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Modeling of Liquid Film Instabilities with Subsequent Entrainment of Droplets
Knotek, Stanislav ; Kozubková, Milada (referee) ; Čermák, Libor (referee) ; Jícha, Miroslav (advisor)
This dissertation deals with instabilities of thin liquid films up to entrainment of drops. Four types of instabilities have been classified depending on the type of structure and process on the liquid film surface: two-dimensional slow waves, two-dimensional fast waves, three-dimensional waves, solitary waves and entrainment of drops from the film surface. This thesis analyzes the physical principles of instabilities and deals with the mathematical formulation of the problem. Shear and pressure forces acting on the surface of the liquid film are identified as the cause of instabilities. Mathematical models for predicting instabilities are demonstrated using approaches based on solving the Orr-Sommerfeld equation and the equations of motion in integral form. Models of shear and pressure forces acting on the surface of the film and selected models of film thickness are presented. The work is focused on the prediction of the initiation of two-dimensional waves using the integral approach. Shear stress and pressure forces acting on the liquid film surface have been modeled using the simulation of air flow over a solid surface. Finally, criteria for drop entrainment are presented with their dependence on air velocity and film thickness.
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Computational Fluid Dynamic Simulation of Intracranial Aneurysms : analysis of time-depend changes of hemodynamic parameters - the road to clinical use
Sejkorová, Alena ; Hejčl, Aleš (advisor) ; Vaverka, Miroslav (referee) ; Přibáň, Vladimír (referee)
Computational Fluid Dynamic Simulation of Intracranial Aneurysms Analysis of time-dependent changes of hemodynamic parameters - the road the clinical use Hemodynamics are involved in the genesis of intracranial aneurysms and time- dependent changes of their parameters lead to aneurysm growth, stabilization or rupture. Definition of these changes using computational fluid hemodynamics could significantly contribute to the understanding of aneurysmal development and rupture and could enable the routine use of mathematical simulations. In this study, computational fluid dynamics were performed for nine incidental aneurysms. Five aneurysms were monitored throughout time and factors leading to aneurysm rupture were analyzed. In four aneurysms the influence of the hemodynamics on the growth was defined. Major growth occurred in areas of low wall shear stress and oscillatory index. These areas increased in size during growth time. Contrary to this, neck shape remodeling occurred in areas with large wall shear stress and pressure. Throughout the follow-up of ruptured aneurysms, the minimal wall shear stress decreased, and the area of low wall shear stress increased significantly. The results indicate that decreasing values of minimal wall shear stress and increasing values of low wall shear stress area...
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Modeling of Liquid Film Instabilities with Subsequent Entrainment of Droplets
Knotek, Stanislav ; Kozubková, Milada (referee) ; Čermák, Libor (referee) ; Jícha, Miroslav (advisor)
This dissertation deals with instabilities of thin liquid films up to entrainment of drops. Four types of instabilities have been classified depending on the type of structure and process on the liquid film surface: two-dimensional slow waves, two-dimensional fast waves, three-dimensional waves, solitary waves and entrainment of drops from the film surface. This thesis analyzes the physical principles of instabilities and deals with the mathematical formulation of the problem. Shear and pressure forces acting on the surface of the liquid film are identified as the cause of instabilities. Mathematical models for predicting instabilities are demonstrated using approaches based on solving the Orr-Sommerfeld equation and the equations of motion in integral form. Models of shear and pressure forces acting on the surface of the film and selected models of film thickness are presented. The work is focused on the prediction of the initiation of two-dimensional waves using the integral approach. Shear stress and pressure forces acting on the liquid film surface have been modeled using the simulation of air flow over a solid surface. Finally, criteria for drop entrainment are presented with their dependence on air velocity and film thickness.
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Application of the Electrodiffusion Method for Near-Wall Flow Diagnostics
Tihon, Jaroslav
The electrodiffusion method is presented as a measuring technique suitable for the flow measurement done in close proximity of the wall, thus in the region difficultly accessible by standard anemometric techniques (e.g. PIV, LDA). The experimental results obtained in different flow configurations (backward-facing step flow, wavy film flow, turbulent channel flow, rising Taylor bubbles) document application capabilities of this technique, e.g. for mapping of wall shear stresses, delimitation of stable flow-recirculation zones, detection of short-time reversal of the near-wall flow, investigation of the near-wall turbulence, or detection of moving fluid particles. A new technology of sensor fabrication based on photolithography is introduced and possible applications of microsensors in microfluidic devices discussed.
Fulltext: content.csg -  PDF
Plný tet: SKMBT_C22013110513500 - PDF
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Wall Shear Stress Induced by Taylor Bubbles in Inclined Flow Channels
Tihon, Jaroslav ; Pěnkavová, Věra ; Vejražka, Jiří
The motion of single air bubbles in flat channels is experimentally investigated. The electrodiffusion technique of near-wall flow diagnostics is applied to measure the wall shear stress distribution under large rising bubbles. The measurements are synchronized with the visual observation of bubble movement by a high-speed camera. The analysis of video records provides information on the bubble shape and terminal velocity. The experiments are carried out for three different channel configuration (with heights of 1.5, 4, and 8 mm), cover a wide range of channel inclination angles (from horizontal to vertical position), and dealing with both the bubbles in stagnant and in co-flowing water.
Fulltext: content.csg - PDF
Plný tet: SKMBT_C22012112614030 - PDF
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The Transitional Regime of Backward-Facing Step Flows
Tihon, Jaroslav ; Pěnkavová, Věra ; Havlica, Jaromír ; Šimčík, Miroslav
The backward-facing step flow is investigated experimentally (electrodiffusion technique) and numerically (CFD software Fluent) at moderate Reynolds numbers achieved in a water channel. The direction sensitive sensors are used to measure the wall shear rate behind the step and detect the near-wall extent of different flow-recirculation regions. 2D numerical simulations provide additional information on the global flow rearrangement caused by the change of operational parameters (channel expansion ratios, inlet flow conditions). At the transitional flow regime, the near-wall flow exhibits an unsteady character with a high sensitivity to external low-frequency perturbations (intrinsic flow instability). The inlet pulsatile forcing is found to affect strongly the overall flow structure behind the step, resulting in the reattachment length reduction and the reverse flow intensification.
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Střihové napětí na stěně pod velkou bublinou stoupající v nakloněném plochém kanále
Tihon, Jaroslav ; Pěnkavová, Věra ; Vejražka, Jiří
The motion of single air bubbles in an inclined flat water channel is investigated experimentally. The electrodiffusion technique of near-wall flow diagnostics is applied to measure the wall shear stress distribution under large bubbles rising along the upper channel wall. The electrodiffusion measurements are synchronized with the visual observation of bubble movement done by a high-speed camera. The analysis of video records provides information on the bubble shape and terminal velocity. The experiments are carried out for three different channel heights and cover a wide range of channel inclination angles (from horizontal to vertical position). The experimental results on bubble motion characteristics obtained in stagnant and upward-flowing liquid arrangements are confronted with the recent predictions and scaling provided by theoretical models.
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