PMM testing is a method used to identify the added mass and damping coefficients used in the equations of motion of a vehicle by attempting to decouple the forces on a body due to velocity and acceleration as a result of creating "hydrodynamically pure" velocities and accelerations. This makes it possible to use quasi-steady state models with terms independent of both velocity and acceleration. This paper explores the ability of simple damping models (solely a function of velocity) with added mass terms (solely a function of acceleration) to simulate the heave force of an 8:1 ellipsoid undergoing PMM testing. In order to help explain the complexity of the flow during PMM tests, a flow analysis of the 8:1 spheroid is provided, which discusses the flow topology of spheroids at steady angle of attack, validity of quasi-steady models, and some other basic flow features seen in PMM testing. In this paper, a simple proportionality relationship between a linear and quadratic damping model is revealed. It is also shown that variations in the heave force response during PMM tests are most heavily influenced by viscous effects, especially cross flow separation. Finally, it is shown where these models break down, owing to the increasing nonlinearity of the flow induced by the harsher motions of large amplitude and/or large frequency tests. / Master of Science

Rubber like materials find wide applications in damping treatment of structures, vibration isolations and they appear prominently in the form of hoses in many structures such as aircraft engines. The study reported in this thesis addresses a few issues in computational modeling of vibration of structures with some of its components made up of rubber like materials. Specifically, the study explores the use of fractional derivatives in representing the constitutive laws of such material and focuses its attention on problems of parameter identification in linear time invariant systems with fractional order damping models. The thesis is divided into four chapters and two annexures. A review of literature related to mathematical modeling of damping with emphasis on fractional order derivative models is presented in chapter 1. The review covers lternatives available for modeling energy dissipation that include viscous, structural and hybrid damping models. The advantages of using fractional order derivative models in this context is pointed out and papers dealing with solution of differential equations with fractional order derivatives are reviewed. Issues related to finite element modeling and random vibration analysis of systems with fractional order damping models are also covered. The review recognizes the problems of system parameter identification based on inverse eigensensitivity and inverse FRF sensitivity as problems requiring further research. The problem of determination of derivatives of eigensolutions and FRF-s with respect to system parameters of linear time invariant systems with fractional order damping models is considered in chapter 2. The eigensolutions here are obtained as solutions of a generalized asymmetric eigenvalue problem. The order of system matrices here depends upon the mechanical degrees of freedom and also somewhat artificially on the fractional order of the derivative terms. The formulary for first and second order eigenderivatives are developed taking account of these features. This derivation also takes into account the various orthogonality relations satisfied by the complex valued eigenvectors. The system FRF-s are obtained by a straight forward inversion of the system dynamic stiffness matrix and also by using a series solution in terms of system eigensolutions. As might be expected, the two solutions lead to identical results. The first and the second order derivatives of FRF-s are obtained based on system dynamic matrix and without taking recourse to modal summation. Numerical examples that bring out various facets of eigensolutions, FRF-s and their sensitivities are presented with reference to single and multi degree freedom systems. The application sensitivity analysis developed in chapter 2 to problems of system parameter identification is considered in chapter 3. Methods based on inverse eigensensitivity and inverse FRF sensitivity are outlined. The scope of these methods cover first and second order analyses and applications to single and multi degree freedom systems. While most illustrations are based on synthetic measurement data, limited efforts are also made to implement the identification methods using laboratory measurement data. The experimental work has involved the measurement of FRF-s on a system consisting of two steel tubes connected by a rubber hose. The two system identification methods are shown to perform well especially when information on second order sensitivity are included in the analysis. The method based on inverse eigensolution is shown to become increasingly unwieldy to apply as the order of the system matrices increases while the FRF based method does not suffer from this drawback. The FRF based method also has the advantage that the prior knowledge of order of fractional order derivative terms is not needed in its implementation while such knowledge is assumed in the method based on eigensolutions. While the methods are shown to perform satisfactorily when synthetic measurement data is used, their success is not uniformly good when laboratory measurement data are employed. Chapter 4 presents a summary of contributions made in the thesis and also enlists a few suggestions for further research. Annexure I provides a précis of elementary notion of fractional order derivatives and integrals. A case study on finite element analysis of aircraft engine component made up of metallic and rubber materials is outlined in Annexure II and the study points towards possible advantages of using fractional order damping models in the study of such structures.

Modern volume carriers such as container vessels, cruise ships and Pure Car and Truck Carriers (PCTC’s) have become more vulnerable to critical roll responses as built in margins have been traded against increased transport efficiency. The research presented in this doctoral thesis aims at enhancing the predictability and control of these critical roll responses. The thesis presents a holistic framework for predicting and assessing the roll damping, which is a crucial parameter for predicting roll motions, based on a unique combination of full scale trials, model tests, semi-empirical methods and computational fluid dynamics. The framework is intended to be used from the early design stage and gradually improved until delivery of the ship and finally to provide input for decision support in the operation. The thesis also includes a demonstration of a new application for rudder control to be used for mitigation of parametric roll. This is performed by simulating real incidents that have occurred with PCTC’s in service. Simulations with rudder roll control show promising results and reveal that the approach could be very efficient for mitigation of parametric roll. Finally, an approach for monitoring of roll induced stresses, so-called racking stresses in PCTC’s, is presented. The approach involves measurement of the ship motions and scaling of pre-calculated structural responses from global finite element analysis. Based on full scale motion and stress measurements from a PCTC in-service the approach is evaluated and demonstrated to be an efficient alternative to conventional methods. / <p>QC 20170516</p>

Most present day spacecrafts have large interconnected solar panels. The dynamic behavior of the spacecraft in orbit can be modeled as a free rigid mass with flexible elements attached to it. The natural frequencies of such spacecrafts with deployed solar panels are very low. The low values of the natural frequencies pose difficulties for maneuvering the spacecraft. The control torque required to maneuver the spacecraft is influenced by the flexibility of the solar arrays. The control torque sets up transient oscillations in the flexible solar panels which in turn induces disturbances in the rigid satellite body and the payload within. Therefore the payload operations can be carried out only after the disturbances die out. For any reduction of the above disturbances it is necessary to understand the dynamic behavior of such systems to an applied torque. The present work first studies the nature of the disturbances. The influence of structural parameters on these disturbances is then investigated. Finally, the use of passive damping treatment using viscoelastic material is investigated for the reduction of the disturbances. In order to understand the nature of vibrations induced in the flexible appendages of a satellite during maneuvers, we model the maneuver loads in terms of applied angular acceleration as well as varying torque. The transient decay of the disturbance of the rigid element is characterized by the dynamic characteristics of the flexible panels or appendages. It is shown that by changing the stiffness of the panel the response of the rigid element can be modified. A simple model consisting of an Euler-Bernoulli beam attached to a free mass is next considered. The influence of various parameters of the EulerBernoulli beam in mitigating vibration and thereby the disturbance in the rigid mass is investigated. As the response of the rigid system mounted with the large flexible panels are influenced by the dynamics of the flexible panels, reduction of these disturbances can be achieved by reducing the vibration in the flexible panels. Therefore application of viscoelastic materials for passive damping treatment is investigated. The loss factor of a structure is significantly improved by using constrained viscoelastic layer damping treatment. However providing a constrained layer damping treatment on the entire structure is very inefficient in terms of the additional mass involved. Therefore damping material is applied at suitable optimal locations. In previous studies reported in literature, modal strain energy distribution in the viscoelastic material as well as the base structure is used as a tool to arrive at the optimum location for the damping treatment. It is shown in this study that such locations selected are not the optimum. A new approach is proposed in this study by which both the above shortcomings are overcome. It is shown that use of a parameter that is the ratio of the strain in the viscoelastic material to the angle of flexure is a more reliable measure in arriving at optimal locations for the application of constrained viscoelastic layers. The method considers the deformations in the viscoelastic material and it is shown that significant values of loss factors are achieved by providing material in a small region alone. We also show that loss factor can be improved by providing damping material near the interface region. The loss factor can be further improved by incorporating spacers by using spacer material having higher extensional modulus. Also shown is the fact that loss factor is unaffected by the shear modulus of the spacer material. Experiments have been conducted to validate these results. In a related study we consider honeycomb type flexible structures since in most of the spacecraft applications honeycomb sandwich constructions are employed. But loss factors of sandwich panels with constrained layer damping treatment are seldom discussed in the literature. Use of viscoelastic layers to improve the loss factors of the honeycomb sandwich beams is explored. The results show that the loss factors are enhanced by increasing the inplane stiffness of the constraining layer. These conclusions too are validated by experimental results. Finally a typical satellite with flexible solar panels is considered, and the use of the viscoelastic material for improving the damping is demonstrated.

Lead-lag dampers are present in most rotor systems to provide the desired level of damping for all flight conditions. These dampers are critical components of the rotor system, and the performance of semi-active Coulomb-friction-based lead-lag dampers is examined for the UH-60 aircraft. The concept of adaptive damping, or “damping on demand,” is discussed for both ground resonance and forward flight. The concept of selective damping is also assessed, and shown to face many challenges. In rotorcraft flight dynamics, optimized warping twist change is a potentially enabling technology to improve overall rotorcraft performance. Research efforts in recent years have led to the application of active materials for rotorcraft blade actuation. An innovative concept is proposed wherein the typically closed section blade is cut open to create a torsionally compliant structure that acts as its own amplification device; deformation of the blade is dynamically controlled by out-of-plane warping. Full-blade warping is shown to have the potential for great design flexibility. Recent advances in rotorcraft blade design have also focused on variable-camber airfoils, particularly concepts involving “truss-core” configurations. One promising concept is the use of hexagonal chiral lattice structures in continuously deformable helicopter blades. The static behavior of passive and active chiral networks using piezoelectric actuation strategies is investigated, including under typical aerodynamic load levels. The analysis is then extended to the dynamic response of active chiral networks in unsteady aerodynamic environments.

A series of resonant column and torsional shear (RCTS) and large scale resonant column (LSRC) tests were performed to investigate the dynamic properties (shear modulus and material damping ratio) of municipal solid waste (MSW). the MSW materials were recovered from the Tri-Cities landfill adjacent to the San Francisco Bay in California. A total of 30 specimens 2.8-in. (71.1-mm) and 6.0-in. (152.4-mm) of old, fresh, and mixed MSW were reconstituted in accordance with established sample preparation procedures. Ten of specimens were small-diameter (2.8-in. (71.1-mm)) RCTS specimen and 20 specimens were larger (6.0-in. (152.4-mm)) LSRC specimens. Dynamic laboratory measurements were performed in the linear and nonlinear strain ranges. Test parameters affecting the dynamic properties in the linear range included: (1) duration of confinement, (2) isotropic total confining pressure, [sigma]o, (3) excitation frequency, f, and (4) specimen size. Other test parameters affecting dynamic properties in the nonlinear strain range were: (1) shearing strain amplitude, [gamma], (2) isotropic total confining pressure, (3) overconsolidation ratio, (4) number of loading cycles, and (5) excitation frequency. In addition, the effects on dynamic properties of MSW specimens of material parameters such as (1) waste composition, (2) water content, (3) unit weight of waste, and (4) particle size were evaluated. The total unit weights of old, fresh, and mixed MSW specimens were estimated during testing in the RCTS and LSRC devices. These estimated total unit weights in the laboratory were compared with those measured at other MSW landfills and were found to generally be less than the field measurements. At a given [sigma]o, Gmax decreases with decreasing weight percentage of soil-size (passing the 3/4-in. (19.1-mm) sieve) material. However, Dmin increases slightly with decreasing weight percentage of soil-size material. Another relationship was developed between estimated total unit weight, [gamma]t, and confining pressure, including weigh percentage of soil-size material. The Vs profiles of old, fresh, and mixed MSW specimens obtained in the laboratory tests were compared with those measured at other MSW landfills in situ. The 62 to 76% soil-size material groups are in good agreement with in-situ Vs profiles. The variation in normalized shear modulus and material damping ratio curves were patterned after the Darendeli model (2001) for different weight percentages of soilsize material. An empirical relationship between normalized shear modulus (G/Gmax) and modified material damping ratio (D-Dmin) was developed in the nonlinear strain range. As part of collaborative research project, nonlinear shear modulus reduction and material damping curves generated by The University of Texas at Austin (UT) and The University of California at Berkeley (UCB) were compared according to different weight percentages of soil-size material. Furthermore, nonlinear shear modulus reduction and material damping ratio curves generated by UT were also compared with ones previously proposed by other researchers.

New materials and new fabrication techniques in turbomachinery lead to monolithic<p>structures with extremely low damping which may be responsible for severe vibrations<p>and possible high-cycle fatigue problems. To solve this, various techniques<p>of damping enhancement are under investigation. The present work is focused on<p>piezoelectric shunt damping.<p>This thesis considers the RL shunt damping of rotationally periodic structures using<p>an array of piezoelectric patches, with an application to a bladed drum representative<p>of those used in turbomachinery. Due to the periodicity and the cyclic symmetry of<p>the structure, the blade modes occur by families with very close resonance frequencies,<p>and harmonic shape in the circumferential direction; the proposed RL shunt<p>approaches take advantage of these two features.<p>When a family of modes is targeted for damping, the piezoelectric patches are<p>shunted independently on identical RL circuits, and tuned roughly on the average<p>value of the resonance frequencies of the targeted modes. This independent<p>configuration offers a damping solution effective on the whole family of modes, but<p>it requires the use of synthetic inductors, which is a serious drawback for rotating<p>machines.<p>When a specific mode with n nodal diameters has been identified as critical and<p>is targeted for damping, one can take advantage of its harmonic shape to organize<p>the piezoelectric patches in two parallel loops. This parallel approach reduces considerably<p>the demand on the inductors of the tuned inductive shunt, as compared<p>to independent loops, and offers a practical solution for a fully passive integration<p>of the inductive shunt in a rotating structure.<p>Various methods are investigated numerically and experimentally on a cantilever<p>beam, a bladed rail, a circular plate, and a bladed drum. The influence of blade<p>mistuning is also investigated. / Doctorat en Sciences de l'ingénieur / info:eu-repo/semantics/nonPublished

The aim of the thesis is to propose active damping as a potential control strategy for chatter instability in machine tools.<p>The regenerative process theory explains chatter as a closed loop interaction between the structural dynamics and the cutting process. This is considered to be the most dominant reason behind machine tool chatter although other instability causing mechanisms exist.<p>The stability lobe diagram provides a quantitative idea of the limits of stable machining in terms of two physical parameters: the width of contact between tool and the workpiece, called the width of cut and the speed of rotation of the spindle. It is found that the minimum value of the stability limit is proportional to the structural damping ratio for turning operations. This important finding provides the motivation of influencing the structural dynamics by active damping to enhance stability limits of a machining operation.<p>A direct implementation of active damping in an industrial environment may be difficult. So an intermediate step of testing the strategy in a laboratory setup, without conducting real cutting is proposed. Two mechatronic "Hardware in the Loop" simulators for chatter in turning and milling are presented, which simulate regenerative chatter experimentally without conducting real cutting tests. A simple cantilever beam, representing the MDOF dynamics of<p>the machine tool structure constitutes the basic hardware part and the cutting process is simulated in real time on a DSP board. The values of the cutting parameters such as spindle speed and the axial width of cut can be changed on the DSP board and the closed loop interaction between the structure and the cutting process can be led to instability.<p><p>The demonstrators are then used as test beds to investigate the efficiency of active damping, as a potential chatter stabilization strategy. Active damping is easy to implement, robust and does not require a very detailed model of the structure for proper functioning, provided a collocated sensor and actuator configuration is followed. The idea of active damping is currently being implemented in the industry in various metal cutting machines as part of the European Union funded SMARTOOL project (www.smartool.org), intended to propose smart chatter control technologies in machining operations. / Doctorat en sciences appliquées / info:eu-repo/semantics/nonPublished

The scope of this research concerns the passive damping of structural vibrations by the use of viscoelastic layers. It is motivated by the need for efficient numerical tools to deal with the medium frequency behaviour of industrial viscoelastic sandwich products. The sandwich modelling technique is based on the use of an interface element: the two deformable plates are modelled by special plate elements while the intermediate dissipative layer is modelled with interface elements. This interface element is based on the first-order shear deformation theory and assume constant peel and shear stresses in the polymer thickness. This element couples the lower and upper layers without additional degrees of freedom. The partition of unity finite element method (PUFEM) is applied to the development of enriched Mindlin plate elements. The element shape functions are obtained as the product of<p>partition of unity functions with arbitrary chosen enrichment functions. Polynomial enrichment leads to the generation of high-order polynomial shape functions and is therefore similar to a p-FEM technique. Numerical examples illustrate the use of both PUFEM Mindlin plate elements and interface elements for the simulation of viscoelastic sandwich structures. / Doctorat en sciences appliquées / info:eu-repo/semantics/nonPublished

[pt] A célula solar de banda intermediária (IBSC) é um dispositivo de terceira geração alternativo à célula solar de junção única e permite atingir maior eficiência mantendo a simplicidade de ter apenas uma junção pn, garantindo baixo custo e baixa complexidade de fabricação. Nesta tese, um extenso trabalho experimental é apresentado, utilizando as técnicas de microscopia de força atômica, microscopia eletrônica de transmissão, catodoluminescência e fotoluminescência, além de extenso trabalho teórico baseado em simulações realizadas com os programas nextnano e SCAPS. Através dos dados obtidos, é discutida a escolha do InGaP para a matriz da célula solar e do InAs para os pontos quânticos; a inclusão das field damping layers, que minimizam o efeito negativo do campo elétrico sobre os pontos quânticos; o desordenamento do InGaP bulk; como pontos quânticos pequenos e com cap layers de menor espessura alteram a tendência de ordenamento das camadas subsequentes de InGaP; a inclusão de uma camada de GaP para garantir a qualidade das interfaces durante o crescimento da estrutura; e a otimização dos pontos quânticos para atingir a energia ideal teórica para a banda intermediária. Cinco estruturas completas de células solares de referência e de banda intermediária baseadas nas discussões apresentadas são então propostas para crescimento futuro. Estas estruturas de IBSC devem apresentar figuras de mérito interessantes, como VOC entre 1,32 eV e 1,44 eV (1; 2), aumento entre 5 por cento e 50 por cento na ISC (3) e baixos efeitos resistivos, garantindo FF alto e eficiências superiores à das células solares de referência. / [en] The intermediate band solar cell (IBSC), an alternative to the single junction solar cell, is a third generation device that achieves greater efficiency while maintaining the simplicity of having only one pn junction, guaranteeing low cost and low complexity to manufacture. In this thesis, an extensive experimental work is presented, using atomic force microscopy, transmission electron microscopy, cathodoluminescence and photoluminescence techniques, in addition to an extensive theoretical work based in simulations performed with nextnano and SCAPS softwares. Through the obtained data, the choice of InGaP for the solar cell matrix and InAs for the quantum dots; the inclusion of field damping layers to minimize the negative effect of the electric field on the quantum dots; the disordering of bulk InGaP; the way small quantum dots with thinner cap layers alter the ordering tendency of subsequent layers of InGaP; the inclusion of a GaP layer to ensure the interfaces’ quality during the structure s growth; and the quantum dots optimization to reach the intermediate band ideal theoretical energy are discussed. Five complete structures for reference and intermediate band solar cells based in the presented discussions are then proposed for future growth. These IBSC structures should present interesting figures of merit, such as a VOC ranging between 1,32 eV and 1,44 eV (1; 2), an increase between 5 per cent and 50 per cent in ISC (3) and low resistance effects, ensuring a high FF and efficiencies superior to the reference solar cells.