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Published Articles


The Volume 8, No 1, March 2003




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Active Vibration Absorber for an Oscillating Structure Using Nonlinear Control

Benjamin Vazquez-Gonzalez, Gerardo Silva-Navarro


https://doi.org/10.20855/ijav.2003.8.1123


The paper addresses the attenuation problem of harmonic vibrations affecting an oscillatory mechanical system using passive and active vibration control methods based on nonlinear control techniques. The methodology starts from a proper design of a passive vibration absorber, which by the addition of an actuated degree of freedom is transformed into an active vibration absorber described by a nonlinear model. The active vibration control scheme employs the measurement of the excitation frequency and the open-loop frequency response to compute an optimal attenuation condition, which is achieved with the application of a nonlinear controller (partial feedback linearisation or output regulation control techniques). The active vibration absorber can attenuate harmonic vibrations with varying excitation frequency as well as parameter variations on the system.


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Vibration of the London Millennium Bridge: Cause and Cure

David E. Newland


https://doi.org/10.20855/ijav.2003.8.1124


When the London Millennium Bridge was opened in June 2000, it swayed alarmingly. This generated huge public interest and the bridge became known as London?s ?wobbly bridge.? Pedestrians unwittingly excited the bridge?s lateral vibration modes. Although previously documented, this phenomenon was not well-known. Selfexcitation occurs only when such a bridge?s damping is small, so the solution was to increase damping artificially by an amount that had to be determined. This proved to be a challenging design task. This paper presents a new feedback model to describe how pedestrian motion synchronises with bridge lateral movement to become a source of self-excitation.


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Optimisation by Evolutionary Algorithms of Free-Layer Damping Treatments on Plates

Luigi Bregant and Simone Puzzi


https://doi.org/10.20855/ijav.2003.8.1125


The problem of controlling or reducing vibration levels is very common in many different applications. From costly aeroplane fuselages to cheap domestic appliances the prob- lem can be tackled with the appropriate use of free layer damping treatments (FLDT). This technique, based on the distribution of layers of visoelastic material on the vibrating parts, is frequently adopted since it is cheap, efficient and easily applicable either at the design stage or in situ as a cor- rective measure. Due to these advantages and few limitations, not much care is taken to optimise the material distribution. Conversely, the necessity to produce lighter and quieter products will force the designer to optimise these treatments too. Under these circumstances, the most appropriate optimi- sation objectives seem to be the maximisation of the achiev- able damping levels and the minimisation of the amount of added material. In this work the potential of the genetic algorithm for the determination of the optimal damping material distribution on vibrating plates is tested. The selection of this optimisa- tion procedure is dictated by the high number of control vari- ables involved and by the nature of the energetic formulation adopted. The latter leads to multi-modal solution spaces where, unfortunately, classical search procedures, based ei- ther on the gradient or hill climbing procedures, tend to fail.


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Analysis of Tram-induced Vibrations Affecting Roads and Buildings in Standard Urban Sites

Cristina Pronello


https://doi.org/10.20855/ijav.2003.8.1126


The passage of trams in towns and cities can cause considerable disturbance to people living or working on streets with tramways. This study investigates the impact of tram-induced vibrations in streets with different ar- rangements, i.e. different road layout and building types, and measured at varying distances from and heights above the tram lines. The first step involved the definition of a methodology for choosing test sites and defining ways of measuring the vibrations. In the second step, a series of measurements was made in order to compile a database which could provide initial indications regarding the impact of vibrations on different types of site and buildings. These results enabled the planning of a new series of in situ measurements using the proposed meth- odology, as well as a programme of measurements to be carried out in laboratory conditions in order to study the effects of wheel and track conditions on vibration.


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The Sound Power Radiated by Some Axisymmetric Vibration Modes of a Guided Annular Plate

Wojciech P. Rdzanek Jr.


https://doi.org/10.20855/ijav.2003.8.1127


This paper presents an asymptotic approach to find the values of the active and reactive sound power of a fully guided annular plate. The results obtained are valid for some time-harmonic axisymmetric high-frequency proc- esses. The plate is embedded in a planar rigid baffle. Low fluid loading and low internal friction are assumed. The self power of the plate has been presented in its elementary form together with its approximation error. Some sample numerical results obtained from the integral and asymptotic formulae show good agreement within the whole high frequency range. The elementary formulae are highly efficient, and therefore they can be used for computing the total sound power radiated by the plate.


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Active Control of Enclosed Sound Fields Using Three-axis Energy Density Sensors: Rigid Walled Enclosures

Ben S. Cazzolato and Colin H. Hansen


https://doi.org/10.20855/ijav.2003.8.1128


The use of three-axis energy density sensors for controlling the sound field within a rigid walled enclosure is in- vestigated numerically. It is shown that energy density sensors perform as well as, if not better than, an equiva- lent number of microphones. The effect on control performance using multiple energy density sensors is also in- vestigated, as is the effect of multiple control sources. The control mechanisms that occur when using energy density sensors with multiple control sources are analysed, and it is shown that the control mechanism moves from modal control to modal rearrangement as the number of control sources is increased. In doing so, the con- trol becomes more local to the sensors. The zone of local control around the energy density sensors is measured, and it is shown that the zone of control is similar in size to that in a diffuse field.


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Acoustic Energy Concept for the Design of a Flow Meter

N.A. Ahmed


https://doi.org/10.20855/ijav.2003.8.1129


An acoustic energy source concept is used to develop simple theoretical expressions for the optimisation of the dimensions for the design of a flow meter. Basic equations are first generated for the prediction of vortex shedding frequency due to a bluff body placed inside a pipe or a channel flow. The Strouhal number predicted using these expressions is compared with limited experimental data for a 6-inch commercial vortex flow meter operating at different Reynolds numbers. The results show good agreement between theory and experiment.


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Lessons Learned from CM-2 Modal Testing and Analysis

Mark E. McNelis, Thomas W. Goodnight, Kelly S. Carney, Kim D. Otten


https://doi.org/10.20855/ijav.2003.8.1130


The Combustion Module-2 (CM-2) is a space experiment that was launched on shuttle mission STS-107 in the SPACEHAB Double Research Module. The CM-2 flight hardware is installed in SPACEHAB single and double racks. The CM-2 flight hardware was vibration tested in the launch configuration to characterise the structure?s modal response. Cross-orthogonality between test and analysis mode shapes was used to assess model correlation. Lessons learned for pre-test planning and model validation are discussed.


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