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Published Articles

The Volume 7, No 2, June 2002

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Resonance with Respect to Angular Positions of an Unbalance of a Cracked Rotor in a Nonlinear Rotor System

Feng Lu and Nong Zhang


Cracked rotors have nonlinear spring characteristics of a piecewise linear type due to an open-closed mechanism of cracks. There have been many studies on the dynamic behaviour of cracked rotors in order to develop fault diagnosis systems for detecting cracks. However, most of these studies concern the change in resonance phenomena mainly due to cracks in rotor systems with linear supports. In many practical rotor systems, various kinds of nonlinear spring characteristics may exist due to mechanical elements, such as nonlinear bearing supports, and the rotor systems become nonlinear. Therefore, existing fault diagnosis systems are unable to detect cracks in such nonlinear rotor systems which are supported by nonlinear bearings. In this paper, we study the vibrational behaviour of a cracked rotor with nonlinear bearing supports and focus on the effect of combined resonances caused by a crack and nonlinear bearing supports. In particular, we investigate in detail the resonance phenomena of a harmonic resonance and a 1/2-order subharmonic resonance by numerical simulations using a PWL model and theoretical solutions using a PS model. The results show that the dynamic behaviours of a cracked rotor in a nonlinear rotor system are obviously different from those in a rotor system with linear supports, and the change in vibrational behaviour of a harmonic resonance and a 1/2-order subharmonic resonance are significant due to changes of angular positions of an unbalance.

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A State Estimation Method for Sound Environment Based on Fuzzy Observation

Akira Ikuta, Mitsuo Ohta, M.O. Tokhi


In the measurement of actual sound environments, the observed data exhibit various probability distribution forms, and often contain fuzziness due to confidence limitations in sensing devices, permissible errors in experi- mental data, and quantisation errors in digitised observations. In this study a fuzzy state estimation method is proposed on the basis of these fuzzy observations, from both the static and dynamic viewpoints. The validity and effectiveness of the proposed method are confirmed experimentally in the evaluation of road traffic noise.

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Active Control of Sound Radiation from a Small Transformer Using Near-field Sensing

Xun Li, Xiaojun Qiu, Colin H. Hansen, Yanting Ai


The active control of noise radiated from a small transformer using near-field sensing strategies is investigated. Two cost functions that are minimised are the sum of the sound intensities at the error sensors and the sum of the squared sound pressures at error sensors in the near-field. The effects of the sensing strategies and the error sen- sor arrangement (location and number) on the control performance were studied numerically using transfer func- tion measurements. To verify the numerical simulation, experiments were carried out using a real time control system. A small transformer was located in an anechoic room for testing. The control system consists of eight control shakers mounted on the transformer, eight error microphones located in the near-field and a ten channel controller. Both predicted and test results are given.

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Active Control of Sound Radiation by Structures Using Near-field Sensing Strategies

Xun Li, Xiaojun Qiu and Colin H. Hansen


The active control of sound radiated from structures using two near-field sensing strategies is investigated theo- retically. The two sensing strategies involve minimising the sum of the sound intensities and minimising the sum of the squared sound pressures in the acoustic field close to the surface of a rectangular panel. A new quadratic expression is derived for the minimisation of the sum of the sound intensities in the near-field. The formulation is expressed in terms of the primary sound field and the measured or calculated transfer functions from the con- trol inputs to the error sensor outputs. Thus, the expression given here can be used to solve any complex prob- lem where the control source to error sensor transfer functions and primary sound field pressures can be deter- mined. In this example, a flat panel is modelled with a harmonic point force excitation and several point force control sources. To compare the control performance achieved by the proposed near-field sensing strategies, the radiated sound field is minimised in the near-field, and then the control results are evaluated in the far-field for each test case.

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Numerical Evaluation of a Decentralised Feedforward Active Control System for Electrical Transformer Noise

Julio A. Cordioli, Colin H. Hansen, Xun Li, Xiaojun Qiu


The implementation of a decentralised architecture for an active system to control electrical transformer noise is discussed and a general theoretical analysis of the performance and convergence of a decentralised system is de- veloped. Using the equations derived in the analysis, together with transfer function and noise level data meas- ured for a large electrical transformer, the performance and the convergence characteristics are calculated and compared for different control system configurations. It is demonstrated that the behaviour of a decentralised system with a leakage coefficient incorporated can be very different from the behaviour of a fully coupled sys- tem.

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Vibration Reduction of Flexible Structures During Slew Operations

G. Song and B. Kotejoshyer


Vibration reduction of flexible structures during slew operations poses a challenging task. Slew of flexible structures arises in many situations such as attitude control of spacecraft with large solar array panels and rotation of flexible robotic manipulators. In this paper, a new approach for vibration reduction of flexible structures during slew operations is proposed. The new approach integrates the method of command input shaping and the technique of active vibration suppression. In this new approach, the method of command input shaping is used to modify the existing command so that less vibration will be caused by the command itself. The technique of active vibration suppression using smart materials is used actively to control the vibration during and after the slew. To verify this new approach experimentally, an apparatus of a flexible rotating beam with a smart material actuator and sensor was built. In this setup, two piezoelectric patches were bonded on the surface of the flexible beam near its cantilevered end and were used as the smart actuator and the smart sensor respectively. With this pair of smart actuator and smart sensor, a strain rate feedback (SRF) controller was designed for active vibration suppression. With the Zero Vibration Derivative (ZVD) shaper and the SRF controller, the proposed new approach can significantly reduce the vibration of the flexible beam during slew operations.

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Hydroacoustic Source Characterisation of Centrifugal Pumps

Fabrice Carta, Stephane Bolpaire, Jacques Charley and Guy Caignaert


This paper presents a study of the hydroacoustic behaviour of radial flow pumps in the low frequency range. New data on ?source terms? that have been obtained on two different test rigs: the first is a pump operating with water (test rig DERAP), and the second is a pump operating with air (SHF impeller, test rig VENTIL). In both cases, the experimental methodology is based on the use of pressure measurements in various points of the suc- tion and delivery pipes, and plane wave propagation is assumed. An intensimetry technique, based upon the con- trol of the wave speed, has been performed to obtain the state vector at different cross-sections. With such a hy- pothesis, the hydroacoustic behaviour of a pump is defined using a two-port model. This paper focuses on the determination of the transfer matrix and source terms of pressure and flow-rate fluctuations, in the frequency do- main, in various operating conditions.

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Adequate Reverberation Time for Brass Band Auditoriums

Ignacio Guillen, Ana Llopis, Antonio Uris


Music bands are deeply rooted in the Valencian Community. According to the particular acoustical characteris- tics of brass bands, some special requirements have to be considered regarding their indoor performances. This paper studies two different auditoriums designed for brass band performances. 'Palau de la Música' Auditorium (Bunyol. Valencia) was built in 1950 and had a reverberation time of around 0.80 s at 1000 Hz. The original acoustical conditions gave good overall results and accordingly, they were maintained after the Auditorium's re- construction in 1995-1997. 'Quart de Poblet Moli Vell' Auditorium (Quart de Poblet. Valencia) was built in 1994-1998, to be used both for brass band concerts and theatrical performances. Its acoustic design was carried out following the 'Palau de la Música' Auditorium, whose performance results were considered satisfactory.

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Comparison of Two Analytical Methods for Numerical Hypersensitivity of Vibration Modes with Close Frequencies

X.L. Liu, C.S. Oliveira


This paper provides a comparison of numerical approaches, in terms of qualitative and quantitative analyses, to study the hypersensitivity of modes with close frequencies. The existence of close frequencies and strong modal couplings can cause rapid changes under a small variation of parameters, observed as the curve veering of fre- quencies and the mode localisation. Perturbation analysis with a discrete frequency is used for qualitative analy- sis, and shows a tendency of veering and localisation of large but miscalculated modal changes. On the other hand, quantitative analysis, which considers the specific characteristics of the modes of close frequencies, can give a more precise measurement. Three examples are given, which show the numerical differences between qualitative and quantitative analyses.

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