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Experimental study of journal bearings dynamics
Tkadlec, Josef ; Vimmr, Jan (referee) ; Omasta, Milan (advisor)
This work deals with experimental analysis of fluid film instability in rotor bearing systems stored in hydrodynamic plain bearings with focus on bearing surface modification. The experiments were performed on a journal bearing dynamics simulator. The aim was to determine the influence of individual operating parameters on the occurrence and course of turbulence instability. The emphasis here was on the effect of surface micro textures. The work also included the preparation of textured samples. Experiments were divided into experiments monitoring the influence of temperature, pressure, bearing geometry and surface modification. The output of the individual experiments were the values of the threshold speed of formation and termination of instability or the magnitude of the amplitude depending on the above described parameters. These values were plotted in graphs. This data was then confronted with previously published articles. The results showed that instabilityin rotor systems is very sensitive to temperature, pressure and bearing geometry changes. The fundamental influence on the unstable behavior of the rotor system has been demonstrated by the use of textured surfaces, where the threshold speed was significantly shifted to a higher speed range compared to the non-textured bearing.
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Studies on Non-Newtonian Behavior in Slit-flow rheometer at Unsteady flow
Halama, Lukáš ; Vimmr, Jan (referee) ; Roupec, Jakub (advisor)
The thesis deals with the description of the unstable flow of non-Newtonian fluid in a slit-flow rheometer, which negatively affects its behaviour. The initiators of unstable fluid flow are the roughness of the rheometer slit walls, the slip on the rheometer walls, and the influence of the inlet and outlet region geometry of the rheometer slit. The work contains methodical procedures for mathematical consideration of individual unstable fluid flow initiators and design of change of slit geometry of slit-flow rheometer. Part of the work is also a comparison of the most commonly used rheological models, derivation of general relations for the creation of the velocity profile of individual rheological models and their subsequent implementation in the rheological application, which significantly simplifies the process of evaluation of measured data when measured on slit-flow rheometer. This application can be used to determine basic parameters in CFD simulations or as a teaching aid.
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Analysis of Influence of Collagen Organization on Mechanical Properties of Arteries
Novák, Kamil ; Daniel, Matej (referee) ; Vimmr, Jan (referee) ; Burša, Jiří (advisor)
This dissertation thesis concerns with Analysis of Influence of Collagen Organization on Mechanical Properties of Arteries and it is divided into three main parts. Motivation for this dissertation thesis was in a study reviewing effect of material model upon resulting stresses in AAA. The effect was calculated in 70 patient-specific geometries of AAA, which exceeds the number of geometries in other scientific papers by one order. Within this study, two material models were used, i.e. real one and 100× stiffer, and obtained stresses were mutually compared. It was quantified that peak stress difference can be higher than 20 % in 10% of patients and therefore the real material model should be preferred over the artificial one although operation with this model is more demanding. The second part of this thesis deals with an identification of structural parameters (orientation and dispersion of collagen fibres) of porcine aortic tissue by using adjusted Fast Fourier Transform based algorithm. The extracted structural parameters were inserted into two-layer structure-motivated constitutive model Martufi-Gasser. This model was validated and its predictive capabilities were also tested with fine results. The most important information obtained from the digital image processing of ~9000 micrographs is existence of only one family of dispersed collagen fibres which breaks the current dogma present in many scientific papers about two families of collagen fibres. The third part concerns with a proposal of an automated phase-correlation based algorithm for obtaining collagen fibre direction from polarized light microscopy images. The proposed algorithm was verified and validated and it yields histograms of collagen fibre directions with overall number of measured points larger than it would be possible to get from any manual measurement. The limitation of the original proposed algorithm is in 90° period of polarized light intensity, thus the method results in angles in the range of 0°–90. Therefore the end of the thesis is dedicated resolving this problem and obtaining real angles in a span of 0°–180°. To this end, the microscope set-up was changed and the algorithm was adjusted accordingly. The original and the adjusted algorithms are collagen-specific, fast and an operator independent. Despite all the author´s effort put into collagen fibre waviness quantification directly from the histograms, the waviness has not been quantified yet in this way and it remains at the stage of research.
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Lubrication of plastic gears
Poledník, Radim ; Vimmr, Jan (referee) ; Šperka, Petr (advisor)
The purpose of this diploma thesis is to describe the influence of input parameters (rolling speed, load, temperature, dynamic viscosity and pressure coefficient of the lubricant) on film thickness in point and elliptical contacts using optical interferometry. Regression based film thickness formula has been obtained using universal form of non-dimensional parameters of entrainment speed U, load W and material G. Equations were also complemented by the function of ellipticity k. New regression formulas for central and minimum film thickness have been compared with existing EHL thickness equations. These film thickness measurement and new regression formulas have applicability to our understanding of the performance and more effective design of lubricated gears from polymeric materials.
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Behaviour of lubricated contacts under high-sliding conditions
Adam, Jakub ; Vimmr, Jan (referee) ; Omasta, Milan (advisor)
The aim of this diploma thesis is to describe the influence of individual operating parameters on thickness and temperature distribution of elastohydrodynamic contact under high slip ratio. In this work, the distribution of the lubricant film thickness is determined by optical interferometry and the temperature mapping in EHL contact is realized by infrared microscopy. Two optical filters were used to separate the radiation of contact surfaces and oil. Temperature of disk surface was measured using a sapphire disk with chromium layer that shielded radiation from ball and oil. The experimental device was modified to simulate the conditions of the various vectors between entrainment and sliding speed. The results extend existing information about behavior of EHL contact under high slip ratio conditions.
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Surface tension measurements
Hála, Ondřej ; Vimmr, Jan (referee) ; Košťál, David (advisor)
The thesis deals with study of surface tension of liquids and measuring possibilities of this parameter. The paper is then focused on design of measuring apparatus based on pendant drop method. The apparatus is controlled with software written in Matlab. Several measurements are done at the end of this work, which confirm possibility to practical use of this apparatus for surface tension measurements. A part of this work is manual to use the apparatus and complete design documentation.
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The study of MR fluids sedimentation
Berka, Pavel ; Vimmr, Jan (referee) ; Roupec, Jakub (advisor)
This diploma thesis investigates the accelerated sedimentation of the magnetorheological fuids (MRFs) using centrifuge. Experiments are carried out at room temperature as well as at higher temperatures (40°C, 60°C and 80°C). The development of the new improved measuring device for the sedimentation evaulation is described. Magnetic components of the device were designed using FEM analysis. Newly designed device is verified. To accomplish the experiments at the high temperatures the new heated chamber for experimental centrifuge is designed and constructed. In the next part of the work full experiment of the accelerated sedimentation is performed. At final the empirical model of the sedimetation velocity for various accelerations and temperatures is synthesized.
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Large Eddy Simulation of Turbulent Two-Phase Flow
Volavý, Jaroslav ; Fürst, Jiří (referee) ; Vimmr, Jan (referee) ; Jícha, Miroslav (advisor)
Doctoral thesis deals with the numerical simulations of two-phase flows, especially with prediction of movement of dispersed phase (particles) carried by fluid. The Euler-Lagrange approach was applied for description of the system fluid-particles. It means that the fluid is considered to be continuum and its movement is described using Euler approach. Particles are regarded as mass points and their movement is solved using Lagrangian approach. The Large Eddy Simulation method was adopted for solution of the fluid flow. The series of simulations of the backward-facing step flow laden with particles were performed. The concentration of the particles in the flow was high enough for consideration of the influence of particles on the turbulence of the carrier phase. The developed scheme for generation of turbulence on the inlet is applied. The influence of anisotropic decomposition of subgrid energy on movement of particles was studied in the frame of this work.
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