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Published Articles

The Volume 9, No 3, September 2004

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Dynamic Behaviour Analysis of a Dual-Rotor System Using the Transfer Matrix Method

B.B. Maharathi, P.R. Dash, A.K. Behera


This paper presents a general formulation for the problem of the steady-state unbalance response of a dual rotor system with a flexible intershaft bearing using an ?extended? transfer matrix method, where the transfer matrix assumes a dimension of (33%33). The validity of the formulation is established by comparing the results obtained through a computer program with closed form solutions available for some simple cases. Some interesting phenomena of steady-state whirl orbits of the dual rotor system are described.

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A New Hybrid Approach for the Thermo-acoustic Modelling of Engine Exhaust Systems

R. N. Hota and M. L. Munjal


The time-domain method of characteristics and the frequency-domain linear acoustics method are the two different methods of thermo-acoustic modelling of engine exhaust systems in order to predict exhaust noise. To overcome the disadvantages of both the time-domain and the frequency-domain approaches, a hybrid approach has been developed which couples the acoustical description of the muffler piping system to the acoustic source more realistically than the usual time invariant linear model. A time domain model and the corresponding frequency domain model of a linear dynamical system are interrelated by the Fourier transform pair. In the present work, the cylinder/cavity is analysed in the time domain to calculate exhaust mass flux history at the exhaust valve by means of the method of characteristics, solving a number of equations simultaneously at the valve junction. This analysis has been done by making use of an interrelationship between progressive wave variables of linear acoustic theory and those of the method of characteristics. In this approach, nonlinear propagation in the exhaust pipe is neglected and radiation impedance at the end of the exhaust tail pipe is duly taken into account. Apart from this, actual reflection of the forward wave due to the presence of muffler is incorporated to make the analysis more realistic. Damping effects present in the fluid have also been taken into account. Computational results have been corroborated by experimental data for a single-cylinder, four-stroke cycle diesel engine.

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Order and Realisability of Impulse Response Filters for Accurate Identification of Acoustical Multi-ports from Transient CFD

Wolfgang Polifke, Alexander Gentemann


So-called network models are popular tools for the analysis of acoustic phenomena, e.g. in mufflers, in ventilation or pipeline systems, and in combustors (thermo-acoustic instabilities). The building blocks of such models are multi-ports, represented mathematically by their respective transfer matrices. Within the limitations of linear acoustics, transfer matrices provide a complete description of the dynamic characteristics of the individual multiports. They may be determined experimentally or in an approximate manner by analytical means. Alternatively, transfer matrices may be reconstructed from transient CFD simulation data with the help of system identification tools. Specifically, it is possible to determine the unit impulse responses of a multi-port with correlation analysis and then obtain transfer matrix coefficients via the z-transform. The present study is concerned with the optimal choice of parameters for accurate transfer matrix identification. Recommendations for the optimal choice of acoustic variables, sample increment, and sample length, as well as filter order, are formulated. Remarkably, it is found that in many cases the use of formally non-causal filters is advantageous. It is argued that this is a consequence of the fact that causal interrelationships imposed by the underlying laws of fluid mechanics are not always represented properly with the standard acoustic variables.

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Original Derivation of the Modal Decomposition Analysis for Solving Mixed Boundary-Initial Value Problems

A. Moura


A semi-analytical formulation is proposed to take into account the transient aspect before a stationary regime is established in a bounded linear inhomogeneous elastic structure. This paper using an original approach introduces the well-known modal decomposition analysis for solving mixed boundary-initial value problems.

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