Efficient duct treatments for broadband acoustic noise suppression of environmentally acceptable commercial aircraft. To this end, an accurate knowledge of duct treatment impedance is a critical aspect of duct treatment design. To validate liner impedance prediction models, which must include grazing-flow effects, a flow-duct apparatus is arranged to allow acoustic field measurements to be conducted conveniently under controlled laboratory conditions. From these measurements, the normal incidence impedance in a grazing incidence and grazing flow environment and for locally reacting test materials can, in theory, be educed. A new approach to educing normal incidence acoustic impedance of nonuniform test specimens in grazing incidence and grazing-flow environments is described. An optimization algorithm is shown to provide an efficient means for searching out an impedance distribution to match the measured acoustic field along the upper wall of a duct opposite the test specimen. The flow duct is allowed to transmit multimodal, nonprogressive acoustic waves in a flow environment; however, only a no-flow environment is discussed. A key feature of the method is the expansion of the impedance function as a piecewise continuous polynomial with undetermined coefficients The upper wall acoustic pressure is computed numerically as a function of these coefficients by using a finite element method. The Davidon-Fletcher-Powell optimization algorithm is used to educe the normal incidence impedance by determining the values of the undetermined coefficients that minimize the difference between the measured and the numerically computed upper wall pressure. Results show that this more robust method reduces by a factor of 30 the time required to make impedance determinations, compared with the contour deformation method and is better suited for liners with spatially varying impedance.
Optimization methods for educing variable-impedance liner properties
Optimierung schallschluckender Auskleidungen für Hohlleiter
AIAA Journal ; 36 , 1 ; 18-23
1998
6 Seiten, 10 Bilder, 8 Quellen
Aufsatz (Zeitschrift)
Englisch
Impedanz (akustisch) , Auskleidung , Schubstrahlflugzeug , Lärmminderung , Verkehrsflugzeug , Versuchsmodell , Kanal (Hohlleitung) , Luftströmung , Schallmessung , Einfallswinkel , Laboratoriumsversuch , Polynom , Näherungsverfahren , Rechenzeit , Algorithmus , Schallausbreitung , Schalldämpfung , Schallübertrager , akustische Eigenschaft
Optimization Method for Educing Variable-Impedance Liner Properties
Online Contents | 1998
|FLUID DYNAMICS - Comparison of Two Waveguide Methods for Educing Liner Impedance in Grazing Flow
Online Contents | 2004
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