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Published Articles

The Volume 7, No 3, September 2002

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Measures of Location Effectiveness of Vibration Absorbers

Jerome K. Vethecan, Aleksandar Subic


The use of dynamic vibration absorbers is common in engineering applications where there is a need to attenuate high vibration levels due to resonant response. Criteria for the optimal specification of absorber properties are well known, being specified in terms of natural frequency, mass ratio and damping. Apart from this, the selec- tion of the most effective attachment location is an important function of the design, and will be subject to the structural dynamics of the system to be treated. While it is commonly held that anti-nodal locations are best for attachment, the mode shape does not provide a true measure of the effectiveness when the device is attached elsewhere. In this paper, the issue of determining the location-effectiveness of dynamic vibration absorbers is examined using structural dynamic coupling principles. Specifically, the modal-coupling method which utilises modal models of both primary and auxiliary systems is considered. A measure of effectiveness is developed us- ing synthesised frequency response functions of the coupled system to indicate the effectiveness of the device over a subset of response coordinates and candidate attachment locations. This information supplements the mode shape and in addition provides a tool for the analyst to make a quantitative assessment of all available op- tions, which is especially useful when the attachment options are severely constrained.

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Computation of Broadband Noise Radiated by a Ducted Fan

Serge Lewy


Broadband fan noise has become the major component of sound radiated by high-bypass-ratio turbofans, be- cause of the past progress in reducing the tones. The main difficulties in predicting it arise from the facts that there are several competing noise generation mechanisms, and that the input data are generally poorly known. The method proposed here is aimed at avoiding these drawbacks, and at being easy to implement. It is based on dipole radiation from a ducted rotor. It assumes that the blade loading spectrum is flat, according to previous tests in a SNECMA facility. The sound pressure is computed using the Ffowcs Williams and Hawkings model in which the Green?s function in free space is replaced by the Green?s function in a cylindrical hard-walled duct. Free-field radiation is derived using the Tyler and Sofrin model. It has been checked that the acoustic power bal- ance between the induct and free-field sound fields is excellent if the mean flow is neglected inside the duct. Di- rectivity patterns, sound power spectra, and the variation of the overall sound power level versus the rotation speed are in good agreement with FANPAC upstream acoustic measurements which were analysed in a previ- ous IJAV article. The effect of the radial turbulence length scale along the blade span is also predicted.

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The Inverse Impact-Induced Vibration Problem in Constrained Rotating Systems

Wei-Hsin Gau, Ching-Yu Yang


The analysis presented in this investigation is aimed at the development of a direct scheme to deal with the in- verse impact-induced longitudinal vibration problem in constrained rotating systems. An inverse method is de- veloped to estimate the impact conditions, such as the coefficient of restitution, the velocity of the impact mass, and the mass ratio between two bodies. The angular velocity of the rotating body and the properties of the body can also be estimated. The modal expansion method is employed to analyse the problem domain, and then a lin- ear inverse model is constructed to identify the unknowns. The linear least-squares error method is adopted for the linear model and thus the number of iterations can be limited to one and the uniqueness of the solutions can be determined. The use of the procedure described in this paper is demonstrated using a radially-rotating rod subjected to an axial impact. The results presented in this paper show that the scheme is stable and the solution estimated converges to the exact result without the inclusion of the measurement errors.

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Comparison of Sound Fields in Regularly-shaped, Long and Flat Enclosures with Diffusely Reflecting Boundaries

Jian Kang


A radiosity-based simulation model has been developed to study the sound fields in rooms having diffusely reflecting boundaries. Using the model, sound fields in four hypothetical spaces, a cube, a long room and two flat rooms, have been compared. In the cube the spatial distribution of sound pressure level and reverberation time is rather even. In the long rooms and flat rooms, the sound pressure level decreases continuously with increasing source-receiver distance, whilst the reverberation time increases steadily until it reaches a maximum and then decreases slightly. This means that in long and flat rooms the sound fields are fundamentally different from the diffuse field even when the boundaries are diffusely reflective ? the reverberation time in these rooms is dependent on the source and receiver location.

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A Theoretical Study of Mufflers with Open-ended Intruding Perforated Tubes

Chao-Nan Wang, Malcolm J. Crocker


This paper presents an analysis of the performance of an open-ended intruding perforated-tube muffler. A theo- retical formulation of the transfer matrix for such a muffler is given. The derivation is based on the assumption of plane wave propagation. By using continuity and momentum equations and the equation of isentropicity at the open end, the transfer matrix of the inlet open-end intruding tube can be derived. Then the transfer matrix and also the transmission loss of the whole muffler can be evaluated. The predicted results are shown to agree well with the experimental measurements presented in the literature. Finally, the effects of the mean flow veloc- ity, porosity, and length of the intruding perforated-tube are investigated analytically.

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An Experimental Study of Acoustical Parameters in Churches

A. Magrini, P. Ricciardi


This paper reports the acoustical parameters measured in ten churches in the city of Genoa, Italy. Churches are often selected for use as auditorium, theatres, concert halls, and other non-liturgical uses without any technical knowledge about their acoustical properties. Wiser selections could be made if the decisions were informed by knowledge of their acoustical parameters. This paper presents the results of a wide investigation on the various acoustical parameters such as clarity index, definition index, centre time and various reverberation times (e.g. EDT, RT20, RT30), which can give more reliable information on the acoustical response of an enclosed envi- ronment. This problem has already been studied by some authors, but mostly for the case of church restoration, connected with the changes in uses for auditorium functions. The selected churches (XI-XVI sec.), are mainly located in the historic centre of Genoa, and are all characterised by a longitudinal plans having two aisles and by cubic volumes ranging from 1500 to 20.000 m3. A wide experimental study has been carried out with the im- pulse response analysis measurement system in each church. A comparison with data available in the literature (reverberation time, RT, and clarity index, C80) regarding churches and also theatres is presented; measure- ments of RT and C80 show a dependency on the ratio of the volume to the floor seating area (V/S). The variation field of RT, C80 and D50 related to frequency are reported. Further on, a mathematical correlation between C80 and RT is proposed.

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Cooling System Rotor Dynamics in Military Combat Vehicle

Samo Zupan, Ivan Prebil, Brane Sirok and Marko Hocevar


To increase the power of the T 72 tank engine, it has been necessary to redesign the ventilator rotor of the cool- ing system. The basic mechanical loads have been analysed and the newly designed rotor has been tested with these loads. Dynamical properties have been determined at the different rotation speeds and with different bear- ing types and arrangements. Characteristic frequencies and transitional phenomena have been determined from the experimental data and the numerical analytical results. Boundary and initial conditions have been taken from the available experimental data and from relevant military standards. Numerical analysis has been carried out us- ing finite element analysis software. In this paper the emphasis is on the analysis of the rotor-bearing system in the movable ventilator test stand housing. The results are presented in the form of transfer functions showing the dynamical properties of the rotor as a function of the excitation of the ventilator housing for different types of bearings and rotational speeds.

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