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Effective control of vibration in rotating machinery is a major concern in many industries and research institutions. With the ever-increasing drive for higher operating speeds, the need for developing vibration mitigation methods that cater for the arduous operating conditions that consequently arise is paramount. Phenomena that may have been insignificant in relatively low-speed rotating machines begin to gain importance with increasing speed, an example of which is the gyroscopic effect. This thesis is aimed at enriching the knowledge base on rotordynamic vibration control. Independent Modal Control (IMC) is addressed, within the context of rotating machinery. A study is performed on various actuation technologies that may be used to implement active vibration control. The well-known problem of balancing rotating machinery is also considered. The first-order modal filters based on Structure Preserving Transformations (SPTs) are capable of decoupling a rotor dynamic system into individual modes of vibration, such that IMC may be performed. Unlike traditional control schemes, the method based on first-order modal filters does not require the imposition of highly restrictive conditions on the system (classical damping). As a result, gyroscopic effects - which are substantial in high-speed rotating machinery - and non-classical damping may be fully accounted for in the modal domain. The main problem pertaining to this method arises from the fact that the response of the controlled system is linked with the stability of the modal filters. As such, if the filters are unstable, the controlled response is eventually overcome by noise. This thesis explores the spectral properties of the modal filter with a view to understanding the factors that affect its stability; some interesting findings on the filter eigenvalues are presented. Furthermore, the question as to whether filter stability is an essential requirement is addressed. The relationship between the rotordynamic system and the modal filter is also investigated. An illustration of the techniques developed in relation to IMC using first-order modal filters is presented in the form of a FE simulation on a realistic aero-engine model. The implementation of active vibration control in a dynamic system is realised through the application of control forces by actuators. In the case of rotating machines, these would normally be located at the bearings. Actuation may be achieved from a variety of technologies such as electromagnetic, piezoelectric, magnetostrictive, ultrasonic etc. This thesis conducts a study on some popular actuation technologies, with the aim of finding an effective alternative to the ubiquitous squeeze film damper. The merits and drawbacks of the various technologies are compared. Also, some novel design concepts are proposed, and (in some cases) their viability demonstrated through calculations. It is well-known that rotor unbalance is usually the main source of vibration in rotating machines. Thus, improvements in procedures for balancing such machines are continuously being sought. With increasing in-service operating speeds and ever more stringent standards, traditional balancing methods progressively become inadequate. One of the reasons for this is the inability of balancing tests to capture the contribution of patterns of unbalance that excite higher modes of vibration, as the tests speeds are usually lower than in-service speeds. This thesis proposes a robust balancing approach that utilises additional information on rotor unbalance, in the form of a covariance matrix, to improve the balancing procedure. The method is illustrated in a FE model of a rotating machine, and is shown to be superior to the traditional method.

Assembly plays a vital role in the quality of a final product and has a great impact on the manufacturing cost. The mechanical assemblies consist of parts that inevitably have variations from their ideal dimensions. These variations propagate and accumulate as parts are assembled together. Excessive amount of variations in an assembly may cause improper functionality of the product being assembled. Improving assembly quality and reducing the assembly time and cost are the main objectives of this thesis. The quality of an assembly is determined in terms of variations in critical assembly dimensions, also known as Key Characteristics (KCs). Key Characteristics are designated to indicate where excess variation will affect product quality and what product features and tolerances require special attention. In order to improve assembly quality and reduce assembly time and cost, it is necessary to: (1) model non-ideal parts based on tolerances defined in design standards or current industrial practice of component inspection, (2) model assemblies and their associated assembly processes to analyse tolerance stack-up in the assembly, (3) develop probabilistic model to predict assembly variation after product assembly, and (4) implement control strategies for minimising assembly variation propagations to find optimum configuration of the assembly. Two assembly models have been developed, a linear model and a fully non-linear model for calculating assembly variation propagations. The assembly models presented in this thesis also allows for inclusion of geometric feature variation of each assembly component. Methods of incorporating geometric feature variations into an assembly variation model are described and analysis techniques are explained. The assembly variation model and the geometric variation models have been developed for 20 and 3D assemblies. Modelling techniques for incorporating process and measurement noise are also developed and described for the nonlinear assembly model and results are given to demonstrate the calculation of assembly variations while considering part, process and measurement errors. Two assembly case studies originating in sub-assemblies of aero-engines have been studied: Case Study 1, representing the rotating part (rotor) of an aero-engine, and Case Study 2, representing non-rotating part (stator) of an aero-engine. A probabilistic method based on the linear model is presented as a general analytical method for analysis of 3D mechanical assemblies. Probability density functions are derived for assembly position errors to analyse a general mechanical assembly, and separate probability functions are derived for the Key Characteristics (KCs) for assembly in Case Studies 1 and 2. The derived probability functions are validated by using the Monte Carlo simulation method based on the exact (full non-linear) model. Results showed that the proposed probabilistic method of estimating tolerance accumulation in mechanical assemblies is very efficient and accurate when compared to the Monte Carlo simulation method, particularly if large variations at the tails of the distributions are considered. Separate control strategies have been implemented for each case study. Four methods are proposed to minimise assembly variations for Case Study 1, and one error minimisation method is suggested for assemblies of Case Study 2. Based on the developed methods to optimise assembly quality, the two case studies were investigated, and it was found that the proposed optimisation methods can significantly improve assembly quality. The developed optimisation methods do not require any special tooling (such as fixtures) and can easily be implemented in practice.

The advantages of isolating vibrating machinery from its supporting structure are that the chances of vibration induced fatigue failure of structural components are reduced, the structure becomes more inhabitable for people due to less vibration exposure and the sound radiated by the structure into the environment is reduced. This last point is especially important for machinery operating in a marine environment because low frequency sound propagates very well underwater, and the machinery induced sound radiated from a ship or submarine is a primary detection and classification mechanism for passive sonar systems. This thesis investigates the control of vibration from an elastic support structure upon which multiple vibrating systems are passively mounted. The excitations are assumed to occur at discrete frequencies with a finite number of harmonic components and the machines are all assumed to be supplied with power from the same electrical supply. Active vibration control may be achieved by adjusting the phase of the voltage supplied to one or more of the machines, so that a minimum value of a measurable cost function is obtained. Adjusting the phase of a machine with respect to a reference machine is known as synchrophasing and is a well established technique for controlling the sound in aircraft cabins and in ducts containing axial fans. However, the use of the technique for reducing the vibration of machinery mounted on elastic structures seems to have received very little attention in the literature and would appear to be a gap in the current knowledge. This thesis aims to address that gap by investigating theoretically and experimentally how synchrophasing can be implemented as an active structural vibration control technique.

La défaillance des pièces mécaniques est très souvent initiée par un défaut de surface. Par conséquent, la génération de contraintes résiduelles compressives sur des pièces mécaniques via une déformation plastique hétérogène améliore la tenue en fatigue et augmente la durée de vie des pièces. Parmi les procédés permettant d'introduire des contraintes résiduelles dans les pièces, le traitement par choc laser est plus intéressant à plusieurs titres. D'une part, il permet de produire des pressions en surface du matériau de l'ordre de 1 à 6 GPa sur de courtes durées d'impulsion allant de 3 à 30 nanosecondes. D'autre part, il offre la possibilité d'introduire des contraintes résiduelles de compression sur une certaine profondeur tout en conservant l'état initial de la pièce traitée. Ainsi, les simulations numériques par réalisation de modèles simples permettent de cerner les physiques mises en jeux. Dans cette perspective, la méthode semi-analytique offre d'énormes avantages, notamment la simplicité des modèles et la réduction des temps de calcul. Cependant, cette méthode n’a jamais été étendue aux problème dynamiques. Dans cette thèse la méthode semi-analytique a été étendue aux problèmes dynamiques et le modèle mis en place été appliqué pour la simulation du procédé de choc Laser / The failure of the mechanical parts is very often initiated by a surface defects. Consequently, the generation of compressive residual stresses on mechanical parts by introducing a heterogeneous plastic strain improves the resistance to fatigue and increases the lifetime of the parts. Among the processes making it possible to introduce residual stresses into the parts, the laser shock peening is more interesting for several reasons. On the one hand, it makes it possible to produce pressures on the surface of material of about 1 to 6 going GPa over short pulse times from 3 to 30 nanoseconds. In addition, he gives the opportunity of introducing residual stresses of compression on a certain depth while preserving the initial state of the treated part. The numerical simulation becomes necessary to determine the best physical phenomena involved. Thus, the semi-analytical method offers a lot of advantages, in particular the simplicity of the models and the computation times saving. This method was never extended to the dynamic problems. In this thesis the semi-analytical method was extended to the dynamic problems and the model implemented is applied for the simulation of the Laser process of shock.