See Jo Kim

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.

Effect of sloped die lip geometry on the operability window in slot coating flows using viscocapillary and two-dimensional models

As an indicator for determining the operability window in slot coating flow, the viscocapillary model considering various configurations of upstream and downstream slot die lips was tested and compared with Navier–Stokes two-dimensional model. Bead pressure and sloped lip angle conditions for uniform coating operation demarcated from leaking and bead break-up defects were quantitatively predicted from the position of upstream meniscus from both models. By comparing the results, it is confirmed that the viscocapillary model for many kinds of sloped die lips could predict the operability window accurately. It is also found that there exists vortex or recirculation regimes inside upstream and downstream coating bead regions, depending on the angles of sloped die lips, even for the stable coating flow. The flow control by die lip structure will be usefully applied to design the strategy for the reliable and optimal coating process, including vortex-free windows.

Rheological behavior of magnetic powder mixtures for magnetic PIM

Powder injection molding (PIM) is a promising manufacturing technology for the net-shape production of small, complex, and precise metal or ceramic components. In order to manufacture high quality magnets using PIM, the magneto-rheological (MR) properties of the PIM feedstock, i.e. magnetic powder-binder mixture, should be investigated experimentally and theoretically. The current research aims at comprehensive understanding of the rheological characteristics of the PIM feedstock. The feedstock used in the experiment consists of strontium ferrite powder and paraffin wax. Steady and oscillatory shear tests have been carried out using a plate-and-plate rheometer, under the influence of a uniform magnetic field applied externally. Rheological properties of the PIM feedstock have been measured and characterized for various conditions by changing the temperature, the powder fraction and the magnetic flux density.

Modeling and numerical simulation of multiflux die in the multilayer co-extrusion process

It is of great importance to understand the stretching and folding mechanism in the multiflux co-extrusion die to get uniform multilayer distribution at the end of die lip in the multilayer co-extrusion processes. In this work, to understand the mechanism of the layer distribution, modeling and numerical simulation were carried out for three-dimensional flow analysis in the multilayer co-extrusion die. The multilayer flow fields were numerically visualized and analyzed on the arbitrary cross-section of the multiflux die. In addition, numerical results for the multiflux die characteristics were obtained for non-Newtonian fluids in terms of power-law index for the cross model, which will be useful for the optimal design of screw and die, simultaneously, in the multilayer co-extrusion process.

Effect of shim configuration on internal die flows for non-Newtonian coating liquids in slot coating process

In this study, a strategy for designing optimal shim configuration inside a slot die is suggested to assure the uniform coating flow distribution of various non-Newtonian shear-thinning liquids at the die exit in a slot coating system. Flow patterns of non-Newtonian liquids inside the slot die, via three-dimensional computations, have been compared using various shim geometries which can adjust the flow region in a slot manifold. The rather non-uniform (parabolic) velocity distributions of shear-thinning liquids at the die exit under the basic shim condition could be effectively flattened by the modification of shim geometry without the change of die manifold structure. Dimensions of hybrid shims for controlling flow features at edge and center regions within slit channel are positively tuned, according to the shear-thinning level of coating liquids.

Atomistic Simulation of Sintering Mechanism for Copper Nano-Powders

Abstract The sintering mechanisms of nanoscale copper powders have been investigated. A molecular dynamics(MD) simulation with the embedded-atom method (EAM) was employed for these simulations. The dimensional changesfor initial-stage sintering such as characteristic lengths, neck growth, and neck angle were calculated to understand thedensification behavior of copper nano-powders. Factors affecting sintering such as the temperature, powder size, andcrystalline misalignment between adjacent powders have also been studied. These results could provide information ofsetting the processing cycles and material designs applicable to nano-powders. In addition, it is expected that MD sim-ulation will be a foundation for the multi-scale modeling in sintering process.Keywords: Molecular dynamics (MD) simulation, Sintering, Copper ······························································································································· ·································································································

Effect of upstream meniscus shape on dynamic wetting and operating limits of Newtonian coating liquids in slot coating bead flows

The coating bead flow and operability window for Newtonian coating liquids are theoretically and experimentally investigated in the slot coating process, with a focus on the shape of the upstream meniscus and contact angles. From the flow visualization in the coating bead region, the contact angles of the upstream meniscus were measured by changing the flow rate and web speed under uniform operating conditions. It was confirmed that the dynamic contact angle is closely related to the capillary number in this process, based on the Hoffman–Voinov–Tanner model. The viscocapillary and two-dimensional Navier–Stokes models using the experimentally observed contact angles accurately predicted the coating bead dynamics and operability windows for two Newtonian liquids.

Numerical study of the flow smulation and visualization in the co-extrusion die using OpenFOAMⓇ

Abstract. It is of great importance to obtain the uniform layer thickness in the multi-layer co-extrusion processes. In the present study, the three-dimensional numerical simulation was carried out using the open source code named OpenFOAMⓇ to understand the flow characteristics in the multi-layer die. In this numerical study, Multi-thin-layers were successfully computed depending on the number of repeating units. The generation mechanism for the multi-layer was numerically verified by the flow simulation and visualization in the co-extrusion die using OpenFOAMⓇ. The results suggested that the multi-layer has a divided and folded mechanism similar to the stretching and folding in the chaotic flow. Key Words: OpenFOAM(오픈 폼), multi-layer(다층), Computational Fluid Dynamics(전산유체역학), Co-extrusion(공압출), Flow Visualization(유동가시화) 1. 서 론 1) 압출 공정은 산업 현장에서 폴리머를 일정한 두께를 가지는 플라스틱 필름이나 시트 등을 생산하는데 사용이 된다. 압출 공정 관련 연구로 본 연구자에 의하여 단축압출기 내에서의 변형정도를 예측하는 척도로써 변형특성 즉 DC(deformation characteristics)를 정의하고 이로부터 기존의 단축압출기 내에서의 DC의 증가, 즉 혼합 정도의 증가는 시간 또는 압출기의 길이에 대하여 선형적임을 밝혀내었다.

NUMERICAL SIMULATIONS OF DROP DEFORMATION IN A CYLINDRICAL TUBE USING MOVING AND FIXED GRID METHODS

It is quite meaningful in an engineering aspect to investigate liquid drop behaviors in two-phase flow, on which dispersive mixing or overall rheological characteristics heavily depend. There are two approaches numerically to deal with two-phase flow problems, which are the moving- and the fixed-grid methods: that is, interface tracking and interface capturing methods. In this study, we developed multiphase flow codes in both Lagrangian and Eulerian frameworks and compared each solution for the single drop problem in an axisymmetric channel flow. Deformation patterns of drops were observed by these two methods and we have discussed the characteristic features of two methods from the deformation of drop for the moving boundary problems.