. The second is a family of clamshell style nozzles where the source of an unwanted tone is identified. It is found that designs must consider flow separation from the clamshell surface. The central scientific contribution of this work is a fundamental investigation of the influence of the initial shear layer thickness to answer some long-standing questions regarding the role of turbulence inflow conditions on jet noise. These are important both for predictive simulation and lab-scale experiments. One of the principal challenges in the prediction and design of low-noise nozzles is resolving the near-nozzle mixing layers at the high Reynolds numbers of engineering conditions. Faithfully representing the near-nozzle dynamics in a large-eddy simulation is a challenge because the locally largest scales are so small. Model scale experiments will likewise typically have relatively thick near-nozzle shear layers, which can hamper their applicability to high-Reynolds-number design. To quantify the sensitivity of the far-field sound to nozzle turbulent shear layer conditions, a family of nozzles is constructed and studied in which the exit turbulent boundary layer thickness is varied from momentum thickness 0.0042D up to 0.021D for otherwise identical flow conditions. Measurements include particle image velocimetry (PIV) to within 0.04 nozzle diameters of the exit plane and far-field acoustic spectra. The influence of the initial turbulent shear layer thickness is pronounced, though less significant than the well-known sensitivity of the farfield sound to laminar versus turbulent shear layer exit conditions. For thicker shear layers, the nominally missing region where the corresponding thinner shear layer develops is the principal factor that leads to the noise difference. The nozzle-exit momentum thickness successfully scales the high-frequency component of the spectra for nozzles of different sizes and exhaust conditions. Despite the success of this single parameter on the far-field acoustic spectra, detailed turbulence statistics show distinct signatures of both the nozzle boundary layer, which varies significantly from nozzle to nozzle, and the growing shear layer, which seems to develop surprisingly similarly for all the nozzles. An analysis of the mean exit velocity profiles suggest that their linear stability is indeed insensitive to their thickness by x = 0.04D.
Issue Date:2015-01-21
Rights Information:Copyright 2014 Ryan A Fontaine
Date Available in IDEALS:2015-01-21
Date Deposited:2014-12

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