Sound radiation characteristics of a box-type structure

The finite element and boundary element methods are employed in this study to investigate the sound radiation characteristics of a box-type structure. It has been shown [T.R. Lin, J. Pan, Vibration characteristics of a box-type structure, Journal of Vibration and Acoustics, Transactions of ASME 131 (2009) 031004-1–031004-9] that modes of natural vibration of a box-type structure can be classified into six groups according to the symmetry properties of the three panel pairs forming the box. In this paper, we demonstrate that such properties also reveal information about sound radiation effectiveness of each group of modes. The changes of radiation efficiencies and directivity patterns with the wavenumber ratio (the ratio between the acoustic and the plate bending wavenumbers) are examined for typical modes from each group. Similar characteristics of modal radiation efficiencies between a box structure and a corresponding simply supported panel are observed. The change of sound radiation patterns as a function of the wavenumber ratio is also illustrated. It is found that the sound radiation directivity of each box mode can be correlated to that of elementary sound sources (monopole, dipole, etc.) at frequencies well below the critical frequency of the plates of the box. The sound radiation pattern on the box surface also closely related to the vibration amplitude distribution of the box structure at frequencies above the critical frequency. In the medium frequency range, the radiated sound field is dominated by the edge vibration pattern of the box. The radiation efficiency of all box modes reaches a peak at frequencies above the critical frequency, and gradually approaches unity at higher frequencies

Control of free-flying space robot manipulator systems

New control techniques for self contained, autonomous free flying space robots were developed and tested experimentally. Free flying robots are envisioned as a key element of any successful long term presence in space. These robots must be capable of performing the assembly, maintenance, and inspection, and repair tasks that currently require human extravehicular activity (EVA). A set of research projects were developed and carried out using lab models of satellite robots and a flexible manipulator. The second generation space robot models use air cushion vehicle (ACV) technology to simulate in 2-D the drag free, zero g conditions of space. The current work is divided into 5 major projects: Global Navigation and Control of a Free Floating Robot, Cooperative Manipulation from a Free Flying Robot, Multiple Robot Cooperation, Thrusterless Robotic Locomotion, and Dynamic Payload Manipulation. These projects are examined in detail

Detection of Communities within the Multibody System Dynamics Network and Analysis of Their Relations

Multibody system dynamics is already a well developed branch of theoretical, computational and applied mechanics. Thousands of documents can be found in any of the well-known scientific databases. In this work it is demonstrated that multibody system dynamics is built of many thematic communities. Using the Elsevier’s abstract and citation database SCOPUS, a massive amount of data is collected and analyzed with the use of the open source visualization tool Gephi. The information is represented as a large set of nodes with connections to study their graphical distribution and explore geometry and symmetries. A randomized radial symmetry is found in the graphical representation of the collected information. Furthermore, the concept of modularity is used to demonstrate that community structures are present in the field of multibody system dynamics. In particular, twenty-four different thematic communities have been identified. The scientific production of each community is analyzed, which allows to predict its growing rate in the next years. The journals and conference proceedings mainly used by the authors belonging to the community as well as the cooperation between them by country are also analyzed

ADVANCED NUMERICAL METHODS FOR THE DYNAMIC OPTIMISATION OF MECHANICAL COMPONENTS

This PhD thesis concerns the development and assessment of innovative methodologies for simulating and improving the dynamic behaviour of mechanical components. In particular, two correlated issues are addressed herein: hybrid FE/LP gear pump modelling as a tool for foreseeing and optimising vibration behaviour in operational conditions; a new methodology for vibration reduction by applying damping patches in appropriate positions. In the field of positive displacement pump modelling, external gear pumps were analysed with the aim of developing ! ! "#! advanced methodologies which accurately predict of the dynamic behaviour of these components. Indeed, the first part of this thesis (PART A) is about external gear pumps for steering systems; the research activity concerning gear pumps was carried out in collaboration with the Dept. of Engineering at the University of Ferrara in co-operation with TRW Automotive Italia S.p.A – Divisione Automotive Pumps (Ostellato, Ferrara, Italy). This research pertains to the creation of a hybrid model, obtained through the integration of a nonlinear elastodynamic model with lumped parameters in relation to moving bodies, and an FE pump model. The model referred to bodies in motion takes into account the most important phenomena involved in pump operations, such as time-varying oil pressure distribution on gears, timevarying meshing stiffness, tooth profile errors, the possibility of tooth contact, bush displacement and hydrodynamic journal bearing reactions. Coupling the FE with the various parts which make up the pump, as well as coupling the lumped-parameter model and the FE model required the development of specific advanced techniques; thus several problems related to the combination of the different models employed in order to form a single hybrid LP/FE model were studied and resolved. Using particular techniques based on comparisons between simulations and experimental results concerning acceleration, forces and moments, the model was experimentally validated. Although this hybrid model is an excellent tool for improving the dynamic behaviour of gear pumps and for optimising the early stages of prototype design, some problems can still remain related to unwanted vibrations into precise frequency ranges. Thus, once the first part of the research was completed, it was decided to delve into the problem of structural optimisation. In particular, a methodology for surface damping treatment was created and applied. Indeed, the second part of the research activity (PART B) was about the optimisation of mechanical components and systems through the application of high damping material components known as patches; this research activity is being ! ! "! carried out by the Dept. of Mechanical Engineering (laboratoire vibrations acoustique) at the INSA institute (Institute National des Sciences Appliquées) in Lyon (France) where I spent thirteen months between the second and third year of my doctoral studies. Such treatment can be applied to existing structures and provides high damping capability over wide temperature and frequency ranges. In many practical plate and machinery casing structures, it is difficult to treat the whole surface with constrained layer viscoelastic material, due to reduced areas or inaccessible parts. Furthermore, it may indeed be desirable to selectively apply one or more damping patches to control certain resonances. Patch damping design is an efficient and cost effective concept for solving noise and vibration problems. As a result of these considerations, the research was focused on finding a general methodology, based on a purely energetic approach, to reduce the unwanted amplitude vibration level in mechanical components through the application of appropriate elements characterized by high damping properties. The methodology was enforced using IDEAS v7! software which makes it possible to address modelling in terms of energy distribution within a structure. Advanced methodologies were developed to reduce the vibration amplitude in components such as plate and bracket by applying patches. Specifically, potential energy estimations will precisely and accurately define the exact locations on the surface of the components which should be covered by the patches. As a result, these studies enable a reduction in vibration amplitude, in reference both to a single component and/or a complex system. In addition, this methodology makes it possible to improve the vibratory behaviour of a component in certain frequency ranges while reducing, at the same time, the effect of dangerous resonances, acting specifically on the location, extent and quantity of the patches to be applied on the surface of the base component. During this thesis, different fields were contemporarily studied: definition and identification of structural modification ! ! "#! methods, theoretical aspects of material damping characteristics, vibrational propagation methods and applicative aspects relating to the implementation of models for the vibratory optimisation of mechanical components. This thesis was developed within the LVA research and technology transfer laboratories at the INSA institute (Lyon, France) and InterMech (Division Acoustic and Vibrations – LAV); and was carried out with the contribution of the Emilia Romagna Region – Assessorato Attività Produttive, Sviluppo Economico, Piano telematico, PRRIITT misura 3.4 azione A