Researcher:
Olgaç, Ufuk

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Master Student

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Ufuk

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Olgaç

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Olgaç, Ufuk

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2006 - 200922010 - 20137

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Researcher Of Publications: search.filters.isAuthorOfPublication.90e87685-7b41-4304-83cf-a2a53583fc5e

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Now showing 1 - 9 of 9
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    Effects of surfactant on liquid film thickness in the bretherton problem
    (Pergamon-Elsevier Science Ltd, 2013) N/A; N/A; Department of Mechanical Engineering; Olgaç, Ufuk; Muradoğlu, Metin; Master Student; Faculty Member; Department of Mechanical Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; 46561
    The effects of insoluble and soluble surfactant on the motion of a long bubble propagating through a capillary tube are investigated computationally using a finite-difference/front-tracking method. Emphasis is placed on the effects of surfactant on the liquid film thickness between the bubble and the tube wall. The numerical method is designed to solve the evolution equations of the interfacial and bulk surfactant concentrations coupled with the incompressible Navier-Stokes equations. A non-linear equation of state is used to relate surface tension coefficient to surfactant concentration at the interface. Computations are first performed for soluble cases and then repeated for the corresponding clean and insoluble cases for a wide range of governing non-dimensional parameters in order to investigate the effects of surfactant and surfactant solubility. The computed film thickness for the clean case is found to be in a good agreement with Taylor's law indicating the accuracy of the numerical method. We found that both the insoluble and soluble surfactant generally have a thickening effect on the film thickness, which is especially pronounced at low capillary numbers. This thickening effect strengthens with increasing sensitivity of surface tension to interfacial surfactant coverage mainly due to the enhanced Marangoni stresses along the liquid film. It is also observed that film thickening shows a non-monotonic behavior for variations in Peclet number. The validity of insoluble surfactant assumption is assessed for various non-dimensional numbers and it is demonstrated that insoluble assumption is valid only when capillary number is very low, i.e., Ca << 1 and when surface tension is highly sensitive to interfacial surfactant coverage, i.e., the elasticity number is large.
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    Computed high concentrations of low-density lipoprotein correlate with plaque locations in human coronary arteries
    (Elsevier, 2011) Knight, Joseph; Poulikakos, Dimos; Saur, Stefan C.; Alkadhi, Hatem; Desbiolles, Lotus M.; Cattin, Philippe C.; Kurtcuoglu, Vartan; Department of Mechanical Engineering; Olgaç, Ufuk; Researcher; Department of Mechanical Engineering; College of Engineering; N/A
    Subendothelial accumulation of low-density lipoprotein (LDL) in arterial walls is an initiator of atherosclerotic plaque formation. We report here on the correlation between healthy state subendothelial LDL concentration distribution and sites of subsequent plaque formation in coronary arteries of patients with coronary artery disease (CAD). We acquired left (LCA) and right coronary artery (RCA) and atherosclerotic plaque geometries of 60 patients with CAD using dual-source computed tomography angiography. After virtually removing all plaques to obtain an approximation of the arteries' healthy state, we calculated LDL concentration in the artery walls as a function of local lumen-side shear stress. We found that maximum subendothelial LDL concentrations at plaque locations were, on average, 45% (RCA) and 187% (LCA) higher than the respective average subendothelial concentration. Our results demonstrate that locally elevated subendothelial LDL concentration correlates with subsequent plaque formation at the same location. © 2011 Elsevier Ltd.
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    Direct numerical simulation of an oscillating droplet in partial contact with a substrate
    (Pergamon-Elsevier Science Ltd, 2013) N/A; N/A; N/A; Department of Mechanical Engineering; Olgaç, Ufuk; Izbassarov, Daulet; Muradoğlu, Metin; Master Student; PhD Student; Faculty Member; Department of Mechanical Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; N/A; 46561
    Small-amplitude oscillations of a viscous drop that is in a partial contact with a flat substrate are investigated computationally using a finite-difference/front-tracking method. Emphasis is placed on the first mode resonance frequency response of the droplet for a wide range of contact angles. It is found that numerical results converge to the theoretical predictions of Strani and Sabetta (Strani M, Sabetta F. J Fluid Mech 1984;141:223-47) for high contact angles, whereas considerable discrepancy is observed as contact angle decreases. However, the dependence of the frequency on the drop radius, drop density and surface tension coefficient remains the same as predicted by the Strani and Sabetta theory. It is also found that the effects of density and viscosity ratio become insignificant for the density and viscosity ratios larger than 10. The oscillations are found to be damped exponentially in time due to viscous dissipation similar to the case of an isolated droplet and the damping rate decreases with increasing contact angle.
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    Effects of surfactant on the motion of a large bubble in a capillary tube
    (ASME, 2011) N/A; Department of Mechanical Engineering; N/A; N/A; Muradoğlu, Metin; Olgaç, Ufuk; Gürsel, Gökalp; Faculty Member; Master Student; Master Student; Department of Mechanical Engineering; College of Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; 46561; N/A; N/A
    The finite-difference/front-tracking method is used to study the motion and deformation of a large bubble moving through a capillary tube in the presence of both insoluble and soluble surfactants. Emphasis is placed on the effects of surfactant on the liquid film thickness between the bubble and the tube wall. The numerical method is designed to solve the evolution equations of the interfacial and bulk surfactant concentrations coupled with the incompressible Navier-Stokes equations. A non-linear equation of state is used to relate interfacial surface tension to surfactant concentration at the interface. The film thickness is first computed for the clean bubble case and the results are compared with the lubrication theory in the limit of small capillary numbers, i.e., Ca << 1, and found to be in good agreement with the predictions of Bretherton [1]. Thereafter, the method is used to investigate the effects of insoluble and soluble surfactants on the film thickness for a wide range of governing non-dimensional numbers. It is found that both the insoluble and soluble surfactants have a thickening effect on the liquid film, which compares well with both the experimental results of Krechetnikov and Homsy [2] and analytical predictions of Daripa and Pasa [3].
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    Patient-specific modeling of low-density lipoprotein transport in coronary arteries
    (World Scientific Publishing Co., 2012) N/A; N/A; Olgaç, Ufuk; Master Student; Graduate School of Sciences and Engineering; N/A
    The following sections are included: Introduction Methods Acquisition and processing of anatomy data Reconstruction of the flow field in the artery lumen Determination of blood plasma and LDL fluxes through the endothelium Reconstruction of flow and LDL concentration fields in the arterial wall Results Summary References
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    A finite-volume/front-tracking method for computations of multiphase flows in complex geometries
    (Springer, 2006) N/A; Department of Mechanical Engineering; N/A; N/A; Muradoğlu, Metin; Olgaç, Ufuk; Kayaalp, Arif Doruk; Faculty Member; Master Student; Master Student; Department of Mechanical Engineering; College of Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; 46561; N/A; N/A
    A finite-volume/front-tracking (FV/FT) method is developed for computations of multiphase flows in complex geometries. The front-tracking methodology is combined with a dual time-stepping based FV method. The interface between phases is represented by connected Lagrangian marker points. An efficient algorithm is developed to keep track of the marker points in curvilinear grids. The method is implemented to solve two-dimensional (plane or axisymmetric) dispersed multiphase flows and is validated for the motion of buoyancy-driven drops in a periodically constricted tube with cases where drop breakup occurs.
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    Computational modeling of surfactant-laden liquid plug propagation in capillary tubes
    (ASME, 2012) N/A; Department of Mechanical Engineering; Muradoğlu, Metin; Olgaç, Ufuk; Faculty Member; Master Student; Department of Mechanical Engineering; College of Engineering; Graduate School of Sciences and Engineering; 46561; N/A
    Pulmonary surfactant is of essential importance in reducing the surface tension on the liquid film that coats the inner surface of the airways and thus making the lung more compliant. Surfactant-deficiency may result in respiratory distress syndrome (RDS), which is especially common in prematurely born neonates. Surfactant replacement therapy (SRT) is a standard treatment, in which a liquid plug with exogenous surfactant is instilled in the trachea, which subsequently propagates by inspiration and spreads the exogenous surfactant to the airways. The efficacy of the treatment depends on various parameters such as the size of the liquid plug, inspiration frequency and the physical properties of the exogenous surfactant. Unsteady simulations are performed to study surfactant-laden liquid plug propagation using finite difference/front-tracking method in order to shed light on the surfactant replacement therapy.
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    Buoyancy-driven motion and breakup of viscous drops in constricted capillaries
    (Pergamon-Elsevier Science Ltd, 2006) N/A; N/A; N/A; Department of Mechanical Engineering; Olgaç, Ufuk; Kayaalp, Arif Doruk; Muradoğlu, Metin; Master Student; Master Student; Faculty Member; Department of Mechanical Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; N/A; 46561
    Buoyancy-driven motion and breakup of viscous drops in sinusoidally constricted channels are studied computationally using a finite-volume/front-tracking (FV/FT) method. Computational results are first compared with the available experimental data and then the conditions for a drop breakup are examined. The effects of the channel geometry, drop size and Bond number on drop breakup are investigated. It is found that the present computational results are in a good agreement with the available experimental data and drop breakup occurs in various modes depending on the channel geometry, the relative drop size and Bond number. It is observed that a drop breaks up into two or more drops when its size is larger than a critical value. It is also found that large drops undergo successive breakups to produce monodispersed small droplets. Critical non-dimensional drop size and critical capillary number for the onset of drop breakup are reported over a wide range of geometrical parameters and Bond numbers.
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    PublicationOpen Access
    Computational modeling of unsteady surfactant-laden liquid plug propagation in neonatal airways
    (American Institute of Physics (AIP) Publishing, 2013) Department of Mechanical Engineering; Olgaç, Ufuk; Muradoğlu, Metin; Faculty Member; Department of Mechanical Engineering; College of Engineering; N/A; 46561
    Surfactant-free and surfactant-laden liquid plug propagation in neonatal airways in various generations representing the upper and lower airways are investigated computationally using a finite-difference/front-tracking method. Emphasis is placed on the unsteady surfactant-laden plug propagation as a model for Surfactant Replacement Therapy (SRT) and airway reopening. The numerical method is designed to solve the evolution equations of the interfacial and bulk surfactant concentrations coupled with the incompressible Navier-Stokes equations. Available experimental data for surfactant Survanta are used to relate surface tension coefficient to surfactant concentration at the interface. It is found that, for the surfactant-free case, the trailing film thickness is in good agreement with Taylor's law for plugs with plug length greater than the airway width. Mechanical stresses that could be injurious to epithelial cells such as pressure and shear stress and their gradients are maximized on the front and rear menisci with increasing magnitudes in the lower generations. These mechanical stresses, especially pressure and pressure gradient, are diminished with the introduction of surfactants. Surfactant is absorbed onto the trailing film and thickens it, eventually leading to either plug rupture or, if totally consumed prior to rupture, to steadily propagating plug. In the upper airways, initially small plugs rupture rapidly and plugs with comparable initial plug length with the airway width persist and propagate steadily. For a more effective SRT treatment, we recommend utilization of plugs with initial plug length greater than the airway width. Increasing surfactant strength or increasing the initially instilled surfactant concentration is found to be ineffective. (C) 2013 AIP Publishing LLC.