Browsing by Author "Misbah, Chaouqi"

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A coupled immersed boundary and immersed interface method for interfacial flows with soluble surfactant
(2018-05-30)
Hu, Wei-Fan
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Lai, Ming-Chih
;
Misbah, Chaouqi
;
應用數學系
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Department of Applied Mathematics
In this paper, we develop a coupled immersed boundary (IB) and immersed interface method (IIM) to simulate the interfacial flow problems with soluble surfactant. That is, a coupled system of surface-bulk convection-diffusion equations must be solved not only on a moving interface but also in an evolving irregular domain. Based on the immersed interface framework, we first begin with the numerical development for the diffusion equation in a fixed irregular domain and then extend the scheme to solve the convection-diffusion equation in an evolving domain. The fluid motion governed by the incompressible Navier-Stokes equations is solved by using traditional immersed boundary method while the bulk surfactant is solved by the proposed immersed interface method. A series of numerical tests for the present scheme have been conducted to illustrate the accuracy and applicability of the method. We first perform the accuracy and efficiency tests for the present IIM solver. We then check the convergence of the surface and bulk surfactant and the fluid variables in the interfacial flow problems. We further run a series of numerical simulations for a suspended droplet under shear flow with presence of soluble surfactant to study the effects of the dimensionless Biot number and the bulk Peclet number on the droplet deformation in details. (C) 2018 Elsevier Ltd. All rights reserved.
A direct Poisson solver in spherical geometry with an application to diffusiophoretic problems
(2020-05-15)
Lin, Te-Sheng
;
Hu, Wei-Fan
;
Misbah, Chaouqi
;
應用數學系
;
Department of Applied Mathematics
We propose a simple and efficient class of direct solvers for Poisson equation in finite or infinite domains related to spherical geometry. The solver was developed based on truncated spherical harmonics expansion, where the differential mode equations were solved by second-order finite difference method without handling coordinate singularities. The solver was further extended to study the dynamics of a diffusiophoretic particle suspended in Stokes flow. Numerical experiments suggested that the particle can achieve a self-sustained unidirectional motion at moderate Peclet numbers, whereas the particle motion becomes chaotic in high Peclet number regimes. The statistical analysis illustrates the run-and-tumble-like nature at short times and diffusive nature at long times without any source of noise. (C) 2020 Elsevier Inc. All rights reserved.
Amoeboid swimming in a channel
(2016)
Wu, Hao
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Farutin, Alexander
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Hu, Wei-Fan
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Thiebaud, Marine
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Rafai, Salima
Several micro-organisms, such as bacteria, algae, or spermatozoa, use flagellar or ciliary activity to swim in a fluid, while many other micro-organisms instead use ample shape deformation, described as amoeboid, to propel themselves either by crawling on a substrate or swimming. Many eukaryotic cells were believed to require an underlying substratum to migrate (crawl) by using membrane deformation (like blebbing or generation of lamellipodia) but there is now increasing evidence that a large variety of cells (including those of the immune system) can migrate without the assistance of focal adhesion, allowing them to swim as efficiently as they can crawl. This paper details the analysis of amoeboid swimming in a confined fluid by modeling the swimmer as an inextensible membrane deploying local active forces (with zero total force and torque). The swimmer displays a rich behavior: it may settle into a straight trajectory in the channel or navigate from one wall to the other depending on its confinement. The nature of the swimmer is also found to be affected by confinement: the swimmer can behave, on average over one swimming cycle, as a pusher at low confinement, and becomes a puller at higher confinement, or vice versa. The swimmer\'s nature is thus not an intrinsic property. The scaling of the swimmer velocity V with the force amplitude A is analyzed in detail showing that at small enough A, V similar to A(2)/eta(2) (where eta is the viscosity of the ambient fluid), whereas at large enough A, V is independent of the force and is determined solely by the stroke cycle frequency and the swimmer size. This finding starkly contrasts with models where motion is based on ciliary and flagellar activity, where V similar to A/eta. To conclude, two definitions of efficiency as put forward in the literature are analyzed with distinct outcomes. We find that one type of efficiency has an optimum as a function of confinement while the other does not. Future perspectives are outlined.
Chaotic Swimming of Phoretic Particles
(2019-12-03)
Hu, Wei-Fan
;
Lin, Te-Sheng
;
Rafai, Salima
;
Misbah, Chaouqi
;
應用數學系
The swimming of a rigid phoretic particle in an isotropic fluid is studied numerically as a function of the dimensionless solute emission rate (or Peclet number Pe). The particle sets into motion at a critical Pe. Whereas the particle trajectory is straight at a small enough Pe, it is found that it loses its stability at a critical Pe in favor of a meandering motion. When Pe is increased further, the particle meanders at a short scale but its trajectory wraps into a circle at a larger scale. Increasing even further, Pe causes the swimmer to escape momentarily the circular trajectory in favor of chaotic motion, which lasts for a certain time, before regaining a circular trajectory, and so on. The chaotic bursts become more and more frequent as Pe increases, until the trajectory becomes fully chaotic, via the intermittency scenario. The statistics of the trajectory is found to be of the run-and-tumble-like nature at a short enough time and of diffusive nature at a long time without any source of noise.