Hyungson Ki

Level set method for two-phase incompressible flows under magnetic fields

This article presents a level set method for two-phase, incompressible flows under magnetic fields. The magnetic force caused by the difference in magnetic permeability of the fluids are derived and incorporated into the momentum equation using the level set function. The governing equation for magnetic fields (magnetostatic equation) is also obtained by considering the discontinuity in the spatial distribution of magnetic permeability using the level set function. To test the method, falling droplet, droplet oscillation and stretching, and rising bubble problems in the presence of a magnetic field are simulated.

On vaporization in laser material interaction

In this article, vaporization processes in the laser interaction with materials are studied theoretically and computationally, focusing on evaporation and homogeneous bubble nucleation. Simulations are carried out using the Redlich–Kwong equation of state and temperature-dependent material property models that can be used up to the critical point. From theoretical considerations, four important temperatures are identified in the understanding of laser material interaction. This study also shows that there are upper limits to the amount of energy that can be consumed by vaporization, which takes place at a temperature that is lower than the material’s critical point. This study also discusses the transition from the thermal mode of ablation to the nonthermal mode in terms of the energy capacity of homogeneous boiling.

FDTD method for laser absorption in metals for large scale problems.

The FDTD method has been successfully used for many electromagnetic problems, but its application to laser material processing has been limited because even a several-millimeter domain requires a prohibitively large number of grids. In this article, we present a novel FDTD method for simulating large-scale laser beam absorption problems, especially for metals, by enlarging laser wavelength while maintaining the materials reflection characteristics. For validation purposes, the proposed method has been tested with in-house FDTD codes to simulate p-, s-, and circularly polarized 1.06 μm irradiation on Fe and Sn targets, and the simulation results are in good agreement with theoretical predictions.

Computation of flow-induced stresses in a structure using a hybrid momentum equation approach

In this paper, we present a method to formulate momentum equations for computing flow-induced stresses in a structure. The momentum equations for fluids and solids are cast into a common form, which when integrated over a computational cell generates a hybrid momentum equation customised to fluid-solid interface configurations. The resulting equation can be solved by a proper numerical method for computational fluid dynamics, such as the SIMPLE algorithm. This approach has been applied to Poiseuille flow between two parallel plates and lid-driven cavity flow in a solid container. Simulation results show a good agreement with the ABAQUS™ results.

A unified momentum equation approach for computing thermal residual stresses during melting and solidification

Abstract In this article, we present a novel numerical method for computing thermal residual stresses from a viewpoint of fluid–structure interaction (FSI). In a thermal processing of a material, residual stresses are developed as the material undergoes melting and solidification, and liquid, solid, and a mixture of liquid and solid (or mushy state) coexist and interact with each other during the process. In order to accurately account for the stress development during phase changes, we derived a unified momentum equation from the momentum equations of incompressible fluids and elastoplastic solids. In this approach, the whole fluid–structure system is treated as a single continuum, and the interaction between fluid and solid phases across the mushy zone is naturally taken into account in a monolithic way. For thermal analysis, an enthalpy-based method was employed. As a numerical example, a two-dimensional laser heating problem was considered, where a carbon steel sheet was heated by a Gaussian laser beam. Momentum and energy equations were discretized on a uniform Cartesian grid in a finite volume framework, and temperature-dependent material properties were used. The austenite–martensite phase transformation of carbon steel was also considered. In this study, the effects of solid strains, fluid flow, mushy zone size, and laser heating time on residual stress formation were investigated.

Water-assisted pulsed Er:YAG laser interaction with silicon

Silicon is virtually transparent to the Er:YAG laser with a wavelength of 2.94u2009μm. In this study, we report that moderately doped silicon (1–10 Ωu2009cm) can be processed by a pulsed Er:YAG laser with a pulse duration of 350u2009μs and a peak laser intensity of 1.7u2009×u2009105u2009W/cm2 by applying a thin water layer on top of silicon as a light absorbing medium. In this way, water is heated first by strongly absorbing the laser energy and then heats up the silicon wafer indirectly. As the silicon temperature rises, the free carrier concentration and therefore the absorption coefficient of silicon will increase significantly, which may enable the silicon to get directly processed by the Er:YAG laser when the water is vaporized completely. We also believe that the change in surface morphology after melting could contribute to the increase in the laser beam absorptance. It was observed that 525u2009nm-thick p-type wafer specimens were fully penetrated after 15 laser pulses were irradiated. Bright yellow flames were observed duri...