An in situ method of wall acoustic impedance measurement in flow ducts

Effects of wall admittance changes on duct transmission and radiation of sound

The development of computational methods in acoustics has led to the introduction of analysis and design procedures which model the turbofan inlet as a coupled system, simultaneously modeling propagation and radiation in the presence of realistic internal and external flows. Such models are generally large, require substantial computer speed and capacity, and can be expected to be used in the final design stages, with the simpler models being used in the early design iterations. Emphasis is given to practical modeling methods that have been applied to the acoustical design problem in turbofan engines. The mathematical model is established and the simplest case of propagation in a duct with hard walls is solved to introduce concepts and terminologies. An extensive overview is given of methods for the calculation of attenuation in uniform ducts with uniform flow and with shear flow. Subsequent sections deal with numerical techniques which provide an integrated representation of duct propagation and near- and far-field radiation for realistic geometries and flight conditions.

/pdf/theoretical-models-for-duct-acoustic-propagation-and-1d2jsi8cr3.pdf

Theoretical models for duct acoustic propagation and radiation

Analytical and experimental studies for predicting noise attenuation in acoustically treated ducts for turbofan engines

https://ntrs.nasa.gov/api/citations/19690027596/downloads/19690027596.pdf

This paper describes the application of the finite-element method in combination with Galerkin's method in the determination of the acoustic properties of turbofan inlets containing high subsonic Mach number steady flows. An approximate solution for the steady inviscid f Ion field is obtained using an integral method for calculating the potential flowfield in the inlet with a correction to account for compressibility effects. The accuracy of the finite element technique in predicting the acoustic properties of annular ducts has been checked by comparison with available analytical solutions for the problems of plane and spinning wave propagation through a hard-walled annular duct with a constant mean flow. Results are presented comparing low-frequency plane wave propagation through a hard-walled turbofan inlet containing a one-dimensional steady flow with the same inlet containing a fully two-dimensional axisymmetric steady flow. It is shown that when one-dimensional steady flow is assumed to exist in the duct, the plane wave propagates with relatively little distortion. However, propagation of a plane wave through the fully two-dimensional flowfield in the inlet produces severe distortion due to the excitation of higher order modes.

Determination of Turbofan Inlet Acoustics Using Finite Elements

Linearized wave equation with associated boundary conditions for predicting sound attenuation in soft walled circular ducts

Attenuation of sound in soft-walled circular ducts

Broadband sound attenuation in soft walled /low acoustic impedance/ circular ducts found dependent on frequency

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Attenuation of sound in soft-walled circular ducts

Attenuation of sound in soft-walled circular ducts