• Transactions of the Korean Society of Mechanical Engineers B
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• v.40 no.11
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• pp.689-695
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• 2016

The performance of proton exchange membrane fuel cells (PEMFC) is strongly related to the water flow and accumulation in the gas diffusion layer (GDL) and catalyst layer. Understanding the behavior of fluid from the characteristics of the media is crucial for the improvement of the performance and design of the GDL. In this paper, a numerical method is proposed to calculate the design parameters of the GDL, i.e., permeability, tortuosity, and effective diffusivity. The fluid flow in a channel filled with randomly packed hard spheres is simulated to validate the method. The flow simulation was performed by lattice Boltzmann method with bounce back condition for the solid volume fraction in the porous media, with different values of porosities. Permeability, which affects the flow, was calculated from the average pressure drop and the velocity in the porous media. Tortuosity, calculated by the ratio the average path length of the randomly injected massless particles to the thickness of the porous media, and the resultant effective diffusivity were in good agreement with the theoretical model. The suggested method can be used to calculate the parameters of real GDL accurately without any modification.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.35 no.2
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• pp.169-178
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• 2011

The dispersion of a pollutant in a turbulent boundary layer has been described in this study by using a two-dimensional lattice Boltzmann method (LBM) and the Smagorinsky sub-grid-scale (SGS) model. The scalar transport equation corresponding to the pollutant concentration is adopted; the pollutant is considered to be in a continuous phase. The pollutant source is classified as ground-level source (GLS) and elevated-point source (ES). Air velocity and particle concentration profile for the pollutant are compared with the respective results and profiles obtained in the experiments of Fackrell and Robins (1982) and Raupach and Legg (1983). The numerical results obtained in this study, i.e., the simulation and the experimental data for the mean flow velocity profiles and the pollutant concentration profiles, are in good agreement with each other.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.40 no.6
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• pp.347-355
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• 2016

This study presents a Lattice Boltzmann Method (LBM) coupled with a momentum-exchange approach/fictitious domain (MEA/FD) method for the simulation of particle suspensions. The method combines the advantages of the LB and the FD methods by using two unrelated meshes, namely, a Eulerian mesh for the flow domain and a Lagrangian mesh for the solid domain. The rigid body conditions are enforced by the momentum-exchange scheme in which the desired value of velocity is imposed directly in the particle inner domain by introducing a pseudo body force to satisfy the constraint of rigid body motion, which is the key idea of a fictitious domain (FD) method. The LB-MEA/FD method has been validated by simulating two different cases, and the results have been compared with those through other methods. The numerical evidence illustrated the capability and robustness of the present method for simulating particle suspensions.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.37 no.7
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• pp.683-693
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• 2013

In this study, the mixing characteristics in lid-driven cavity flows were studied numerically by using a hybrid lattice Boltzmann method (HLBM). First, we compared the numerical results from single-relaxation-time (LB-SRT) and multi-relaxation-time (LB-MRT) models to examine their reliability. In most of the cavity flow, the results from both the LB-SRT and the LB-MRT models were in good agreement with those using a Navier-Stokes solver for Re=100-5000. However, the LB-MRT model was superior to the LB-SRT model for the simulation of higher Reynolds number flows having a geometrical singularity with much lesser spatial oscillations. For this reason, the LB-MRT model was selected to study the mass transport in lid-driven cavity flows, and it was demonstrated that mass transport in the fluid was activated by a recirculation zone in the cavity, which is connected from the top to the bottom surfaces through two boundary layers. Various mixing characteristics such as the concentration profiles, mean Sherwood (Sh) numbers, and velocity were computed. Finally, the detailed transport mechanism and solutions for the concentration profile in the cavity were presented.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.37 no.6
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• pp.591-597
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• 2013

Because most existing non-Newtonian models are not suitable for application to the lattice Boltzmann method, theoretical and numerical studies in this regard remain challenging. In this study, the hydrokinetic (HK) model was modified and applied to a 3D sudden contraction-expansion channel flow, and the characteristics of the HK model flow were evaluated to generate non-trivial predictions in three-dimensional strong shear flows. The HK model is very efficient for application to the lattice Boltzmann method because it utilizes the shear rate and relaxation time. However, the simulation would be unstable in a high shear flow field because the local relaxation time sharply decreases with an increase in the shear rate in a strong shear flow field. In the HK model, it may become necessary to truncate the relaxation time and non-dimensional parameter to obtain stable numerical results.

• Journal of the Institute of Electronics Engineers of Korea TC
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• v.42 no.12
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• pp.103-110
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• 2005

The performance of maximum likelihood(ML) detection for the given channel is analyzed in spatially multiplexed MIMO system. In order to obtain the vector symbol error rate, we define error vectors which represent the geometrical relation between lattice points. The properties of error vectors are analyzed to show that all lattice points in infinite lattice almost surely have four nearest neighbors after random channel transformation. Using this information and minimum distance obtained by the modified sphere decoding algorithm, we formulate the analytical performance of vector symbol error over the given channel. To verify the result, we simulate ML performance over various random channel which are classified into three categories: unitary channel, dense channel, and sparse channel. From the simulation results, it is verified that the derived analytical result gives a good approximation about the performance of ML detector over the all random MIMO channels.

• Communications for Statistical Applications and Methods
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• v.17 no.2
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• pp.193-204
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• 2010

Recently, researchers of the various fields where the spatial analysis is needed have more interested in spatial statistics. In case of data with spatial correlation, methodologies accounting for the correlation are required and there have been developments in methods for spatial data analysis. Lattice data among spatial data is analyzed with following three procedures: (1) definition of the spatial neighborhood, (2) definition of spatial weight, and (3) the analysis using spatial models. The present paper shows a spatial statistical analysis method superior to a general statistical method in aspect estimation by using the trimmed mean squared error statistic, when we analysis the spatial lattice data that outliers are included. To show validation and usefulness of contents in this paper, we perform a small simulation study and show an empirical example with a criminal data in BusanJin-Gu, Korea.

• Nuclear Engineering and Technology
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• v.55 no.9
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• pp.3450-3463
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• 2023

In the two-step analysis of Pebble-Bed type High-Temperature Gas-Cooled Reactor (PB-HTGR), the lattice physics calculation for the generation of homogenized cross-sections is based on the fuel pebble. However, the randomly-dispersed fuel particles in the fuel pebble introduce double heterogeneity and randomness. Compared to the deterministic method, the Monte Carlo method which is flexible in geometry modeling provides a high-fidelity treatment. Therefore, the Monte Carlo code NECP-MCX is extended in this study to perform the lattice physics calculation of the PB-HTGR. Firstly, the capability for the simulation of randomly-dispersed media, using the explicit modeling approach, is developed in NECP-MCX. Secondly, the capability for the generation of the homogenized cross-section is also developed in NECP-MCX. Finally, simplified PB-HTGR problems are calculated by a two-step neutronics analysis tool based on Monte Carlo homogenization. For the pebble beds mixed by fuel pebble and graphite pebble, the bias is less than 100 pcm when compared to the high-fidelity model, and the bias is increased to 269 pcm for pebble bed mixed by depleted fuel pebble. Numerical results show that the Monte Carlo lattice physics calculation for the two-step analysis of PB-HTGR is feasible.

• Journal of Aerospace System Engineering
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• v.17 no.2
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• pp.26-37
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• 2023

Urban air mobility (UAM) is being developed as part of the next-generation aircraft, which could be a viable solution to entrenched problems of urban traffic congestion and environmental pollution. A new airport platform called vertiport as a space where UAM can take off and land vertically is also being introduced. Noise regulations for UAM will be strict due to its operation in a highly populated urban area. Ground effects caused by vertiport can directly affect aerodynamic forces and noise characteristics of UAM. In this study, ground effects of vertiport on aerodynamic loads, vorticity field, and far-field noise were analyzed using Lattice-Boltzmann Method (LBM) simulation and Ffowcs Williams and Hawkings (FW-H) acoustic analogy with a permeable surface method.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.32 no.9C
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• pp.888-894
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• 2007

The computational complexity of a sphere decoder (SD) is conventionally reduced by decoding order scheme which sorts candidate symbols in the ascending order of the Euclidean distance from the output of a zero-forcing (ZF) receiver. However, since the ZF output may not be a reliable sorting reference, we propose two types of sorting schemes to allow faster decoding. The first is to use the newly found lattice points in the previous search round instead of the ZF output (Type I). Since these lattice points are closer to the received signal than the ZF output, they can serve as a more reliable sorting reference for finding the maximum likelihood (ML) solution. The second sorting scheme is to sort candidate symbols in descending order according to the number of candidate symbols in the following layer, which are called child symbols (Type II). These two proposed sorting schemes can be combined with layer sorting for more complexity reduction. Through simulation, the Type I and Type II sorting schemes were found to provide 12% and 20% complexity reduction respectively over conventional sorting schemes. When they are combined with layer sorting, Type I and Type II provide an additional 10-15% complexity reduction while maintaining detection performance.