Wook Ryol Hwang

Numerical simulations of Stokes–Brinkman equations for permeability prediction of dual scale fibrous porous media

A new finite-element scheme to solve the Stokes–Brinkman equation for flow analyses in dual scale porous media is presented and has been applied to predict the effective permeability of dual scale fibrous media. Both continuous and discontinuous stress conditions at the interface between a porous media and a surrounding fluid are explored by introducing an equivalent momentum equation for the Brinkman equation. The proposed scheme uses a uniform structured regular rectangular mesh to discretize the domain and employs the level-set method to describe the porous media allowing for inclusion of complex geometrical features easily. Biperiodic boundary conditions have been applied to conduct the flow analysis in a representative volume of mesoscale porous structures. Numerical solutions in a parallel channel flow over a porous media are presented and compared with analytic solutions to assess the accuracy of the proposed scheme. The scheme is then applied to flow past two regular periodic geometries of ellipti...

Permeability Prediction of Fibrous Porous Media in a Bi-Periodic Domain:

A finite-element scheme is presented to investigate the permeability of fibrous porous medium in a bi-periodic domain such that a single cell problem with a small number of fiber filaments may represent a large number of repeated structures. Based on the fictitious domain method and the rigid-ring description, we use a fixed regular mesh for the entire domain, including the interior of fibers, and the fiber is described by its boundary only. A formal bi-periodicity has been implemented with the mortar-element method and fibers split by the domain boundary have been treated in a relatively easy way. To demonstrate the feasibility, examples of the regular fiber packing, the random fiber distribution and the fiber bundle problems have been investigated using the bi-periodic domain and compared with the literature.A finite-element scheme is presented to investigate the permeability of fibrous porous medium in a bi-periodic domain such that a single cell problem with a small number of fiber filaments may represent a large number of repeated structures. Based on the fictitious domain method and the rigid-ring description, we use a fixed regular mesh for the entire domain, including the interior of fibers, and the fiber is described by its boundary only. A formal bi-periodicity has been implemented with the mortar-element method and fibers split by the domain boundary have been treated in a relatively easy way. To demonstrate the feasibility, examples of the regular fiber packing, the random fiber distribution and the fiber bundle problems have been investigated using the bi-periodic domain and compared with the literature.

Direct numerical simulations of droplet emulsions in sliding bi-periodic frames using the level-set method

We present a direct numerical simulation technique for droplet emulsions of Newtonian-Newtonian system in simple shear flow. The Lees-Edwards type boundary condition has been incorporated with the sliding bi-periodic frame of Hwang et al. [W.R. Hwang, M.A. Hulsen, H.E.H. Meijer, Direct simulation of particle suspensions in sliding bi-periodic frames, J. Comput. Phys. 194 (2004) 742] for the continuous flow field problem and the level-set method with the continuous surface stress (CSS) formulation has been used for accurate description of the sharp interfaces. Based on the standard velocity-pressure formulation of the finite-element method, we use the mortar element method for the implementation of the sliding periodicity and employ the discontinuous Galerkin (DG) method for the stabilization of the interface advection equation. We present numerical results on the morphological development for a single, two and multiple drops in sliding bi-periodic frames for the demonstration of the feasibility of the present method in investigation of the relationship between the morphology and the bulk material responses such as the shear viscosity and the first normal stress difference.

Chaotic advection in a cavity flow with rigid particles

Experiments by Maric and Macosko 1 show that the addition of a small number of balls into a minimixer significantly improves the dispersion characteristics in polymer blends. Their results show that the balls enhance the circulation of materials from low to high shear rate regions and promote breakup of drops. In this work, we examine the direct influence of the addition of such a ball on distributive mixing. We study chaotic advection of fluids in a simple lid-driven cavity flow containing freely suspended inertialess rigid particles using dynamical systems theory and numerical simulations. A large number of papers are published which deal with the influence of a time-periodic movement of different walls 2,3 on chaotic advection. 4 Other studies show the influence of changes in the geometric aspect ratio of the cavity, 5 while some others show that a single oscillating wall can induce chaotic advection if inertia becomes important. 6 Vikhansky 7 also studied cavity flows with rigid particles and claimed that the Lagrangian chaos of the particle motion induces Eulerian chaos of the flow. Admittedly, this argument seems evident especially in flows possessing many particles with complicated time-dependent particle movements, because of complex interactions between particle/particle and particle/fluid sreferred to Lagrangian chaos of the particle motion in Ref. 7d. In this work, however, we report that even a regular periodic motion of a single particle can induce chaotic advection of the fluid material as well. We concentrate on understanding the mechanism how the existence of a particle affects the dynamical systems of the flow, by visual

Transverse mobility prediction of non-Newtonian fluids across fibrous porous media

This work is an extension of our previous work [Wang JF and Hwang WR. Permeability prediction of fibrous porous media in a bi-periodic domain. J Compos Mater 2007; 42: 909—929], in which a finite-element fictitious-domain mortar-element technique was developed to investigate the permeability of fibrous porous media in the bi-periodic domain, to non-Newtonian shear-thinning fluid. Considering the amount of shear-thinning, the pressure drop, the fiber microstructure, and the porosity as parameters, we investigate (i) the (normalized) mobility and its dependence on both the amount of shear-thinning and the given pressure drop; (ii) mechanisms leading to the main flow path in a highly shear-thinning fluid in randomly distributed fiber problems, and (iii) inter-tow and intra-tow non-Newtonian flow characteristics in a fiber bundle problem. The dependence of the mobility on shear-thinning has been found to appear completely opposite according to given pressure drop values.This work is an extension of our previous work [Wang JF and Hwang WR. Permeability prediction of fibrous porous media in a bi-periodic domain. J Compos Mater 2007; 42: 909—929], in which a finite-element fictitious-domain mortar-element technique was developed to investigate the permeability of fibrous porous media in the bi-periodic domain, to non-Newtonian shear-thinning fluid. Considering the amount of shear-thinning, the pressure drop, the fiber microstructure, and the porosity as parameters, we investigate (i) the (normalized) mobility and its dependence on both the amount of shear-thinning and the given pressure drop; (ii) mechanisms leading to the main flow path in a highly shear-thinning fluid in randomly distributed fiber problems, and (iii) inter-tow and intra-tow non-Newtonian flow characteristics in a fiber bundle problem. The dependence of the mobility on shear-thinning has been found to appear completely opposite according to given pressure drop values.

EXPERIMENTAL INVESTIGATION OF ELECTROSTATIC SPRAY OF TWIN-FLUID ATOMIZATION

An electrostatic atomizer of twin-fluid atomization with combined needle and annular electrode was developed, from which a relatively high speed electrostatic spray was obtained. The flow field of charged droplets of the electrostatic spray under different charging voltages was measured by the PIV technique. The characteristics of atomization and motion of droplets of uncharged and charged spray were investigated systematically and analyzed by PIV and PDA systems. The flow field of the twin-fluid jet of electrostatic spray and droplet size distribution was visualized by PIV and PDA measurement techniques. The results indicate that the flow field of spray is clearly affected by the electrical field. The velocity in the spray center becomes higher and the flow field around the spray shows a change of fluctuation with the increase of charging voltage. The diameter of spray droplets on the centerline increases when the spray is charged and keeps the trend of gradual increase from the center point to the edge of the spray. The spray expands when it is charged and the spray angle becomes difficult to define since more particles are distributed around the spray core.

Experiments on Characteristic Deformation and Breakup Behaviors of an Immiscible Drop in a Screw Channel Flow

Abstract Deformation and breakup mechanisms of an immiscible drop were investigated experimentally in a screw channel flow, which mimics the flow in the metering section of the single screw extruder. It was observed that the deformation/breakup mechanisms are far distinct from the well-known routes for the linear flow, mainly because of the complex flow history that can be characterized by the repetition of strong/weak shear, the continuously varying shear direction and the acceleration/deceleration. The experimental observations include: comet-like thread formation with the large bulbous head and the long narrow tail; the initiation of the breakup at the entrance of the low-shear zone; breakup by asymmetric end pinching; breakup by the growth of the capillary wave, propagating from the tail toward the head; monotonically decreasing droplet size distribution from the head to tail. The mechanism for the comet-like thread formation has been investigated by using the numerically obtained flow field.

Hydrodynamic extensional stress during the bubble bursting process for bioreactor system design

Cell damage, one of critical issues in the bioreactor design for animal cell culture, is caused mainly from the bubble bursting at the free surface subjected to strong extensional flows. In this work, extensive computational studies are performed to investigate bubble bursting process in great details. Extensive numerical simulations are performed for a wide range of bubble diameters (from 0.5 to 6 mm) and the surface tension values (from 0.03 to 0.072 N/m), with which effects of the bubble size and surfactant (PF68) concentration on the hydrodynamic stress are investigated. For all the cases, the maximum extensional stress appears at the instance when receding films impact each other at the bottom of the bubble. A model equation based on numerical simulations is presented to predict the maximum extensional stress as a function of the bubble diameter and the surface tension. The bubble diameter has turned out to contribute significantly the maximum hydrodynamic extensional stress. In addition, the bubble collapsed time and the affected volume around a bubble subjected to the critical extensional stress are investigated. The extensional stress estimation is reported as a function of the bubble size and the surface tension. The influence of the bubble size on the maximum stress dominates and extensional stress reaches up to the order of 104 Pa for bubble size of 0.5 mm.

Hydrodynamics of CNT dispersion in high shear dispersion mixers

In this work, we investigate the carbon nanotube (CNT) fragmentation mechanism and dispersion in high shear homogenizers as a plausible dispersion technique, correlating with device geometries and processing conditions, for mass production of CNT-aluminum composites for automobile industries. A CNT dispersion model has been established in a turbulent flow regime and an experimental method in characterizing the critical yield stress of CNT flocs are presented. Considering CNT dispersion in ethanol as a model system, we tested two different geometries of high shear mixers — blade-stirrer type and rotor-stator type homogenizers — and reported the particle size distributions in time and the comparison has been made with the modeling approach and partly with the computational results.

Numerical simulation of a shear-thinning fluid through packed spheres

Flow behaviors of a non-Newtonian fluid in spherical microstructures have been studied by a direct numerical simulation. A shear-thinning (power-law) fluid through both regular and randomly packed spheres has been numerically investigated in a representative unit cell with the tri-periodic boundary condition, employing a rigorous three-dimensional finite-element scheme combined with fictitious-domain mortar-element methods. The present scheme has been validated for the classical spherical packing problems with literatures. The flow mobility of regular packing structures, including simple cubic (SC), body-centered cubic (BCC), face-centered cubic (FCC), as well as randomly packed spheres, has been investigated quantitatively by considering the amount of shear-thinning, the pressure gradient and the porosity as parameters. Furthermore, the mechanism leading to the main flow path in a highly shear-thinning fluid through randomly packed spheres has been discussed.