To see the other types of publications on this topic, follow the link: Friction models.
Dissertations / Theses on the topic 'Friction models'
Create a spot-on reference in APA, MLA, Chicago, Harvard, and other styles
Consult the top 50 dissertations / theses for your research on the topic 'Friction models.'
Next to every source in the list of references, there is an 'Add to bibliography' button. Press on it, and we will generate automatically the bibliographic reference to the chosen work in the citation style you need: APA, MLA, Harvard, Chicago, Vancouver, etc.
You can also download the full text of the academic publication as pdf and read online its abstract whenever available in the metadata.
Browse dissertations / theses on a wide variety of disciplines and organise your bibliography correctly.
1
Liu, Chi-hong, and 廖志航. "Base friction modelling of discontinuous rock masses." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2002. http://hub.hku.hk/bib/B42577123.
Full text
2
Do, Nguyen Ba. "Modeling of Frictional Contact Conditions in Structures." Thesis, Georgia Institute of Technology, 2005. http://hdl.handle.net/1853/7123.
Full textAbstract:
This thesis explores two aspects of modeling the behavior of joint friction in structures. The first aspect deals with the accurate and efficient simulation of a simple system that incorporates the LuGre friction law. Energy transfer and dissipation in a structural joint model is the second topic of this thesis. It is hypothesized that friction could serve to pump energy from one frequency to higher frequencies where it might be dissipated more quickly. Motivation for this study stems from the need to have accurate models of high-precision space structures. Because friction at connecting joints plays a major role in the damping capacity of the structure, a good understanding of this mechanism is necessary to predict the vibratory response and enhance the energy dissipation of the structure.
Simulation results of a dynamic system with LuGre friction show that the system is relatively well-conditioned when the slip velocity is small, and ill-conditioned for large slip velocities. Furthermore, the most efficient numerical method to simulate this system is determined to be an implicit integration scheme. To study the energy transfer and dissipation, two models of a jointed structure with friction are considered. Results from the steady-state forced responses of the two structural systems indicate that friction converted low frequency, single harmonic excitation to multi-harmonic response through internal resonances. However, differences in energy dissipation results between the models show that the response of a frictional system is highly sensitive to system parameters and friction laws. Conclusions and suggestions for future research are also discussed.
3
Söderberg, Anders. "Interface modeling - friction and wear." Doctoral thesis, KTH, Maskinkonstruktion (Avd.), 2009. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-9861.
Full textAbstract:
The general trend toward increased use of computer models and simulations during product development calls for accurate and reliable product models. The function of many products relies on contact interfaces between interacting components. Simulating the behavior of such products requires accurate models of both components and interfaces. Depending on the purpose of the simulation, interface models of different degrees of detail are needed. In simulating very large systems with many interfaces, it might be computationally expensive to integrate detailed models of each individual interface. Condensed models, or abstractions, that describe the interface properties with the fewest degrees of freedom are therefore required. This thesis deals with the modeling and simulation of mechanical interfaces in a systems context. The five appended papers discuss the issue from both the simulation and tribological points of view. The aim is to study how friction and wear can be modeled in the behavioral simulation of technical systems and to discuss the convenience and applicability of using different types of models as building blocks of a system model in simulations. Paper A reviews existing friction models of sliding contacts under different running conditions. Paper B uses a simplified contact model, the elastic foundation model, to model friction in a boundary-lubricated rolling and sliding contact. The model is integrated into a dynamic rigid body model of a mechanical system, and the system behavior is simulated. Paper C discusses the application of the elastic foundation model to rough surface contact problems and investigates how the error in its results depends on surface roughness. Papers D and E address how the wear of the contact surfaces at the pad-to-rotor interface in a passenger car disc brake can be simulated using finite element analysis (FEA).QC 20100811
4
Thon, Bjørn Olav. "Friction Models for Oscillating Flow in a U-tube." Thesis, Norges teknisk-naturvitenskapelige universitet, Institutt for fysikk, 2014. http://urn.kb.se/resolve?urn=urn:nbn:no:ntnu:diva-24656.
Full textAbstract:
The basic theory of friction in pipe flow was reviewed in the first part of this thesis. Using the results from this review, steady state friction relations were derived for laminar and turbulent pipe flow.Steady state turbulent friction relations were tested in 1D simulations of oscillating flow in a U-tube with a constant inner diameter. The results show that steady friction relations do not provide sufficient damping compared to experimental data. In order to improve the simulation results, systematic tests of some common dynamic friction relations were conducted. The concluding solution to the friction issue is a combination of laminar and turbulent friction relations, with a dynamic correction. The new friction relation was tested in 1D simulations of oscillating flow in a U-tube with a constant inner diameter. Results show that there is very good accordance between simulations and experimental data. The computational results were validated by experimental data from flow cases of varying Reynolds numbers and three different liquids.In the second part of this thesis, experimental work was conducted on a U-tube with an inner diameter change at the bottom of the bend. The damping of the oscillations was observed to be much greater than in the U-tube with a constant inner diameter. A computer program for image analysis was developed to allow for continuous measurements of the liquid interface level in the U-tube experiments. The program returned satisfying results, both with respect to accuracy and reliability. However, it was concluded that the results from the program could have been better if the U-tube had been made from another material.Simulations of oscillating flow in the U-tube with an inner diameter change were done in order to further investigate the new dynamic friction relation. Comparisons of simulated and experimental results indicated that the friction loss in the diameter change coupling caused inaccuracies in the simulations.
5
Colville, Kevin William. "Friction models in the solution of nonstationary contact problems." Master's thesis, University of Cape Town, 1993. http://hdl.handle.net/11427/17334.
Full textAbstract:
Bibliography: pages 82-83.In most implementations of the finite element method for the solution of contact problems the model of friction used is the classic Amontons-Coulomb. This dissertation is an attempt to rectify the current situation by considering four more advanced friction models, and coding them in FORTRAN for use with the finite element program ABAQUS. The new models are: a quasi-steady-state sliding model proposed by Zhang, Moslehy and Rice; a nonlinear pressure-dependent model proposed by Wriggers, vu Van and Stein; and a model that includes a film of lubricant proposed by Wilson, Hsu and Huang. The friction models are described in detail, including the algorithmic implementation. The contact problem is then formulated in the Total Lagrangian and Updated Lagrangian formulations for contact between an elastic-plastic (Mises plasticity) body and a rigid tool. The variational (weak) form of the formulation is given and this is then discretised by the finite element method. To test and compare the models three common metal forming processes are simulated: hemispherical punching of a disk, two-dimensional plane strain and three-dimensional cold rolling of a strip, and axisymmetric cup deep-drawing. The results are presented in the form of contour plots of the second invariant of stress (Mises), and the plastic yield and maximum stress. Also graphs for the thickness strain are given. These results are presented for each combination of friction model and process to allow easy comparison of frictional behaviour.
6
Kim, Dooroo. "Dynamic modeling of belt drives using the elastic/perfectly-plastic friction law." Thesis, Atlanta, Ga. : Georgia Institute of Technology, 2009. http://hdl.handle.net/1853/29637.
Full textAbstract:
Thesis (M. S.)--Mechanical Engineering, Georgia Institute of Technology, 2010.Committee Chair: Leamy, Michael; Committee Member: Costello, Mark; Committee Member: Ferri, Aldo. Part of the SMARTech Electronic Thesis and Dissertation Collection.
7
Sextro, Walter. "Dynamical contact problems with friction : models, methods, experiments and applications /." Berlin Heidelberg New York, NY Springer, 2002. http://deposit.d-nb.de/cgi-bin/dokserv?id=2889583&prov=M&dok_var=1&dok_ext=htm.
Full text
8
Sextro, Walter. "Dynamical contact problems with friction : models, methods, experiments and applications /." Berlin [u.a.] : Springer, 2007. http://www.loc.gov/catdir/toc/fy0711/2006940067.html.
Full text
9
Sun, Tek-kei, and 孫廸麒. "Numerical modeling of skin friction and penetration problems in geotechnical engineering." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2013. http://hdl.handle.net/10722/195991.
Full textAbstract:
Numerical modeling using finite element method (FEM) is well-recognized as a powerful method for both engineers and researchers to solve boundary value problems. In the modeling of geotechnical problems, the analyses are often limited to simple static problems with either steady-state effective or total stress approach while the transient response (development and dissipation of excess pore water pressure, uex) is seldom considered. Besides, infinitesimal small soil deformation is usually assumed. The simulation is further complicated when the soil-structure interaction problems involve significant soil displacements; like a pile subject to negative skin friction (NSF) and a cone/pile penetration. However, conventional FEM analysis prematurely terminates due primarily to excessive mesh distortion. One could see that simulating a transient problem with large deformation and distortion remains a great challenge. In this study, advanced FE simulations are performed to give new insights into the problems of (1) a pile subject to NSF; and (2) a cone penetration. The transient response of the NSF problem is modeled with the fluid-coupled consolidation technique and geometric nonlinearity. The fluid-coupled cone penetration problem is modeled with a newly developed adaptive approach.
The NSF and cone penetration simulations involve complex soil-structure interface modeling. Two types of modified interface responses are developed and verified which consider fluid coupling. The developed algorithm is applied to back analyze a case history of a pile subject to NSF induced by surcharge loading. Promising results were shown. Development of dragload and neutral plane (NP) with time is studied. NP locates at 75% of the pile embedded length (D) in long-term. Next, a parametric study is performed to investigate the influences of pile geometries, ground compressibility and loading conditions towards the pile responses. The long-term NP locates at around 0.55D to 0.65D in the studied engineering scenarios. The maximum downdrag can be up to 10% of the pile diameter. NP shifts upward when the head load increases. A simple design chart is proposed which helps engineers to estimate the long-term axial load distribution. An illustrative example is given to demonstrate the application and performance of the chart.
The study is extended to investigate the cone penetration problem. An advanced adaptive method is developed and implemented into the FE package ABAQUS to resolve the problems of numerical instability, excessive mesh distortion and premature termination. The proposed method is verified by modeling a ground consolidation problem. Next, total stress back analysis of cone penetration is conducted with the proposed method. The development of cone factor predicted by the proposed method gives a better match with the laboratory result when comparing with the built-in ALE method. Next, the development and dissipation of uex during cone advancing with the proposed method and fluid-coupled technique is investigated. uex develops dramatically around the cone tip. The soil permeability is back calculated from the dissipation test and agrees well with the input value. It is believed that the construction effects of a press-in pile and the subsequence NSF on that pile can be modeled by utilizing the finding of this study.published_or_final_version
Civil Engineering
Doctoral
Doctor of Philosophy
10
Serafin, Stefania. "The sound of friction : real time models, playability and musical applications /." May be available electronically:, 2004. http://proquest.umi.com/login?COPT=REJTPTU1MTUmSU5UPTAmVkVSPTI=&clientId=12498.
Full text
11
Khatibi, Rahman Haghi. "Mathematical open channel flow models and identification of their friction parameters." Thesis, University of London, 1989. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.263145.
Full textAbstract:
This thesis l concerned with the mathematical modelling of open channel flows governed by the Saint-Venant equations, which are used as a prediction or identification tools. A survey of the literature in these fields identified the problems in need of Immediate research. Numerical test runs were then devised which led to projecting a clear picture as follows. The performance of twn widely used Implicit finite difference schemes, the 4-point box and 6-point staggered schemes were compared In a wide range of circumstances. it is concluded that both schemes produce 'very close results, but the staggered scheme is prone to convergence problems In some extreme cases. It was also noted that a sharp change in geometric configuration of compound channels produced discontinuous features on the aim ulated depth and discharge hydrographs. The inability of the staggered scheme In handling a head-discharge relationship as a downstream boundary condition was tackled by proposing and implementing a scheme of second order accuracy. As model data are generally corrupted withh errors and noise, their effects together with that of other factors on the Identified friction parameters we Investigated. The results demonstte the paramount Importance of the effect of a choice of objective function on the Identified parameters. While the individual values of the identified M2nning n may vary from one flood event to another, their mean is shown both numerically and rigorously to be dependent upon the choice of objective function. It is shown that an objective function formulated by using absolute errors performs ideally and produces reliable results even in the presence of autocorrelated Gaucian noise samples. The mean of the Identified parameters is also found to be adversely affected if the observation station is affected by localized disturbances. Sensitivity of objective functions to the variation In the value of the friction parameter Is also found to be an Important factor, as Insensitivity leads to ill-conditioning.
12
Schmid, Florian. "Analysis of geometrically non-linear models for contact with dry friction." Berlin Logos-Verl, 2008. http://d-nb.info/99054186X/04.
Full text
13
Zhao, Ming. "Unsteady pipe friction : formulation of efficient models and investigation of existing modeling assumptions /." View abstract or full-text, 2004. http://library.ust.hk/cgi/db/thesis.pl?CIVL%202004%20ZHAO.
Full textAbstract:
Thesis (Ph. D.)--Hong Kong University of Science and Technology, 2004.Includes bibliographical references (leaves 175-184). Also available in electronic version. Access restricted to campus users.
14
Pemberton, W. Patrick. "Predictive relationships in friction stir processing of nickel-aluminum bronze." Thesis, Monterey, Calif. : Naval Postgraduate School, 2005. http://handle.dtic.mil/100.2/ADA441369.
Full textAbstract:
Thesis (M.S. in Mechanical Engineering)--Naval Postgraduate School, September 2005.Thesis Advisor(s): Terry R. McNelley. "September 2005." Includes bibliographical references (p. 45-47). Also available in print.
15
Livingston, Richard Verile. "Comparison of Heat Generation Models in Finite Element Analysis of Friction Welding." BYU ScholarsArchive, 2019. https://scholarsarchive.byu.edu/etd/7686.
Full textAbstract:
Finite element models of friction welding can be used to estimate internal conditions of welds which are useful for weld analysis and developing experimental welding procedures. Many modeling techniques are used to accomplish these goals, each with relative strengths and weaknesses. A comparative analysis of friction welding models using different heat generation methods is presented. The three different heat generation methods examined were viscoplastic friction, constant steady-state generation, and experimentally measured power data. The models were compared against each other using three output measurements: temperature, axial force, and upset. The friction model predicted temperatures within 40 degrees C. Temperature accuracy improved at a higher upset rate and higher spindle speed, when weld samples heated up faster. The model was excellent at predicting upset, with accuracy within 1.5%. Maximum force was predicted within 9-18%. The constant heat generation model typically predicted temperatures within 30 degrees C. Upset was estimated within 7%. Maximum force was predicted within 12% at high feed rates, but accuracy dropped to 28% when feed rate was reduced. The motor power model was the most accurate model at estimating temperature, with a typical accuracy within 25 degrees C. Axial upset was predicted within 5%. Maximum force was predicted within 1-8%, with greater accuracy occurring at higher feed rates.
16
Arvanitaki, A. "Adhesion and traction of viscoelastic contacts." Thesis, Imperial College London, 1996. http://hdl.handle.net/10044/1/8273.
Full text
17
Gregoris, Daniele. "Cosmological models, nonideal fluids and viscous forces in general relativity." Doctoral thesis, Stockholms universitet, Fysikum, 2014. http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-107130.
Full textAbstract:
This thesis addresses the open questions of providing a cosmological model describing an accelerated expanding Universe without violating the energy conditions or a model that contributes to the physical interpretation of the dark energy. The former case is analyzed considering a closed model based on a regular lattice of black holes using the Einstein equation in vacuum. In the latter case I will connect the dark energy to the Shan-Chen equation of state. A comparison between these two proposals is then discussed. As a complementary topic I will discuss the motion of test particles in a general relativistic spacetime undergoing friction effects. This is modeled following the formalism of Poynting-Robertson whose link with the Stokes’ formula is presented. The cases of geodesic and non-geodesic motion are compared and contrasted for Schwarzschild, Tolman, Pant-Sah and Friedman metrics respectively.
18
Baykara, Berkay. "Control Of Systems Under The Effect Of Friction." Master's thesis, METU, 2009. http://etd.lib.metu.edu.tr/upload/12611327/index.pdf.
Full textAbstract:
Precision control under the effect of friction requires an effective compensation of friction. Since friction has a complex and highly nonlinear behaviour, it is generally insufficient to represent the friction in a dynamic control
system only with a linear viscous model, which is mostly valid in high-velocity motions. Especially when the control system moves near zero velocity regions or changes the direction of motion, an accurate modelling of friction including the lowvelocity dynamic behaviour is a prerequisite to obtain a more complete and realistic dynamic model of the system. Furthermore, the parameters of the friction model
should be identified as accurate as possible in order to attain a satisfactory performance. Therefore, the parameters of the friction should be estimated regarding the working conditions. The estimated friction force can then be used to improve the
controlled performance of the dynamic system under consideration.
In this thesis, the modelling, identification and compensation of friction in a rotary mechanical system are studied. The effectiveness of the existing friction models in the literature are investigatednamely the classical Coulomb with viscous friction model, the Stribeck friction model, the LuGre friction model, and the Generalized Maxwell-Slip (GMS) friction model. All friction models are applied to the system together with the same linear, proportional with derivative (PD)-type and proportional with integral and derivative (PID)-type feedback control actions for the sake of being faithful in comparison. The accuracy of the friction compensation methods is examined separately for both the low-velocity and high-velocity motions of the system. The precision of friction estimation is also shown in the case of using both the desired velocity and measured velocity as an input to the friction models. These control studies are verified in simulation environment and the corresponding results are given. Furthermore, an experimental set-up is designed and manufactured as a case study. The parameters of the aforementioned friction models are identified and the control laws with different friction models are applied to the system in order to demonstrate the compensation capabilities of the models. The results of the experiments are evaluated by comparing them among each other and with the simulation results.
19
Gandhi, Satish. "Study of Friction Effects on System Dynamics using Low-Order Lumped-Parameter Models." University of Cincinnati / OhioLINK, 2002. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1029423191.
Full text
20
Kaneko, Yoshihiro Clayton Robert W. Lapusta Nadia. "Investigations of earthquake source processes based on fault models with variable friction rheology /." Diss., Pasadena, Calif. : California Institute of Technology, 2009. http://resolver.caltech.edu/CaltechETD:etd-04282009-202026.
Full text
21
DESHMUKH, DINAR V. "PHYSICS BASED REDUCED ORDER MODELS FOR FRICTIONAL CONTACTS." University of Cincinnati / OhioLINK, 2005. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1115997302.
Full text
22
Mijena, Ermias Hailu. "A comparison of friction piles bearing capacity based on theoretical and empirical mathematical models." Thesis, Norges teknisk-naturvitenskapelige universitet, Institutt for bygg, anlegg og transport, 2012. http://urn.kb.se/resolve?urn=urn:nbn:no:ntnu:diva-19210.
Full textAbstract:
This thesis consists of 8 chapters and each chapter is described as follows:Chapter 2: Presents the different types of piles which exist today and their classification.Chapter 3: Presents the different soil parameters we use in pile analyses and design and the properties of these parameters.Chapter 4: Presents the different empirical and theoretical static pile capacity equation. It also presents the effect of time and plugging on pile capacity.Chapter 5: Presents pile dynamics, static and dynamic load tests on piles.Chapter 6: Presents a case study which is a case of a bridge project on the Drammen selva river in Drammen, Norway.Chapter 7: Presents the analysis and results obtained for the case study described in chapter 6.Chapter 8: It is the last part of the work which presents conclusions and possible future work.
23
KONDEPUDI, RAMABALARAJENDRASESH. "NUMERICAL ANALYSIS OF LUMPED PARAMETER DYNAMIC SYSTEMS WITH FRICTION." University of Cincinnati / OhioLINK, 2004. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1083622496.
Full text
24
Cigeroglu, Ender. "Development of microslip friction models and forced response prediction methods for frictionally constrained turbine blades." Columbus, Ohio : Ohio State University, 2007. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=osu1181856489.
Full text
25
Mastricola, Nicholas Palma. "Nonlinear Stiffness and Edge Friction Characterization of Coned Disk Springs." The Ohio State University, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=osu1480346443676492.
Full text
26
Borst, Rikard. "Utvärdering och implementering av automatiska farthållare i fordonssimulator." Thesis, Linköping University, Department of Electrical Engineering, 2006. http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-6116.
Full textAbstract:
Vehicle simulators are becoming more common in vehicle industries. Company earns lot of money on simulations instead of real tests. Real tests are necessary but not made so extensively as before.
In this thesis the building of an vehice simulator will be described and a comparison between three different cruise controls. The three cruise controls are PI-regulator, a regulator who regulates after positions in the terrain and a MPC-regulator. The reason for choosing this three is to see the difference between simple regulation and more complex regulation with respect to fuel consumption, travel time and complexity.
The vehicle simulator is made in Matlab/Simulink, Visual Studio and Open Scene Graph. The facilities needed for runnning the simulator is a relative good computer with a grapics card on at least 128 MB RAM plus a steering wheel and pedals for brake and gas to achieve best feeling. A keyboard can be used but it reduces almost all feeling.
After several simulations a conclusion was made. The MPC-regulator was the regulator who consumed least fuel and travel time. The regulator who regulates after positions in the terrain was not too far away. It would be interesting to do more research about it. In fact it is only a PI-regulator who makes ``clever'' decisions when a hill with enough slope appears. With enough slope means a downhill where the vehicle can accelerate without the use of fuel and an uphill where the vehicle can not keep its speed with maximum use of fuel.
A conclusion was stated that the friction and height profile influenced on settings for the PI-regulator and with some adjustments on this settings, fuel could be saved.
27
Covington, Joshua L. "Experimental and Numerical Investigation of Tool Heating During Friction Stir Welding." Diss., CLICK HERE for online access, 2005. http://contentdm.lib.byu.edu/ETD/image/etd961.pdf.
Full text
28
Mergel, Janine Cornelia [Verfasser], Roger Andrew [Akademischer Betreuer] Sauer, and Lorenzis Laura [Akademischer Betreuer] De. "Advanced computational models for the analysis of adhesive friction / Janine Cornelia Mergel ; Roger Andrew Sauer, Laura De Lorenzis." Aachen : Universitätsbibliothek der RWTH Aachen, 2017. http://d-nb.info/1187251348/34.
Full text
29
Bhagwat, Pushkaraj. "A Computational Study of the Effects of Plasticity and Damage Models in Microscopic and Macroscopic Static Metal Friction." University of Cincinnati / OhioLINK, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1461593775.
Full text
30
Geier, Martin. "Considerações sobre o atrito para processos de forjamento a frio através do ensaio de compressão do anel." reponame:Biblioteca Digital de Teses e Dissertações da UFRGS, 2007. http://hdl.handle.net/10183/15298.
Full textAbstract:
São realizadas análises teóricas e experimentais do atrito nos processos de forjamento a frio, através do ensaio de compressão do anel. Os modelos de atrito de Amonton-Coulomb (μ), atrito interfacial (m) e de Levanov (f) são investigados com auxílio de softwares comerciais de simulação de processos de forjamento. Diferentes condições de lubrificação foram aplicadas para o aço baixa liga 16MnCr5 e a liga de alumínio AA6351 nas condições recozido e encruado. O atrito foi analisado, utilizando os softwares MSC.Superforge e Qform, em função dos resultados obtidos e as condições de lubrificação e estado do material. Valores paramétricos do atrito adquiridos através de correlação teórico-experimental são condizentes com a literatura, indicando a relação do atrito com o material de trabalho, inclusive com seu grau de encruamento. Os modelos de atrito m e f apresentaram maior sensibilidade com relação ao estado do material.Experimental and theoretical analyses of friction in cold forging process are evaluated by means of the ring compression test. Friction models from Amonton- Coulomb (μ), interfacial friction (m) and Levanov’s model (f) are investigated by aid of numerical simulation software. Different lubricants are applied for low carbon steel alloy 16MnCr5 under annealed and work-hardened conditions. Friction is analyzed by means of lubricant and material conditions using MSC.Superforge and Qform numerical simulation softwares. Acquired frictional parameter values obtained by theoretical-experimental correlation agree with literature, showing friction’s relationship with workpiece material and its hardening level. Friction models m and f presented better sensitivity when concerning the material’s hardening level.
31
Posada, Maria. "Comparison of 3-D Friction Stir Welding Viscoplastic Finite Element Model with Weld Data and Physically-Simulated Data." BYU ScholarsArchive, 2012. https://scholarsarchive.byu.edu/etd/3494.
Full textAbstract:
Models (both physical and numerical) of the friction stir (FS) welding process are used to develop a greater understanding of the influence of independent process parameters on dependent process output variables, such as torque, power, specific weld energy, peak temperature, cooling rates and various metallurgical factors (e.g., grain size and precipitates). An understanding of how the independent process parameters influence output variables and ultimately their effect on resultant properties (e.g., strength, hardness, etc..) is desirable. Most models developed have been validated primarily for aluminum alloys with relatively small amounts of experimental data. Fewer models have been validated for steels or stainless steels, particularly since steels and stainless steels have proven more challenging to friction stir than aluminum alloys. The Gleeble system is also a powerful tool with the capability to perform thermomechanical simulations in a known and controlled environment and provide physical representation of resultant microstructure and hardness values. The coupling of experimental data and physical simulated data can be extremely useful in assessing the capabilities of friction stir numerical process models. The overall approach is to evaluate Isaiah an existing three-dimensional finite element code developed at Cornell University by comparing against experimental and physically-simulated data to determine how well the code output relates to real FS data over a range of nine processing conditions. Physical simulations replicating select thermomechanical streamline histories were conducted to provide a physical representation of resultant metallurgy and hardness. Isaiah shows promise in predicting qualitative trends over a limited range of parameters and is not recommended for use as a predictive tool but rather a complimentary tool, Once properly calibrated, the Isaiah code can be a powerful tool to gain insight into the process, strength evolution during the process and coupled with a texture evolution model may also provide insight into microstructural and texture evolution over a range for which it is calibrated.
32
Schöfisch, Thorben. "Quantifying strain in analogue models simulating fold-and-thrust belts using magnetic fabric analysis." Licentiate thesis, Uppsala universitet, Mineralogi, petrologi och tektonik, 2021. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-451977.
Full textAbstract:
Applying the anisotropy of magnetic susceptibility to analogue models provides detailed insights into the strain distribution and quantification of deformation within contractional tectonic settings like fold-and-thrust belts (FTBs). Shortening in FTBs is accommodated by layer-parallel shortening, folding, and thrusting. The models in this research reflect the different deformation processes and the resulting magnetic fabric can be attributed to thrusting, folding and layer-parallel shortening. Thrusting develops a magnetic foliation parallel to the thrust surface, whereas folding and penetrative strain develop a magnetic lineation perpendicular to the shorting direction but parallel to the bedding. These fabric types can be observed in the first model of this study, which simulated a FTB shortened above two adjacent décollements with different frictional properties. The different friction coefficients along the décollements have not only an effect on the geometric and kinematic evolution of a FTB, but also on the strain distribution and magnitude of strain within the belt. The second series of models performed in this study show the development of a thrust imbricate and the strain distribution across a single imbricate in more detail. Three models, with similar setup but different magnitudes of bulk shortening, show strain gradients by gradual changes in principal axes orientations and decrease in degree of anisotropy with decreasing distance to thrusts and kinkzones. These models show that at the beginning of shortening, strain is accommodated mainly by penetrative strain. With further shortening, formation of thrusts and kinkzones overprint the magnetic fabric locally and the degree of anisotropy is decreasing within the deformation zones. At thrusts, an overprint of the magnetic fabric prior deformation towards a magnetic foliation parallel to the thrust surfaces can be observed. A rather complex interplay between thrusting and folding can be analysed in the kinkzones. In general, this thesis outlines the characteristics of magnetic fabric observed in FTBs, relates different types of magnetic fabric to different processes of deformation and provides insights into the strain distribution of FTBs.
33
Anderson, Warwick Wyndham. "An Investigation of Dividend Signalling on the New Zealand Stock Exchange in the 1990s and of Several New Tools Employable in such an Investigation." Thesis, University of Canterbury. Accountancy, Finance and Information Systems, 2006. http://hdl.handle.net/10092/861.
Full textAbstract:
This thesis investigates the nature of joint dividend-and-earnings signalling in announcements to the New Zealand Stock Exchange in the 1990s. Initially the Market Model is used to compute expected returns, and the abnormal returns derived from these are subjected to restricted least squares regressions to separate out a putative dividend signal from the concurrent earnings signal. But with the Market Model, the zero-value company returns associated with an absence of trading in thinly traded stocks are over-represented in returns distributions leading to problems of bias. New models are developed that explicitly exploit zero returns. The first alternative methodology entails friction modelling, which uses a maximum likelihood estimation procedure to find the relationship coefficients and the range of returns that should be considered as zero, and then proceeds to treat them as a separate category. The second alternative methodology is that of state asset models, which take a fresh new look at investor perceptions of the connection between movements in company returns and those of the concurrent underlying market. Zero-value company returns cease to be zero in value, where a state model is rotated, or alternatively they can be modelled as an extra state. All three methodologies furnish some evidence of dividend signalling; but this evidence is highly dependent on small changes within the given methodology.
34
Veldhuizen, Todd. "Grid Filters for Local Nonlinear Image Restoration." Thesis, University of Waterloo, 1998. http://hdl.handle.net/10012/943.
Full textAbstract:
A new approach to local nonlinear image restoration is described, based on approximating functions using a regular grid of points in a many-dimensional space. Symmetry reductions and compression of the sparse grid make it feasible to work with twelve-dimensional grids as large as 2212. Unlike polynomials and neural networks whose filtering complexity per pixel is linear in the number of filter co-efficients, grid filters have O(1) complexity per pixel. Grid filters require only a single presentation of the training samples, are numerically stable, leave unusual image features unchanged, and are a superset of order statistic filters. Results are presented for additive noise, blurring, and superresolution.
35
Lahey, Timothy. "Modelling Hysteresis in the Bending of Fabrics." Thesis, University of Waterloo, 2002. http://hdl.handle.net/10012/941.
Full textAbstract:
This thesis presents a model of fabric bending hysteresis. The hysteresis model is designed to reproduce the fabric bending measurements taken by the Kawabata Evaluation System (KES) and the model parameters can be derived directly from these property measurements. The advantage to using this technique is that it provides the ability to simulate a continuum of property curves. Results of the model and its components are compared and constrasted with experimental results for fabrics composed of different weaves and yarn types. An attempt to incorporate the bending model as part of a fabric drape simulation is also made.
36
Lynam, Thomas. "Exploitation of Redundant Inverse Term Frequency for Answer Extraction." Thesis, University of Waterloo, 2002. http://hdl.handle.net/10012/1190.
Full textAbstract:
An automatic question answering system must find, within a corpus,short factual answers to questions posed in natural language. The process involves analyzing the question, retrieving information related to the question, and extracting answers from the retrieved information. This thesis presents a novel approach to answer extraction in an automated question answering (QA) system. The answer extraction approach is an extension of the MultiText QA system. This system employs a question analysis component to examine the question and to produce query terms for the retrieval component which extracts several document fragments from the corpus. The answer extraction component selects a few short answers from these fragments. This thesis describes the design and evaluation of the Redundant Inverse Term Frequency (RITF) answer extraction component. The RITF algorithm locates and evaluates words from the passages that are likely to be associated with the answer. Answers are selected by finding short fragments of text that contain the most likely words based on: the frequency of the words in the corpus, the number of fragments in which the word occurs, the rank of the passages as determined by the IR, the distance of the word from the centre of the fragment, and category information found through question analysis. RITF makes a substantial contribution in overall results, nearly doubling the Mean Reciprocal Rank (MRR), a standard measure for evaluating QA systems.
37
Seveso, S. L. "ADVANCES IN MODELS OF PULSAR GLITCHES." Doctoral thesis, Università degli Studi di Milano, 2015. http://hdl.handle.net/2434/255803.
Full textAbstract:
Neutron stars are surely one of the most interesting astronomical objects: in no other place of the observable universe, in fact, matter is so compressed that the density reaches and overcomes the nuclear saturation value. This is a so extreme condition that in no terrestrial laboratory we can directly reproduce it in order to study its properties. Neutron stars are therefore a very fascinating research field that can bring us to a deeper understanding of this exotic matter, by modeling the observations of peculiar phenomena related to these stars. This thesis is focused on the pulsar glitches, which are rapid jumps in the rotational velocity of the star.
Shortly after the first glitches were observed it was suggested that they could be due to a superfluid component in the stellar interior that could store angular momentum thanks to the pinning interaction between superfluid vortexes and crustal lattice. This qualitative idea suggests that the problem must be faced by merging results which come from the microphysics point of view into a more macroscopic simulation. In this thesis we deal with the evaluation of the vortex--nuclei interaction from the mesoscopic point of view: we perform a realistic calculation of the pinning force per unit length for the inner crust of a neutron star. We find that this mesoscopic interaction is $\approx10^{15} \mbox{ dyn/cm}$, much smaller than previous calculations but still enough to explain glitches. A similar approach has been adopted also in the calculation of the pinning force in the core: if protons in the interior are in a type II superconducting state, an interaction between magnetic flux tubes and rotational vortexes is possible and must be estimated (in case of type I superconductivity the interactions are much weaker). Our results indicate that even in this case, the interaction between vortexes and flux tubes will be significantly weaker than in the crust, as the force per unit length is $\approx 10^{12} - 10^{13} \mbox{ dyn/cm}$.
These results are used in static models (based on the "snowplow" model) which can reproduce the physical observable parameters and also provide a mean to infer the masses of the most frequent glitching stars: we propose a unified description of the glitch phenomenon both for small and large glitchers and we analyze the interesting correlation between mass and glitching strength.
The whole temporal evolution of a glitch can be followed thanks to the development of a dynamical model. The multifluids formalism is implemented in a consistent simulation, which take into account realistic physical inputs like equation of state, entrainment and drag forces: the rise and the recovery of a glitch is analyzed in connection with the relevant parameters of the model, in order to understand more deeply all the aspects of this phenomenon. We also compare our simulations with the observational data of the frequent glitchers in order to estimate the masses of the pulsars. It's noteworthy the fact that the results are in good agreement with the ones obtained with the static approach, showing again the same mass--glitching strength relation.
38
Miti, Filippo. "Mathematical models for cellular aggregation: the chemotactic instability and clustering formation." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2016. http://amslaurea.unibo.it/12020/.
Full textAbstract:
In this thesis we present a mathematical formulation of the interaction between microorganisms such as bacteria or amoebae and chemicals, often produced by the organisms themselves. This interaction is called chemotaxis and leads to cellular aggregation. We derive some models to describe chemotaxis. The first is the pioneristic Keller-Segel parabolic-parabolic model and it is derived by two different frameworks: a macroscopic perspective and a microscopic perspective, in which we start with a stochastic differential equation and we perform a mean-field approximation. This parabolic model may be generalized by the introduction of a degenerate diffusion parameter, which depends on the density itself via a power law. Then we derive a model for chemotaxis based on Cattaneo's law of heat propagation with finite speed, which is a hyperbolic model. The last model proposed here is a hydrodynamic model, which takes into account the inertia of the system by a friction force. In the limit of strong friction, the model reduces to the parabolic model, whereas in the limit of weak friction, we recover a hyperbolic model. Finally, we analyze the instability condition, which is the condition that leads to aggregation, and we describe the different kinds of aggregates we may obtain: the parabolic models lead to clusters or peaks whereas the hyperbolic models lead to the formation of network patterns or filaments. Moreover, we discuss the analogy between bacterial colonies and self gravitating systems by comparing the chemotactic collapse and the gravitational collapse (Jeans instability).
39
Nilfouroushan, Faramarz. "Ground Movements in the Zagros Fold-Thrust Belt of SW Iran Measured by GPS and InSAR Compared to Physical Models." Doctoral thesis, Uppsala University, Department of Earth Sciences, 2007. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-7928.
Full textAbstract:
This thesis uses geodetic satellite data to measure present-day crustal deformation in the Zagros fold-thrust belt (SW Iran). Geodetic-type measurements are also used in down-scaled models that simulate the surface deformations seen in convergent settings like the Zagros fold-thrust belt.
Global Positioning System (GPS) measurements of three surveys between 1998 and 2001 indicate 9 ± 3 mm/yr and 5 ± 3 mm/yr shortening across the SE and NW Zagros respectively. GPS results show that in addition to the different rates and directions of shortening on either side of the NS trending Kazerun fault, local along-belt extension occurs to the east.
Differential SAR interferograms of ERS1 & 2 images between 1992 and 1999 detect 8 ± 4 mm/yr uplift rate across a newly recognized fault in SW Qeshm Island. This can be attributed to a steep imbricate thrust that may still represent the local Zagros deformation front.
The salt diapirs in the Zagros rise from a source layer that acts as a low-frictional decollement that decouples the deformation of the cover sediments from their basement in the eastern Zagros whereas the cover to the west deforms above a high-friction decollement. Physical models were prepared to simulate cover deformation in the Zagros by shortening a sand pack above adjacent high- and low-frictional decollements (represented by a ductile layer). The strain distributions differed above the two types of decollements; it was more heterogeneous above the salt where local extension in the shortening direction was dominant. A separate work also investigated systematically the role of basal friction on cover deformation in convergent settings. Accurate height measurements of the model surface by laser-scanner indicated a deformation front more distal than usual, particularly in the low-basal frictional models. The volume reduction in our shortened sand models correlated directly with their basal friction.
40
Miyasato, Hugo Heidy 1986. "Modeling of the clutch squeal phenomenon and practical possibilities for its mitigation = Modelagem do fenômeno de squeal de embreagens e possibilidades práticas para sua atenuação." [s.n.], 2015. http://repositorio.unicamp.br/jspui/handle/REPOSIP/265816.
Full textAbstract:
Orientador: Milton Dias JuniorTese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecânica
Made available in DSpace on 2018-08-27T02:04:03Z (GMT). No. of bitstreams: 1 Miyasato_HugoHeidy_D.pdf: 11029202 bytes, checksum: 6dc0ca80ebb19f846ccecdc9fe6870e3 (MD5) Previous issue date: 2015
Resumo: O trem de potência é um sistema complexo, uma vez que envolve a interação entre componentes tais como o motor, a embreagem, a transmissão e as rodas. Diversos fenômenos de ruído, vibração e sua severidade (NVH) podem surgir durante o seu funcionamento (judder, clonk, shuffle, rattle, etc.). O squeal de embreagens é um fenômeno relativamente novo desse sistema e ele surge como um ruído de tom único, especialmente em situações de partida na fase de modulação da embreagem. Componentes tais como as molas cushion, membrana e de retrocesso foram modeladas a fim de permitir a criação de diferentes projetos e combinações em termos de posição e propriedades físicas. Um mecanismo de estabilização para a placa de pressão foi testado numericamente para servir como uma alternativa para o sistema quando não existem possibilidades de modificações nos seus parâmetros. Discos de embreagens reais contêm imperfeições devido à utilização, tolerâncias da manufatura, etc. Para levar esses fatos em consideração, um elemento rotativo de rigidez/amortecimento viscoso com atrito foi criado para representar as molas cushion e suportar distribuições arbitrárias ao longo do perímetro do disco, permitindo que cada elemento tenha suas próprias características. A suposição de amortecimento viscoso introduziu termos relacionados à velocidade de rotação do disco de embreagem na matriz de rigidez. Simulações com pequenos erros de posicionamento nas molas cushion geraram acoplamento entre a vibração axial da placa de pressão e seus deslocamentos angulares. A mola membrana foi modelada como uma alavanca, permitindo a introdução de medições. Isso foi importante para identificar regiões de rigidez negativa e positiva, explicando sua interação com uma curva de cushion real. As molas de retrocesso foram modeladas, resultando numa representação matricial na qual foi possível gerar termos de rigidez positiva fora da diagonal principal. Finalmente, o mecanismo de estabilização para o sistema consistiu de outro disco conectado à placa de pressão. Quando as propriedades do dispositivo foram ajustadas apropriadamente, ele foi efetivo em uma situação de veering. Nessa condição, as coordenadas de ambos os discos foram combinadas nas formas modais, causando movimentos de wobbling fora de fase que resultaram em dissipação de energia nos elementos de conexão
Abstract: The powertrain is a complex system, once that it involves the interaction between components such as the engine, the clutch, the transmission and the wheels. Several noise, vibration and harshness (NVH) phenomena may appear during its operation (judder, clonk, shuffle, rattle, etc.). Clutch squeal is a relatively new phenomenon of this system and it appears as a single tone noise, especially on a drive-off condition on the modulation phase of the clutch. Components such as the cushion, diaphragm and leaf springs were modelled to allow the creation of different designs and combinations in terms of position or physical properties. A stabilization device for the pressure plate was numerically tested to serve as an alternative to the system when there are no practical possibilities of modifications on its parameters. Real clutch discs contain imperfections due to usage, manufacturing tolerances, etc. To take these facts into consideration, a rotating stiffness/viscous damper element with friction was created to represent the cushion springs and support arbitrary distributions along the perimeter of the disc, allowing each element to have its own characteristics. The assumption of viscous damping introduced terms related to the clutch disc rotating speed on the stiffness matrix. Simulations with small position errors on the cushion spring generated coupling between the pressure plate axial vibration and its angular displacements. The diaphragm spring was modelled as a lever, allowing the introduction of measurements. It was important to identify regions of negative and positive stiffness, explaining its interaction with a real cushion curve. The leaf springs were modelled, which resulted on a matrix representation where it was possible to generate positive stiffness terms out of its main diagonal. Finally, the stabilization device for the system consisted of another disc connected to the pressure plate. When the device properties were properly adjusted, it was affective in a situation of veering. In this condition, the coordinates of both discs were combined on the mode shapes, causing out of phase wobbling motions, that resulted on energy dissipation on the connection elements
Doutorado
Mecanica dos Sólidos e Projeto Mecanico
Doutor em Engenharia Mecânica
41
Seo, Na Jin. "Biomechanical models of hand coupling for axial torque and push exertions effects of torque direction, hand-handle friction, and handle size on axial torque and push exertions for cylindrical handles." Saarbrücken VDM Verlag Dr. Müller, 2007. http://d-nb.info/991124189/04.
Full text
42
Eroz, Murat. "Advanced models for sliding seismic isolation and applications for typical multi-span highway bridges." Diss., Atlanta, Ga. : Georgia Institute of Technology, 2007. http://hdl.handle.net/1853/19709.
Full textAbstract:
Thesis (Ph.D)--Civil and Environmental Engineering, Georgia Institute of Technology, 2008.Committee Chair: DesRoches, Reginald; Committee Member: Goodno, Barry; Committee Member: Jacobs, Laurence; Committee Member: Streator, Jeffrey; Committee Member: White, Donald.
43
Gunawan, Harry Putu. "Numerical simulation of shallow water equations and related models." Thesis, Paris Est, 2015. http://www.theses.fr/2015PEST1010/document.
Full textAbstract:
Cette thèse porte sur l'approximation numérique des équations de Saint-Venant et de quelques problèmes qui leur sont reliés. Dans la première partie, nous analysons les propriétés mathématiques et les applications des schémas numériques sur grilles décalées. La robustesse de ces schémas est prouvée sur des applications telles que les équations de Saint-Venant dans un domaine en rotation, en vue des écoulements géostrophiques, ainsi que l'extension de ces équations au cas visqueux. Dans la seconde partie, nous présentons des modèles basés sur les équations de Saint-Venant. Nous commençons par étudier le couplage avec l'équation d'Exner, qui porte sur le transport des sédiments. Nous observons des propriétés de convergence numérique vers la solution exacte dans un cas de solution analytique, et nous constatons un bon accord avec des données expérimentales dans le cas de la rupture de barrage avec fond érodable. Nous continuons par l'étude d'un schéma numérique, basé sur une méthode de volumes finis colocalisés (HLLC) pour l'approximation du modèle de Richard-Gavrilyuk. Ce modèle étend les équations de Saint-Venant au cas des écoulements avec cisaillement. Des tests numériques montrent la validité du schémaThis thesis is devoted to the numerical approximation of the shallow water equations and of some related models. In the first part, we analyze the mathematical properties and the applications of the staggered grid scheme. The robustness of this scheme is validated on various applications such as the rotating shallow water equations for geostrophic flows model and viscous shallow water equations. In the second part, we consider some related models. Firstly focusing on the coupling between the Exner equation and the shallow water equations, modelling bedload sediment transport, we observe in a particular case the numerical convergence of the scheme to the exact solution, as well as a good agreement with the experimental data in the dam-break with erodible bottom test. Secondly, we present a numerical scheme based on the finite volume collocated scheme (HLLC) in order to approximate the Richard-Gavrilyuk model. This model is an extension of the shallow water model, fit for modelling the shear shallow water flows. Some numerical tests provide a validation of the scheme
44
Rosa, Phillipe Mendes. "Estudo do escoamento turbulento em dutos corrugados com cavidade helicoidal." Universidade Tecnológica Federal do Paraná, 2014. http://repositorio.utfpr.edu.br/jspui/handle/1/1141.
Full textAbstract:
PetrobrásTubos corrugados têm sido utilizados em vários cenários da engenharia como, por exemplo, em linhas flexíveis de transporte de petróleo no mar, onde sua instalação, manuseio e remoção, tornam o processo mais prático devido sua flexibilidade. Entretanto, escoamentos em tubos corrugados, em geral, estão sujeitos a um aumento da perda de carga, aumento da turbulência e a variações dos padrões de escoamento quando comparados ao escoamento normalmente observado em tubos de seção transversal constante. Nesse cenário, o presente trabalho apresenta um estudo numérico e experimental do escoamento turbulento em tubos corrugados com cavidades helicoidais para diferentes ângulos de hélice (ou passo) e números de Reynolds variando entre 7.500 e 100.000. O escoamento é modelado matematicamente a partir das equações de conservação da massa e do balanço da quantidade de movimento, considerando-se o fluido newtoniano e incompressível. Ao escoamento é aplicado o modelo de turbulência de duas equações Baseline Model. O sistema de equações resultante da modelagem matemática é discretizado utilizando o Método Numérico de Volumes Finitos baseado em Elementos (MVFbE), e resolvido através do programa computacional ANSYS-CFX. Para o estudo experimental, que concerne à medição do fator de atrito em geometrias equivalentes às estudadas numericamente, é realizado utilizando a bancada experimental já existente no LACIT/UTFPR. A partir dos resultados numéricos é proposta uma correlação para o fator de atrito, envolvendo parâmetros geométricos do tubo corrugado e número de Reynolds, com um desvio médio de 0,88%. Além disso, é estudada a influência da parede corrugada no comportamento de propriedades como os campos de velocidade, pressão, tensão e outras propriedades turbulentas. Da última análise, constatou-se que a mudança no padrão de escoamento próximo à cavidade é semelhante entre os aumentos do número de Reynolds e largura da cavidade, não apresentando mudanças qualitativas em respeito à variação do ângulo de hélice.
Corrugated pipes have been used in several branches in the engineering, e.g, in transport flex line of oil in sea, where its installation, handling and remotion, become the process more practice due its flexibility. However, flow in corrugated pipes, in general, are subjects to an increase of drop pressure, an increase in the turbulence and deviations in the flow pattern as compared to the flow usually observed in pipes with constant cross section. In this sense, the present work shows an study both numerically and experimental of turbulent flow in corrugated pipes with helical grooves for different helix angles (or pitch) and Reynolds numbers varying between 7.500 and 100.000. The flow is modeled mathematically by the mass conservation and momentum equations, assuming the fluid is Newtonian and incompressible. To the flow, is applied the turbulence model of two equations, Baseline Model. The system equations resulted from mathematical modelling is discretized by using the element-based finite-volume method approach, and solved through the ANSYS-CFX. For the experimental part, is measured the friction factor in geometries equivalent to those studied numerically, where is performed by using the experimental facilities located in the LACIT/UTFPR. With the experimental data and numerical results, an correlation for friction factor is proposed in function of the geometric parameters of the corrugated pipe and Reynolds number, with an average uncertainty 0.88%. Moreover, is studied the groove's influence in the behavior of properties like fields of pressure, velocity, and stresses among another turbulent parameters. From the last analysis, has shown that the flow pattern near to the grooves follows the same behavior when it increases the Reynolds number and the grooves width, not showing qualitative changes when the helix angle has been changed.
45
Rodriguez, obando Diego Jair. "From Deterioration Modeling to Remaining Useful Life Control : a comprehensive framework for post-prognosis decision-making applied to friction drive systems." Thesis, Université Grenoble Alpes (ComUE), 2018. http://www.theses.fr/2018GREAT086/document.
Full textAbstract:
La durée de vie utile résiduelle (RUL) peut être simplement définie comme une prédiction du temps restant pendant lequel un système est capable d'exécuter sa fonction prévue ; elle est mesurée à partir de l'instant présent jusqu'à la défaillance finale. Cette durée prévue dépend principalement de l'état de détérioration des composants du système et de leurs conditions de fonctionnement futures prévues. Ainsi, la prédiction de la RUL est un processus incertain et son contrôle n'est pas une tâche triviale. En général, le but de la prévision de la RUL est d'influencer la prise de décision pour le système. Dans cette thèse, on a présenté un cadre compréhensible pour le contrôle de la RUL. Les incertitudes du modèle ainsi que les perturbations du système ont été prises en compte dans le cadre proposé. Des questions telles que le traitement de l'incertitude et l'inclusion d'objectifs RUL dans la stratégie de contrôle sont étudiées, depuis la modélisation jusqu'à une architecture de contrôle globale finale. On a montré que l'on peut prédire la RUL à partir d'une estimation appropriée de la détérioration et d'hypothèses sur les conditions de fonctionnement futures. Les systèmes d'entraînement par friction sont utilisés pour illustrer l'utilité de l'architecture globale susmentionnée. Pour ce type de système, le frottement est à la fois source du mouvement et source de la détérioration. Ce double caractéristique de frottement est une motivation pour contrôler automatiquement la détérioration du système en maintenant un compromis entre les exigences de mouvement et les valeurs RUL souhaitées. Dans cette thèse, un nouveau modèle orienté contrôle pour les systèmes d'entraînement par friction, qui inclut un modèle dynamique de la détérioration, est proposé. Le degré de détérioration est considéré en fonction de l'énergie dissipée, à la surface de contact, pendant la transmission mécanique de puissance. Une approche est proposée pour estimer l'état actuel de la détérioration d'un système d'entraînement par friction. L'approche est basée sur un Filtre de Kalman Etendu (EKF en anglais) qui utilise un modèle augmenté incluant le système mécanique dynamique et la dynamique de détérioration. L'EKF fournit également des intervalles qui incluent sûrement la valeur de détérioration réelle avec une valeur de probabilité. Une nouvelle architecture de commande de la RUL est proposée, elle comprend : un système de surveillance de l'état de détérioration (par exemple l'EKF proposé), un estimateur de l'état de fonctionnement du système, un système de commande de la RUL et un principe actionneur de la RUL. L'estimateur des conditions de fonctionnement est basé sur l'hypothèse qu'il est possible de quantifier certaines caractéristiques des exigences de mouvement, par exemple le rapport cyclique des couples moteur. Le contrôleur RUL utilise une fonction de coût qui pondère les exigences de mouvement et les valeurs RUL souhaitées pour modifier un filtre à paramètres variables, utilisé ici comme principe actionneur RUL. Le principe actionneur RUL est basé sur une modification des couples exigés, provenant d'un éventuel système de contrôle de mouvement. Les résultats préliminaires montrent qu'il est possible de contrôler la RUL, selon le cadre théorique proposéRemaining Useful Lifetime (RUL) can be simply defined as a prediction of the remaining time that a system is able to perform its intended function, from the current time to the final failure. This predicted time mostly depends on the state of deterioration of the system components and their expected future operating conditions. Thus, the RUL prediction is an uncertain process and its control is not trivial task.In general, the purpose for predicting the RUL is to influence decision-making for the system. In this dissertation a comprehensive framework for controlling the RUL is presented. Model uncertainties as well as system disturbances have been considered into the proposed framework. Issues as uncertainty treatment and inclusion of RUL objectives in the control strategy are studied from the modeling until a final global control architecture. It is shown that the RUL can be predicted from a suitable estimation of the deterioration, and from hypothesis on the future operation conditions. Friction drive systems are used for illustrating the usefulness of the aforementioned global architecture. For this kind of system, the friction is the source of motion and at the same time the source of deterioration. This double characteristic of friction is a motivation for controlling automatically the deterioration of the system by keeping a trade-off, between motion requirements and desired RUL values. In this thesis, a new control-oriented model for friction drive systems, which includes a dynamical model of the deterioration is proposed. The amount of deterioration has been considered as a function of the dissipated energy, at the contact surface, during the mechanical power transmission. An approach to estimate the current deterioration condition of a friction drive system is proposed. The approach is based on an Extended Kalman Filter (EKF) which uses an augmented model including the mechanical dynamical system and the deterioration dynamics. At every time instant, the EKF also provides intervals which surely includes the actual deterioration value which a given probability. A new architecture for controlling the RUL is proposed, which includes: a deterioration condition monitoring system (for instance the proposed EKF), a system operation condition estimator, a RUL controller system, and a RUL actuation principle. The operation condition estimator is based on the assumption that it is possible quantify certain characteristics of the motion requirements, for instance the duty cycle of motor torques. The RUL controller uses a cost function that weights the motion requirements and the desired RUL values to modify a varying-parameter filter, used here as the RUL-actuating-principle. The RUL-actuating-principle is based on a modification of the demanded torques, coming from a possible motion controller system. Preliminary results show that it is possible to control de RUL according to the proposed theoretical framework
46
Done, Vamshidhar. "Numerical modeling of dry wear : Experimental study of fretting wear, fretting wear simulations with debris entrapped and industrial applications of fretting wear models." Thesis, Lyon, 2017. http://www.theses.fr/2017LYSEI137.
Full textAbstract:
De nombreux modèles numériques sont proposés dans la littérature en utilisant des méthodes d'éléments finis et d'éléments finis discrets pour étudier l'usure par frottement, ils incluent à peine l'effet des débris d'usure. Ces modèles étant coûteux en termes de calcul, simuler un grand nombre de cycles d'usure par frottement n'est pas réalisable dans la pratique. Une nouvelle méthodologie est proposée qui ne nécessite que des propriétés de matériau en vrac comme les coefficients de frottement / usure et utilise des méthodes semi-analytiques pour simuler l'usure par frottement avec des débris piégés. Dans cette approche, les débris sont supposés être attachés à l'une des surfaces pendant le processus de fretting. Les résultats obtenus à partir de cette approche ont été comparés avec des expériences de fretting. La méthode proposée permet de saisir la profondeur d'usure et la largeur des cicatrices, et les résultats sont très proches de ceux observés dans les expériences. L'assemblage des soupapes des moteurs à combustion subit une usure par frottement en raison d'un phénomène complexe impliquant une rigidité structurelle et une tribologie de contact. L'usure des soupapes a de nombreux effets néfastes sur les performances du moteur. Il provoque une récession de la soupape entraînant des changements dans les connexions du mécanisme d'entraînement de la soupape, ce qui perturbe l'ouverture et la fermeture des soupapes. Avec des normes d'émission strictes, l'utilisation de lubrifiant pour réduire la friction au contact est limitée. Si l'usure à travers la circonférence n'est pas uniforme, il y aura une fuite de gaz et le moteur donne moins de puissance. Il est nécessaire de bien comprendre la raison de l'usure des soupapes et de développer un modèle numérique capable de prédire l'usure par frottement de la soupape pour le nombre d'heures de fonctionnement donné. Des expériences ont été réalisées pour comprendre le mécanisme d'usure et calculer les coefficients d'usure qui peuvent être utilisés dans le modèle numérique. Un modèle d'usure numérique est construit qui capture la rigidité structurelle de l'ensemble de soupape et le mécanisme d'usure au contact du siègeMany numerical models are proposed in the literature using finite element and finite discrete element methods to study fretting wear, they barely include the effect of wear debris. These models being computationally expensive, simulating large number of fretting wear cycles is not practically feasible. A new methodology is proposed which needs only bulk material properties like friction/wear coefficients and uses semi-analytical methods to simulate fretting wear with entrapped debris. In this approach, debris are assumed to be attached to one of the surfaces during the fretting process. The results obtained from this approach were compared with fretting experiments. The proposed method permits to capture the wear depth and scar width, and results are very close to that observed in the experiments. Valve assembly of combustion engines undergo fretting wear due to a complex phenomenon involving structural stiffness and contact tribology. Valve wear has many detrimental effects on the engine performance. It causes valve recession leading to changes in connections of valve drive train in turn disturbing the opening and closing of valves. With stringent emission norms, usage of lubricant to reduce friction at the contact is restricted. If the wear across the circumference is not uniform, there will be leakage of gas and the engine gives lesser power output. There is a need to thoroughly understand the reason for valve wear and develop a numerical model that can predict valve fretting wear for the given number of operating hours. Experiments were performed to understand the wear mechanism and derive wear coefficients that can be used in the numerical model. A numerical wear model is built that captures structural stiffness of the valve assembly and wear mechanism at seat contact
47
Nguyen, Quoc Lan. "Instabilités liées au frottement des solides élastiques : modélisation de l'initiation des séismes." Université Joseph Fourier (Grenoble), 1999. http://www.theses.fr/1999GRE10016.
Full textAbstract:
Ce travail, qui a pour origine l'etude de l'initiation des seismes comporte trois parties. Une presentation des divers modeles mecaniques du frottement est donnee dans la premiere partie. La deuxieme partie est destinee a l'analyse mathematique des problemes dynamiques de frottement dependant de glissement en elasticite et visco-elasticite. Dans le cas elastique avec contrainte normale imposee, on obtient l'existence de la solution pour les problemes plan et anti-plan. L'etude de l'unicite permet de degager le cadre dans lequel le probleme est bien pose. Dans le cas visco-elastique, on etablit l'existence et l'unicite de la solution. De plus, on montre que la solution converge vers la solution du probleme elastique lorsque les effets de la viscosite diminuent. Dans la troisieme partie, on presente une approche numerique de l'initiation d'une evolution instable du glissement sur une faille finie, basee sur l'analyse spectrale. On calcule une constante universelle de stabilite avec laquelle on peut juger s'il y a ou non une evolution de type catastrophique. On met en evidence la partie dominante qui exprime l'essentiel de l'evolution et qui opere sur une echelle de temps donnee par le spectre. Les simulations numeriques montrent les caracteristiques du glissement pendant la periode de l'initiation dont la duree peut varier fortement de quelques secondes a quelques heures.
48
Smith, Donal. "Financial frictions in macroeconomic models." Thesis, University of York, 2016. http://etheses.whiterose.ac.uk/18226/.
Full textAbstract:
The purpose of this thesis is to examine the interaction between financial frictions and macroeconomic variables. A range models are used which encompasses both theoretical and empirical approaches. The thesis attempts to address several questions with the interaction between changes in borrowing constraints, and so credit markets, and their impact on macroeconomic variables the main themes throughout the chapters. The thesis explores the role of changes in borrowing limits on the amplification of shocks. Utilising the theoretical collateral constraints model it also draws together two separate literatures to examine the interaction between collateral constraints and the steady state interest rate. Finally the thesis conducts an empirical analysis of the impact of an array of financial frictions and financial stress measures on macroeconomic aggregates. The empirical approach used allows this analysis to be conducted in a multi-country setting and so allow the issue of the international propagation of shock to be taken into account. In terms of results, from the theoretical models it is found that financial liberalisation, as modelled by a loosening of the borrowing constraint, leads to a greater amplification of financial shocks compared to conventional shocks. In relation to the steady state interest rate the results suggest that a financial disruption can lead a persistently low interest rate. It is also found that an ageing population and higher debt levels could influence the probability of an economy entering a situation of persistently low interest rates. From the empirical analysis it is found that many measures proposed in the literature are not strong transmitters of financial stress, this is particularity true of credit which is a commonly used shock measure in the literature. It is found that measures of financial frictions that are constructed from corporate bond market data have the most impact on macroeconomic variables.
49
CRUCITTI, FRANCESCA. "HETEROGENEOUS FIRMS MODELS AND FINANCIAL MARKET FRICTIONS." Doctoral thesis, Università degli Studi di Milano, 2019. http://hdl.handle.net/2434/613188.
Full textAbstract:
The common thread in this thesis is represented by general equilibrium models with heterogeneous firms. Initiated by Huggett (1993) and Aiyagari (1994), a strand of general equilibrium literature characterized by the distribution of heterogeneous individuals has been developed. In recent years, the introduction of heterogeneity in macroeconomics increased exponentially. The thesis is developed in this context. The first chapter provides a methodological analysis. It examines the importance of the modelization choice of the idiosyncratic productivity process of individuals. The second chapter proposes a theoretical model which can be able to reconcile four important facts shared by most of the advanced economies around the world: declining labor share of income, rising capital misallocation, low total factor productivity growth and the declining relative price of investment goods.
50
Pisoni, Attilio C. (Attilio Carlo). "A constitutive model for friction in metal-working." Thesis, Massachusetts Institute of Technology, 1992. http://hdl.handle.net/1721.1/12620.
Full text