Browsing by Author "Kim, Yongsam"

Now showing 1 - 12 of 12

A note on pressure accuracy in immersed boundary method for Stokes flow
(2011-06-01)
Chen, Kuan-Yu
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Feng, Ko-An
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Kim, Yongsam
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Lai, Ming-Chih
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應用數學系
In this short note, we provide a simplified one-dimensional analysis and two-dimensional numerical experiments to predict that the overall accuracy for the pressure or indicator function in immersed boundary calculations is first-order accurate in L(1) norm, half-order accurate in L(2) norm, but has O(1) error in L(infinity) Despite the pressure has O(1) error near the interface, the velocity field still has the first-order convergence in immersed boundary calculations. (C) 2011 Elsevier Inc. All rights reserved.
A penalty immersed boundary method for viscoelastic particulate flows
(2018-08-01)
Kim, Yongsam
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Lai, Ming-Chih
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Seol, Yunchang
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應用數學系
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丘成桐中心
We extend the penalty immersed boundary (pIB) method to investigate the interaction between circular rigid particles and a surrounding viscoelastic Oldroyd-B fluid. The basic idea of the pIB method is the splitting of an immersed boundary, which here is a rigid body, notionally into two Lagrangian components: one is a massive component carrying all mass of the rigid body, and the other is massless. These two components are connected by a system of stiff springs with zero rest length. The massive component has no direct interaction with the surrounding fluid and behaves as though in a vacuum, following the dynamics of a rigid body, in which the acting forces and torques are generated from the system of stiff springs that connects the two Lagrangian components. The massless component interacts with the surrounding Oldroyd-B fluid: it moves at the local fluid velocity and exerts force locally on the fluid. We verify the pIB method combined with Oldroyd-B fluid model by investigating the effects of the wall and elasticity of the fluid on the lateral position of a circular particle falling under the influence of gravity and by studying convergence of the numerical solutions. We also simulate the interaction between multiple circular particles and the surrounding Oldroyd-B fluid and compare the dynamics of the particles in various flow conditions.
An immersed boundary method for simulating Newtonian vesicles in viscoelastic fluid
(2019-01-01)
Seol, Yunchang
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Tseng, Yu-Hau
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Kim, Yongsam
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Lai, Ming-Chih
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應用數學系
In this paper, a two-dimensional immersed boundary method is developed to simulate the dynamics of Newtonian vesicle in viscoelastic Oldroyd-B fluid under shear flow. The viscoelasticity effect of extra stress is well incorporated into the immersed boundary formulation using the indicator function. Our numerical methodology is first validated in comparison with theoretical results in purely Newtonian fluid, and then a series of numerical experiments is conducted to study the effects of different dimensionless parameters on the vesicle motions. Although the tank-treading (TT) motion of Newtonian vesicle in Oldroyd-B fluid under shear flow can be observed just like in Newtonian fluid, it is surprising to find that the stationary inclination angle can be negative without the transition to tumbling (TB) motion. Moreover, the inertia effect plays a significant role that is able to turn the vesicle back to positive inclination angle through TT-TB-TT transition as the Reynolds number increases. To the best of our knowledge, this is the first numerical work for the detailed investigations of Newtonian vesicle dynamics suspended in viscoelastic Oldroyd-B fluid. We believe that our numerical results can be used to motivate further studies in theory and experiments for such coupling vesicle problems. (C) 2018 Elsevier Inc. All rights reserved.
An immersed boundary method for simulating the dynamics of three-dimensional axisymmetric vesicles in Navier-Stokes flows
(2014-01-15)
Hu, Wei-Fan
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Kim, Yongsam
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Lai, Ming-Chih
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應用數學系
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數學建模與科學計算所(含中心)
In this paper, we develop a simple immersed boundary method to simulate the dynamics of three-dimensional axisymmetric inextensible vesicles in Navier-Stokes flows. Instead of introducing a Lagrange's multiplier to enforce the vesicle inextensibility constraint, we modify the model by adopting a spring-like tension to make the vesicle boundary nearly inextensible so that solving for the unknown tension can be avoided. We also derive a new elastic force from the modified vesicle energy and obtain exactly the same form as the originally unmodified one. In order to represent the vesicle boundary, we use Fourier spectral approximation so we can compute the geometrical quantities on the interface more accurately. A series of numerical tests on the present scheme have been conducted to illustrate the applicability and reliability of the method. We first perform the accuracy check of the geometrical quantities of the interface, and the convergence check for different stiffness numbers as well as fluid variables. Then we study the vesicle dynamics in quiescent flow and in gravity. Finally, the shapes of vesicles in Poiseuille flow are investigated in detail to study the effects of the reduced volume, the confinement, and the mean flow velocity. The numerical results are shown to be in good agreement with those obtained in literature. (C) 2013 Elsevier Inc. All rights reserved.
An immersed boundary method for simulating vesicle dynamics in three dimensions
(2016-10-01)
Seol, Yunchang
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Hu, Wei-Fan
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Kim, Yongsam
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Lai, Ming-Chih
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應用數學系
We extend our previous immersed boundary (IB) method for 3D axisymmetric inextensible vesicle in Navier-Stokes flows (Hu et al., 2014 [17]) to general three dimensions. Despite a similar spirit in numerical algorithms to the axisymmetric case, the fully 3D numerical implementation is much more complicated and is far from straightforward. A vesicle membrane surface is known to be incompressible and exhibits bending resistance. As in 3D axisymmetric case, instead of keeping the vesicle locally incompressible, we adopt a modified elastic tension energy to make the vesicle surface patch nearly incompressible so that solving the unknown tension (Lagrange multiplier for the incompressible constraint) can be avoided. Nevertheless, the new elastic force derived from the modified tension energy has exactly the same mathematical form as the original one except the different definitions of tension. The vesicle surface is discretized on a triangular mesh where the elastic tension and bending force are calculated on each vertex (Lagrangian marker in the IB method) of the triangulation. A series of numerical tests on the present scheme are conducted to illustrate the robustness and applicability of the method. We perform the convergence study for the immersed boundary forces and the fluid velocity field. We then study the vesicle dynamics in various flows such as quiescent, simple shear, and gravitational flows. Our numerical results show good agreements with those obtained in previous theoretical, experimental and numerical studies. (C) 2016 Elsevier Inc. All rights reserved.
Numerical simulations of three-dimensional foam by the immersed boundary method
(2014-07-15)
Kim, Yongsam
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Lai, Ming-Chih
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Peskin, Charles S.
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Seol, Yunchang
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應用數學系
In this paper, we extend (Kim et al., 2010 [13]) to the three-dimensional dry foam case, i.e., a foam in which most of the volume is attributed to its gas phase. Dry foam dynamics involves the interaction between a gas and a collection of thin liquid-film internal boundaries that partitions the gas into discrete cells or bubbles. The liquid-film boundaries are flexible, contract under the influence of surface tension, and are permeable to the gas which moves across them by diffusion at a rate proportional to the local pressure difference across the boundary. Such problems are conventionally studied by assuming that the pressure is uniform within each bubble. Here, we introduce instead an immersed boundary method that takes into account the non-equilibrium fluid mechanics of the gas. To model gas diffusion across the internal liquid-film boundaries, we allow normal slip between the boundary and the gas at a velocity proportional to the (normal) force generated by the boundary surface tension. We implement this method in the three-dimensional framework, and test it by verifying the 3D generalization of the von Neumann relation, which governs the coarsening of a three-dimensional dry foam. (C) 2014 Elsevier Inc. All rights reserved.
Numerical simulations of vesicle and bubble dynamics in two-dimensional four-roll mill flows
(2017-05-10)
Kim, Yongsam
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Lai, Ming-Chih
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Seol, Yunchang
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應用數學系
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Department of Applied Mathematics
We use a computational technique based on the immersed boundary method to construct a four-roll mill device with which we can generate a broad spectrum of flow types from an extensional flow to a rotational one. We put a vesicle or a bubble in the constructed four-roll mill device to investigate their interaction with the surrounding fluid. The vesicle dynamics are determined by its bending rigidity, inextensibility, and hydrodynamical force, whereas the bubble dynamics is governed by the surface tension and the hydrodynamic interaction. Depending on the type of the flow, these suspended objects go through either a tank-treading motion or a tumbling motion. We validate our numerical method by a convergence study and discuss the transition between tank-treading and tumbling motions for the vesicles and bubbles.
Numerical study of viscosity and inertial effects on tank-treading and tumbling motions of vesicles under shear flow
(2012-12-26)
Kim, Yongsam
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Lai, Ming-Chih
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應用數學系
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數學建模與科學計算所(含中心)
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Department of Applied Mathematics
An inextensible vesicle under shear flow experiences a tank-treading motion on its membrane if the viscosity contrast between the interior and exterior fluids is small. Above a critical threshold of viscosity contrast, the vesicle undergoes a tumbling bifurcation. In this paper, we extend our previous work [Kim and Lai, J. Comput. Phys. 229, 4840 (2010)] to the case of different viscosity and investigate the transition between the tank-treading and tumbling motions in detail. The present numerical results are in a good agreement with other numerical and theoretical studies qualitatively. In addition, we study the inertial effect on this transition and find that the inertial effect might inhibit the tumbling motion in favor of the tank-treading motion, which is observed recently in the literature. The critical viscosity contrast for the transition to the tumbling motion usually increases as the reduced area increases in the Stokes regime. However, we surprisingly observe that the critical viscosity contrast decreases as the reduced area increases to some point in the flow of slightly higher Reynolds number. Our numerical result also shows that the inertial effect has stronger inhibition to tumbling motion when the reduced area is small. DOI: 10.1103/PhysRevE.86.066321
Numerical Study on Statistical Behaviors of Two-Dimensional Dry Foam
(2019-01-01)
Seol, Yunchang
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Kim, Yongsam
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應用數學系
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Department of Applied Mathematics
We investigate the statistical behaviors of two-dimensional dry foam using numerical simulations based on the immersed boundary (IB) method. We model the liquid phase of a foam as a thin elastic boundary with surface tension and the gas phase as a viscous incompressible fluid which can go through the liquid boundary. We present evidence of the existence of a limiting scaling state of the dry foam dynamics in which the asymptotic value of mu 2 , the second moment of the distribution of the numbers of cell sides, lies in the range of 1.3 +/- 0.3. We also numerically verify some well-known formulas derived in the dynamics of two-dimensional dry foam such as von Neumann relation, Lewis law, and Aboav-Weaire law. Our simulation results are comparable to those of soap froth experiments and Potts model simulations. Furthermore, we investigate the statistical behaviors of two-dimensional dry foam in an oscillatory shear flow to show the applicability of our method to more general flow conditions.
Simulating the dynamics of inextensible vesicles by the penalty immersed boundary method
(2010-06-20)
Kim, Yongsam
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Lai, Ming-Chih
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數學建模與科學計算所(含中心)
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Graduate Program of Mathematical Modeling and Scientific Computing, Department of Applied Mathematics
In this paper, we develop an immersed boundary (IB) method to simulate the dynamics of inextensible vesicles interacting with an incompressible fluid. In order to take into account the inextensibility constraint of the vesicle, the penalty immersed boundary (pIB) method is used to virtually decouple the fluid and vesicle dynamics. As numerical tests of our current pIB method, the dynamics of single and multiple inextensible vesicles under shear flows have been extensively explored, and compared with the previous literature. The method is also validated by a series of convergence study, which confirms its consistent first-order accuracy on the velocity field, the vesicle configuration, the vesicle area and the perimeter errors. In addition, the method is also applied to study a binary-component vesicle problem. (C) 2010 Elsevier Inc. All rights reserved.
Simulation of a Soap Film Mobius Strip Transformation
(2017-08-01)
Kim, Yongsam
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Lai, Ming-Chih
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Seol, Yunchang
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應用數學系
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Department of Applied Mathematics
If the closed wire frame of a soap film having the shape of a Mobius strip is pulled apart and gradually deformed into a planar circle, the soap film transforms into a two-sided orientable surface. In the presence of a finite-time twist singularity, which changes the linking number of the film's Plateau border and the centreline of the wire, the topological transformation involves the collapse of the film toward the wire. In contrast to experimental studies of this process reported elsewhere, we use a numerical approach based on the immersed boundary method, which treats the soap film as a massless membrane in a Navier-Stokes fluid. In addition to known effects, we discover vibrating motions of the film arising after the topological change is completed, similar to the vibration of a circular membrane.
The Immersed Boundary Method for Two-Dimensional Foam with Topological Changes
(2012-08-01)
Kim, Yongsam
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Seol, Yunchang
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Lai, Ming-Chih
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Peskin, Charles S.
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數學建模與科學計算所(含中心)
We extend the immersed boundary (IB) method to simulate the dynamics of a 2D dry foam by including the topological changes of the bubble network. In the article [Y. Kim, M.-C. Lai, and C. S. Peskin, J. Comput. Phys. 229: 5194-5207, 2010], we implemented an IB method for the foam problem in the two-dimensional case, and tested it by verifying the von Neumann relation which governs the coarsening of a two-dimensional dry foam. However, the method implemented in that article had an important limitation; we did not allow for the resolution of quadruple or higher order junctions into triple junctions. A total shrinkage of a bubble with more than four edges generates a quadruple or higher order junction. In reality, a higher order junction is unstable and resolves itself into triple junctions. We here extend the methodology previously introduced by allowing topological changes, and we illustrate the significance of such topological changes by comparing the behaviors of foams in which topological changes are allowed to those in which they are not.