Research Project: Biyolojik/Mikro-Akışlarda Newtonsal Olmayan Damlacıkların Sayısal Modellenmesi
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Contributors
Funders
ID
TB.00098
Authors
Muradoğlu, Metin
Faculty Member
Publications
A computational study of two-phase viscoelastic systems in a capillary tube with a sudden contraction/expansion
(American Institute of Physics (AIP) Publishing, 2016) Izbassarov, Daulet; Muradoğlu, Metin; Department of Mechanical Engineering; Yes; College of Engineering
Two-phase viscoelastic systems are computationally studied in a pressure-driven flow with a sudden contraction and expansion using a finite-difference/front-tracking method. The effects of viscoelasticity in drop and bulk fluids are investigated including high Weissenberg and Reynolds number cases up to Wi = 100 and Re = 100. The Finitely Extensible Non-linear Elastic-Chilcott and Rallison (FENE-CR) model is used to account for the fluid viscoelasticity. Extensive computations are performed to examine drop dynamics for a wide range of parameters. It is found that viscoelasticity interacts with drop interface in a non-monotonic and complicated way, and the two-phase viscoelastic systems exhibit very rich dynamics especially in the expansion region. At high Re, the drop undergoes large deformation in the contraction region followed by strong shape oscillations in the downstream of the expansion. For a highly viscous drop, a re-entrant cavity develops in the contraction region at the trailing edge which, in certain cases, grows and eventually causes encapsulation of ambient fluid. The re-entrant cavity formation is initiated at the entrance of the contraction and is highly influenced by the viscoelasticity. Compared to the corresponding straight channel case, the effects of viscoelasticity are reversed in the constricted channel: Viscoelasticity in drop/continuous phase hinders/enhances formation of the re-entrant cavity and entrainment of ambient fluid into main drop. Encapsulation of ambient fluid into main droplet may be another route to produce a compound droplet in microfluidic applications. (C) 2016 AIP Publishing LLC.
Simulations of soluble surfactants in 3D multiphase flow
(Academic Press Inc., 2014) Muradoğlu, Metin; Tryggvason, Gretar; Department of Mechanical Engineering; Yes; College of Engineering
A finite-difference/front-tracking method is developed for simulations of soluble surfactants in 3D multiphase flows. The interfacial and bulk surfactant concentration evolution equations are solved fully 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. Simple test cases are designed to validate different parts of the numerical algorithm and the computational results are found to be in a good agreement with the analytical solutions. The numerical algorithm is parallelized using a domain-decomposition method. It is then applied to study the effects of soluble surfactants on the motion of buoyancy-driven bubbles in a straight square channel in nearly undeformable (spherical) and deformable (ellipsoidal) regimes. Finally the method is used to examine the effects of soluble surfactants on the lateral migration of bubbles in a pressure-driven channel flow. It is found that surfactant-induced Marangoni stresses counteract the shear-induced lift force and can reverse the lateral bubble migration completely, i.e., the contaminated bubble drifts away from the channel wall and stabilizes at the center of the channel when the surfactant-induced Marangoni stresses are sufficiently large.
A front-tracking method for computational modeling of viscoelastic two-phase flow systems
(Elsevier, 2015) Izbassarov, Daulet; Muradoğlu, Metin; N/A; Department of Mechanical Engineering; Yes; College of Engineering
A front-tracking method is developed for direct numerical simulations of viscoelastic two-phase systems in which one or both phases could be viscoelastic. One set of governing equations is written for the whole computational domain and different phases are treated as a single fluid with variable material and rheological properties. The interface is tracked explicitly using a Lagrangian grid while the flow equations are solved on a fixed Eulerian grid. The surface tension is computed at the interface using the Lagrangian grid and included into the momentum equations as a body force. The Oldroyd-B, FENE-CR and FENE-MCR models are employed to model the viscoelasticity. The viscoelastic model equations are solved fully coupled with the flow equations within the front-tracking framework. A fifth-order WENO scheme is used to approximate the convective terms in the viscoelastic model equations and second-order central differences are used for all other spatial derivatives. A log-conformation method-is employed to alleviate the high Weissenberg number problem (HWNP) and found to be stable and very robust for a wide range of Weissenberg numbers. The method has been first validated for various benchmark single-phase and two-phase viscoelastic flow problems. Then it has been applied to study motion and deformation of viscoelastic two-phase systems in a pressure-driven flow through a capillary tube with a sudden contraction and expansion. The method has been demonstrated to be grid convergent with second-order spatial accuracy for all the cases considered in this paper.
A computational study of droplet-based bioprinting: effects of viscoelasticity
(American Institute of Physics (AIP) Publishing, 2019) Izbassarov, Daulet; Muradoğlu, Metin; Nooranidoost, Mohammad; Taşoğlu, Savaş; Department of Mechanical Engineering; Graduate School of Sciences and Engineering; Yes; College of Engineering; GRADUATE SCHOOL OF SCIENCES AND ENGINEERING
Despite significant progress, cell viability continues to be a central issue in droplet-based bioprinting applications. Common bioinks exhibit viscoelastic behavior owing to the presence of long-chain molecules in their mixture. We computationally study effects of viscoelasticity of bioinks on cell viability during deposition of cell-loaded droplets on a substrate using a compound droplet model. The inner droplet, which represents the cell, and the encapsulating droplet are modeled as viscoelastic liquids with different material properties, while the ambient fluid is Newtonian. The model proposed by Takamatsu and Rubinsky ["Viability of deformed cells," Cryobiology 39(3), 243-251 (1999)] is used to relate cell deformation to cell viability. We demonstrate that adding viscoelasticity to the encapsulating droplet fluid can significantly enhance the cell viability, suggesting that viscoelastic properties of bioinks can be tailored to achieve high cell viability in droplet-based bioprinting systems. The effects of the cell viscoelasticity are also examined, and it is shown that the Newtonian cell models may significantly overpredict the cell viability.
Computational modeling of unsteady surfactant-laden liquid plug propagation in neonatal airways
(American Institute of Physics (AIP) Publishing, 2013) Muradoğlu, Metin; Olgaç, Ufuk; Department of Mechanical Engineering; Yes; College of Engineering
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.
