• Journal of the Institute of Electronics Engineers of Korea SD
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• v.45 no.4
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• pp.146-153
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• 2008

In this paper, a integrated phase-locked loop(PLL) as a clock multiphase generator for a high speed serial link is designed. The designed PLL keeps the same bandwidth and damping factor by using programmable current mirror in the whole operation frequency range. Also, the close-loop transfer function and VCO's phase-noise transfer function of the designed PLL are obtained with circuit netlists. The self impedance on board-mounted chip is calculated according to sizes and positions of decoupling capacitors. Especially, the detailed self-impedance analysis is carried out between frequency ranges represented the maximum gain in the close-loop transfer function and the maximum gain in the VCO's phase noise transfer function. We shows PLL's jitter characteristics by decoupling capacitor's sizes and positions from this result. The designed PLL has the wide operating range of 0.4GHz to 2GHz in operating voltage of 1.8V and it is designed 0.18-um CMOS process. The reference clock is 100MHz and PLL power consumption is 17.28mW in 1.2GHz.

• Structural Engineering and Mechanics
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• v.66 no.4
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• pp.505-513
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• 2018

Steam flooding is widely used in heavy oil reservoir with coupling effects among the formation temperature change, fluid flow and solid deformation. The effective stress, porosity and permeability in this process can be affected by the multi-physical coupling of thermal, hydraulic and mechanical processes (THM), resulting in a complex interaction of geomechanical effects and multiphase flow in the porous media. Quantification of the state of deformation and stress in the reservoir is therefore essential for the correct prediction of reservoir efficiency and productivity. This paper presents a coupled fluid flow, thermal and geomechanical model employing a program (MATLAB interface code), which was developed to couple conventional reservoir (ECLIPSE) and geomechanical (ABAQUS) simulators for coupled THM processes in multiphase reservoir modeling. In each simulation cycle, time dependent reservoir pressure and temperature fields obtained from three dimensional compositional reservoir models were transferred into finite element reservoir geomechanical models in ABAQUS as multi-phase flow in deforming reservoirs cannot be performed within ABAQUS and new porosity and permeability are obtained using volumetric strains for the next analysis step. Finally, the proposed approach is illustrated on a complex coupled problem related to steam flooding in an oil reservoir. The reservoir coupled study showed that permeability and porosity increase during the injection scenario and increasing rate around injection wells exceed those of other similar comparable cases. Also, during injection, the uplift occurred very fast just above the injection wells resulting in plastic deformation.

• Journal of computational fluids engineering
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• v.17 no.3
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• pp.99-107
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• 2012

In this paper, the shock waves in compressible solids and liquids are simulated using a six-equation diffuse interface multiphase flow model that is extended to the Cochran and Chan equation of state. A pressure relaxation method based on a volume fraction function and a pressure-correction equation are newly implemented to the six-equation model. The developed code has been validated by a shock tube problem with liquid nitromethane and an impact problem of a copper plate on a solid explosive. In addition, a new problem, an impact of a copper plate on liquid nitromethane, has been solved. The present code well shows the wave structures in compressible solids and liquids without any numerical oscillations and overshoots. After the impact of a solid copper plate on liquid, two shock waves (one propagates into liquid and the other into solid) are generated and a material interface moves to the impacting direction. The computational results show that the shock velocity inside the liquid linearly increases with the impact velocity.

• Journal of Ocean Engineering and Technology
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• v.35 no.2
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• pp.121-130
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• 2021

In this study, a computational fluid dynamics (CFD) simulation based on an Eulerian-Eulerian approach was used to evaluate the mixing performance of a mud agitator through the distribution of bulk particles. Firstly, the commercial CFD software Star-CCM+ was verified by performing numerical simulations of single-phase water mixing problems in an agitator with various turbulence models, and the simulation results were compared with an experiment. The standard model was selected as an appropriate turbulence model, and a grid convergence test was performed. Then, a simulation of the liquid-solid multi-phase mixing in an agitator was simulated with different multi-phase interaction models, and lift and drag models were selected. In the case of the lift model, the results were not significantly affected, but Syamlal and O'Brien's drag model showed more reasonable results with respect to the experiment. Finally, with the properly determined simulation conditions, a multi-phase flow simulation of a mud agitator was performed to predict the mixing time and spatial distribution of solid particles. The applicability of the CFD multi-phase simulation for the practical design of a mud agitator was confirmed.

• Journal of computational fluids engineering
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• v.18 no.1
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• pp.13-21
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• 2013

Cavitating flow is usually formed on the surface of a high speed underwater object. When a object moves near a free surface at very high speed, the cavity signature becomes one of the major factors to be overcome by sensors of military satellite. The present work was to study the free surface effect on the ventilated cavitation. The governing equations were Navier-Stokes equations based on a homogeneous mixture model. The multiphase flow solver used an implicit preconditioning method in the curvilinear coordinate system. The cavitation model used here was the one first presented by Merkle et al.(2006) and redeveloped by Park & Ha(2009). Computations considered the free surface effects were carried out with a NACA0012 hydrofoil and the corresponding results were compared with the experimental data to have a good agreement. Calculations were then performed considering the ventilated cavitation, including the effect of non-condensable gas under the free surface effects.

• Structural Engineering and Mechanics
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• v.23 no.2
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• pp.147-161
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• 2006

Recent developments in fractal theory suggest that fractal may provide a more realistic representation of characteristics of cementitious materials. In this paper, the roughness of fracture surfaces in cementitious material has been characterized by fractal theory. A systematic experimental investigation was carried out to examine the dependency of fracture parameters on the aggregate sizes as well as the loading rates. Three maximum aggregate sizes (4.76 mm, 12.7 mm, and 19.1 mm) and two loading rates (slow and fast loading rate) were used. A total of 25 compression tests and 25 tension tests were performed. All fracture parameters exhibited an increase, to varying degrees, when aggregates were added to the mortar matrix. The fracture surfaces of the specimens were digitized and analyzed. Results of the fractal analysis suggested that concrete fracture surfaces exhibit fractal characteristics, and the fractal geometry provide a useful tool for characterizing nonlinear fracture behavior of concrete. Fractal dimension D was monotonically increased as maximum aggregate sizes increase. A new fractal fracture model was developed which considers the size and shape of aggregate, and the crack paths in the constituent phases. Detailed analyses were given for four different types of fracture paths. The fractal fracture model can estimate fractal dimension for multiphase composites.

• Transactions of the Korean hydrogen and new energy society
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• v.30 no.6
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• pp.585-592
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• 2019

In this study, a flashing boiling phenomenon of pressurized water jet was numerically studied and validated against an experimental data in the literatures. The volume of fluid (VOF) technique was used to consider two-phase behavior of water, while the homogeneous relaxation model (HRM) model was used to provide the velocity of phase change. During the flashing boiling through a nozzle, a mach disk was observed near nozzle exit because of pressure drop resulting from two-phase under-expansion. The flashing jet structure, local distributions of temperature/vapor volume fraction/velocity, and position of the mach disk were examined as nozzle inlet temperature changed.

• Journal of the Korean Society of Visualization
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• v.20 no.3
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• pp.49-57
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• 2022

The interaction of planar shock wave with rectangular water column is investigated numerically. The flow phenomenon like reflection, transmission, cavitation, recirculation of shock wave, and large negative pressure due to expansion waves was discussed qualitatively and quantitatively. The numerical simulation was performed in a shock tube with a water column, and planar shock was initiated with a pressure ratio of 10. Three cases of the water column with different thicknesses, namely 0.5D, 1D, and 2D, were installed and studied. Water naturally has a higher acoustic impedance than air and mitigates the shock wave considerably. The numerical simulations were modelled using Eulerian and Volume of fluids multiphase models. The Eulerian model assumes the water as a finite structure and can visualize the shockwave propagation inside the water column. Through the volume of fluids model, the stages of breakup of the water column and mitigation effects of water were addressed. The numerical model was validated against the experimental results. The computational results show that the installation of a water column significantly impacts the mitigation of shock wave.

• Journal of the Korean Society of Visualization
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• v.15 no.3
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• pp.34-40
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• 2017

Within fractionating devices existing in separation and purification industries, sieve trays are widely used as tower internals and their choice is due to economical attractiveness. While operating a trayed distillation tower weeping phenomenon has a critical effect on the efficiency, in this case study a weeping phenomenon was undertaken by means of numerical model in a rectangular sieve tray. Eulerian-Eulerian Computational Fluid Dynamics (CFD) method was used and the obtained CFD results are in a good agreement with the experimental data in terms of weeping rate and pressure drop.

• Transactions of the Korean Society of Automotive Engineers
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• v.15 no.6
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• pp.62-72
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• 2007

Fracture analysis of human bone is necessary to predict the failure of musculoskeletal structures and to heal them by several possible mechanisms under different loading conditions. But human bone is a complex material, with a multiphase, heterogeneous and anisotropic microstructure. Due to the difficulty of obtaining experimental and clinical results, the importance of numerical analysis and computational simulations in biomechanics are increasing gradually. In this study, stress analysis for human femur model is performed by using the 2-dimensional finite element method(FEM) and its stress distribution is determined. From these results, the fracture mechanic parameters are calculated and the fracture criteria on human femur are investigated and discussed.