1,721,047 research outputs found
Modelling of flow effects on propagation and radiation from bypass ducts
No full textA hybrid numerical procedure is presented for predicting the propagation and radiation of fan noise from an aero-engine bypass duct in the presence of mean flow and a bypass shear layer. The objective is to develop a relatively simple scheme which can predict the effects of bypass duct geometry and acoustic treatment on the radiated sound at modest computational cost. To achieve this, a relatively detailed in-duct numerical model, which captures the effects of the mean flow, liners and non-uniform geometry is coupled to an approximate radiation model which includes the effect of refraction through the bypass shear layer. Two different approaches are proposed for the radiation model. Results are presented which compare the predictions of each scheme for a generic, bypass duct with an impedance distribution characteristic of a modern high bypass ratio turbofan engine
Coupling of mapped wave infinite elements and plane wave basis finite elements for the Helmholtz equation in exterior domains
No full textThe theory for coupling of mapped wave infinite elements and special wave finite elements for the solution of the Helmholtz equation in unbounded domains is presented. Mapped wave infinite elements can be applied to boundaries of arbitrary shape for exterior wave problems without truncation of the domain. Special wave finite elements allow an element to contain many wavelengths rather than having many finite elements per wavelength like conventional finite elements. Both types of elements include trigonometric functions to describe wave behaviour in their shape functions. However the wave directions between nodes on the finite element/infinite element interface can be incompatible. This is because the directions are normally globally constant within a special finite element but are usually radial from the pole within a mapped wave infinite element. Therefore forcing the waves associated with nodes on the interface to be strictly radial is necessary to eliminate this internode incompatibility. The coupling of these elements was tested for a Hankel source problem and plane wave scattering by a cylinder and good accuracy was achieved. This paper deals with unconjugated infinite elements and is restricted to two-dimensional problems
A numerical study on the effects of three-dimensional features of a turbofan aero-engine intake on the far field noise
No full text
A numerical integration scheme for special quadrilateral finite elements for the Helmholtz equation
No full textThis paper is an extension to an earlier paper dealing with the general problem of integrating special wave elements and specifically deals with quadrilateral elements, which have their own unique problems. The theory for integrating quadrilateral wave finite elements for the solution of the Helmholtz equation for very short waves is presented. The results are compared with those obtained using large numbers of Gauss-Legendre integration points
The effects of thickness on the impedance of a rectangular aperture in the presence of a grazing flow
No full textThis paper presents an analytical investigation into the effect of a grazing mean flow upon the acoustic impedance of a rectangular aperture in a plate of finite thickness. Previous similar analytical works each made some simplifying approximation about the variation of the acoustic potential throughout the thickness of the aperture, such that they were valid only in either the thin or thick plate limits. In addition to providing a theoretical framework to correctly allow for a plate of any thickness, the results given here indicate the range of validity of the previous approximate theories.Two different forms of matching conditions across the shear layer have been proposed in previous works. Equations and boundary element method (BEM) solutions for both variants are given here. Results from both forms are compared against some existing measured results. As the approximations regarding aperture thickness have now been removed, it is possible to be definitive as to which form of matching conditions gives the most accurate results for a given application. The analysis is shown to predict the reactance of the aperture very well, although the results for resistance show greater discrepancy against experimental values
- …
