• Transactions of the Korean Society of Mechanical Engineers
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• v.17 no.6
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• pp.1541-1546
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• 1993

To extend the widely used Gibson and Launder's second order closure model to the low-Reynolds-number region near a wall, modifications have been made for velocity pressure-gradient interaction and dissipation terms in the stress equations, and also for the dissipation rate equation. From the computation of fully developed plane channel flow, it is found that the results with present model agree well with the data of direct numerical simulation in the predictions of stress components. And, the computed mean velocity profile coincides with the universal velocity law.

• The KSFM Journal of Fluid Machinery
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• v.14 no.4
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• pp.57-63
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• 2011

Regenerative blower is suitable for hydrogen recirculation in fuel cell vehicle due to its capability of high pressure rise in single stage. Numerical models were applied to investigate inner gap leakage flow characteristics. A leakage flow in the inner gap is dominantly affected by pressure gradient. Therefore a blower with concentric channel type was suggested as one of modified models for reducing the inner gap pressure gradient. Also numerical results such as pressure rise, efficiency, leakage flow rate and torque were compared between modified and reference models. The performance of concentric channel type was improved as a result of reduced leakage flow.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.30 no.3C
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• pp.141-146
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• 2005

The authors investigate the convergence speed problem of nonlinear adaptive equalization. Convergence constraints and time constant of radial basis function network using stochastic gradient (RBF-SG) algorithm is analyzed and a method of making time constant independent of hidden-node output power by using sample-by-sample node output power estimation is derived. The method for estimating the node power is to use a single-pole low-pass filter. It is shown by simulation that the proposed algorithm gives faster convergence and lower minimum MSE than the RBF-SG algorithm.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.33 no.4
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• pp.288-297
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• 2009

A serpentine channel geometry often used in a fuel cell has a strong pressure gradient between adjacent channels in specific regions. The pressure gradient helps some amount of reactant gas penetrate through a gas diffusion layer(GDL). As a result, the overall serpentine flow structure is slightly different from the intention of a designer. The purpose of this paper is to examine the effect of serpentine flow structure on current density distribution. By using a commercial code, STAR-CD, a numerical simulation is performed to analyze the fuel cell with high aspect ratio of active area. To increase the accuracy of the numerical simulation, GDL permeabilities are measured with various compressive forces. Three-dimensional flow field and current density distribution are calculated. For the verification of the numerical simulation results, water condensation process in the cathode channel is observed through a transparent bipolar plate. The result of this study shows that the region of relatively low current density corresponds that of dropwise condensation in cathode channels.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.33 no.7
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• pp.527-534
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• 2009

Flame stabilization characteristics were experimentally investigated in a fuel-air cross flowing mixing layer. A combustor consists of a narrow channel of air steam and a cross flowing fuel. Depending on the flow rates of methane and air, flame can be stabilized in two modes. First is an attached flame which is formulated at the backward step where the methane and air streams meet. Second is a lifted-flame which is formulated within the mixing layer far down steam from backward step. The heights and flame widths of the lifted flames were measured. Flame shapes of the lifted flames were similar to an ordinary edge flame or a tribrachial flame, and their behavior could be explained with the theories of an edge flame. With the increase of the mixing time between fuel and air, the fuel concentration gradient decreases and the flame propagation velocity increases. Thus the flame is stabilized where the flow velocity is matched to the flame propagation velocity in spite of a significant disturbance in the fuel mixing and heat loss within the channel. This study provides many experimental results for a higher fuel concentration gradient, and it can also be helpful for the development and application of a smaller combustor.

• 한국신재생에너지학회:학술대회논문집
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• 2007.11a
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• pp.147-151
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• 2007

A serpentine channel geometry often used in a polymer electrolyte membrane fuel cell has a strong pressure gradient between adjacent channels in specific regions. The pressure gradient helps some amount of reactant gas penetrate through a gas diffusion layer(GDL). As a result, the overall serpentine flow structure is slightly different from intention of a designer. The purpose of this paper is to examine the effect of serpentine flow structure on current density distribution. By using a commercial code, STAR-CD, a numerical simulation is performed to analyze the fuel cell with relatively high aspect ratio active area. To increase the accuracy of the numerical simulation, GDL permeabilities are measured with various compression conditions. Three-dimensional flow field and current density distribution are calculated. For the verification of the numerical simulation results, water condensation process in the cathode channel is observed through a transparent bipolar plate. The result of this study shows that the region of relatively low current density corresponds to that of dropwise condensation in cathode channels.

• The Journal of the Acoustical Society of Korea
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• v.38 no.3
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• pp.283-289
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• 2019

RLS (Recursive-least-squares) algorithm is known to have good convergence and excellent error level after convergence. However, there is a disadvantage that numerical instability is included in the algorithm due to inverse matrix calculation. In this paper, we propose an algorithm with no matrix inversion to avoid the instability aforementioned. The proposed algorithm still keeps the same convergence performance. In the proposed algorithm, we adopt an averaged gradient-based step size as a self-adjusted step size. In addition, a variable forgetting factor is introduced to provide superior performance for time-varying channel estimation. Through simulations, we compare performance with conventional RLS and show its equivalency. It also shows the merit of the variable forgetting factor in time-varying channels.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.31 no.3 s.258
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• pp.209-216
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• 2007

Bacterial chemotaxis is essential to the study of structure and function of bacteria. Although many studies have accumulated the knowledge about chemotaxis in the past, the motion of a single bacterium has not been studied much yet. In this study, we have developed a device microfabricated by soft lithography and consisting of microfluidic channels. The microfluidic assay generates a concentration gradient of chemoattractant linearly in the main channel by only diffusion of the chemicals. Bacteria are injected into the main channel in a single row by hydrodynamic focusing technique. We measured the velocity of bacteria in response to a given concentration gradient of chemoattractant using the microfludic assay, optical systems with CCD camera and simple PTV (Particle Tracking Velocimetry) algorithm. The advantage of this assay and experiment is to measure the velocity of a single bacterium and to quantify the degree of chemotaxis by statistically analyzing the velocity at the same time. Specifically, we measured and analyzed the motility of Escherichia coli strain RP437 in response to various concentration gradients of L-aspartate statistically and quantitatively by using this microfluidic assay. We obtained the probability density of the velocity while RP437 cells are swimming and tumbling in the presence of the linear concentration gradient of L-aspartate, and quantified the degree of chemotaxis by analyzing the probability density.

• 한국전산유체공학회:학술대회논문집
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• 2008.03b
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• pp.436-439
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• 2008

Computer simulation of multiphase flows has grown dramatically in the last two decades. In this work, we have studied the flow characteristics of immiscible two fluids in a 2-D micro channel driven by pressure gradient using multi-phase lattice Boltzmann method suggested by Shan and Chen(1993) considering the fluid-surface interaction. we tried to examine the effects of parameters related to the two phase flow characteristics and pressure drop in the micro channel like contact angle and channel configuration by changing their value. The results of current study could show the lattice Boltzmann method can simulate the behaviors of two phase flow in the region of micro fluidics well.

• The KIPS Transactions:PartB
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• v.13B no.3 s.106
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• pp.197-204
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• 2006

Recent advances in information technology have resulted in rapid growth in the mobile communication. With this explosive growth, reliable transmission and error concealment technique become increasingly important to offer high quality multimedia services. In this paper, we propose an improved BMA(Boundary Matching Algorithm) method using gradient vectors to conceal channel errors in inter-frames of H.264 video images. General BMA method computes the sum of pixel differences of adjacent pixels of the candidate block and its neighbouring blocks, assuming that adjacent pixels have almost the same value. In real images, however, there exist some gradients, which means that the pixel values are increasing or decreasing in a specific direction. In this paper, we develop a precise estimation method of errors in candidates blocks using gradient information and try to recover lost blocks with this technique. Experimental results show the improvement of picture quality about $1{\sim}3dB$ compared to existing methods.