• KSII Transactions on Internet and Information Systems (TIIS)
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• v.14 no.11
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• pp.4595-4610
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• 2020

In the practical communication environment, the accurate channel state information (CSI) is difficult to obtain, which will cause the mismatch of resource and degrade the system performance. In this paper, to account for the channel uncertainties, a robust power allocation scheme for a downlink Non-orthogonal multiple access (NOMA) heterogeneous network (HetNet) is designed to maximize energy efficiency (EE), which can ensure the quality of service (QoS) of users. We conduct the robust optimization model based on worse-case method, in which the channel gains belong to certain ellipsoid sets. To solve the non-convex non-liner optimization, we transform the optimization problem via Dinkelbach method and sequential convex programming, and the power allocation of small cell users (SCUs) is achieved by Lagrange dual approach. Finally, we analysis the convergence performance of proposed scheme. The simulation results demonstrate that the proposed algorithm can improve total EE of SCUs, and has a fast convergence performance.

• Journal of Ship and Ocean Technology
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• v.10 no.4
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• pp.1-11
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• 2006

The finite volume based multi-block RANS code, WAVIS developed at MOERI is applied to the numerical self-propulsion test. WAVIS uses the cell-centered finite volume method for discretization of the governing equations. The realizable $k-{\epsilon}$ turbulence model with a wall function is employed for the turbulence closure. The free surface is captured with the two-phase level set method and body forces are used to model the effects of a propeller without resolving the detail blade flow. The propeller forces are obtained using an unsteady lifting surface method based on potential flow theory. The numerical procedure followed the self-propulsion model experiment based on the 1978 ITTC performance prediction method. The self-propulsion point is obtained iteratively through balancing the propeller thrust, the ship hull resistance and towing force that is correction for Reynolds number difference between the model and full scale. The unsteady lifting surface code is also iterated until the propeller induced velocity is converged in order to obtain the propeller force. The self-propulsion characteristics such as thrust deduction, wake fraction, propeller efficiency, and hull efficiency are compared with the experimental data of the practical container ship. The present paper shows that hybrid RANS and potential flow based numerical method is promising to predict the self-propulsion parameters of practical ships as a useful tool for the hull form and propeller design.

• Journal of Energy Engineering
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• v.29 no.2
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• pp.1-9
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• 2020

In recent days, fuel cell has received attention from the world as an alternative power source to hydrocarbon used in automobile engines. With the industrial advances of fuel cell, There have been a lot of researches actively conducted to find a way of generating hydrogen. Among many hydrogen production methods, Solid Oxide Electrolysis Cell(SOEC) is not only a basic way but also environment-friendly method to produce hydrogen gas. Solid Oxide Electrolysis Cell has lower electrical energy demands and high thermal efficiency since it is possible to operate under high temperature and high pressure conditions. For these reasons, experimental researches as well as studies on numerical modeling for Solid Oxide Electrolysis Cell have been under way. However, studies on numerical modeling are relatively less enough than experimental accomplishments and have limited performance prediction, which mostly is considered as a result from inadequate effects of electrochemical properties by temperature and pressure. In this study, various experimental studies of commercial Membrane Electrode Assembly (MEA) composed of Ni-YSZ (40wt%, Ni-60 wt% YSZ)/8-YSZ (TOSOH, TZ8Y)/LSM (La_0.9Sr_0.1MnO₃) was utilized for improving effectiveness of SOEC model. After numerically analyzing effects of electrochemical properties according to operating temperature, causing the largest deviation between experiments and simulation are that Charge Transfer Coefficient (CTC), exchange current density, diffusion coefficient, electrical conductivity in SOEC. Analyzing temperature effect on parameter used in overpotential model is conducted for modeling of SOEC. cross-validation method is adopted for application of various MEA and evaluating feasibility of model. As a result, the study confirm that the numerical model of SOEC based on structured process of effectiveness evaluation makes performance prediction better.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.22 no.8
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• pp.1755-1764
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• 1997

In this paper, we proposed the mechanism for the buffer allocation and a cell scheduling method with logical separation a single buffer in the ATm switch, and analyzed the cell loss probability and the delay of each trafic (CBR/VBR/ABR) based on the weighted value and the dynamic cell service scheduling algorithm. The proposed switch buffering system classifies composite trafics incoming to the switch, according to the characteristic of traffic, then stores them in the logically separated buffers, and adopts the round-robin service with weighted value in order to transmit cells in buffers though one output port. We analyzed 4 cell service scheduling algorithms with dynamic round-robinfor each logically separated service line of a single buffer, in which buffers have the respective weighted values and 3 classes on mixed traffic which characteristized by traffic descriptor. In simulation, using SIMCRIPT II.5., we model the VBR and the ABR traffics as ON/OFF processes, and the CBR traffic as a Poisson processes. As the results of analysis according to the proposed buffer management mechanism and cell service algorithm, we have found that the required QoS of each VC can be quaranteed depends on a scale of weighted values allocated to buffers that changed the weighted values, and cell scheduling algorithm.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.41 no.4
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• pp.257-268
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• 2017

This paper proposes an efficient two-dimensional simulation model for solid oxide fuel cells (SOFCs) based on the electrochemical effectiveness model. The effectiveness model is known to accurately predict the current generation performance of SOFC electrodes, by considering the complex reaction/transport processes that occur within thin active functional layers near the electrolyte. After validation tests, the two-dimensional simulation model was used to calculate the distribution of current density and oxygen concentration transverse to the flow channel in anode-supported SOFCs, with which the oxygen depletion characteristics were investigated in detail. In addition, simulations were also conducted to determine the minimum number of grids required in the transverse direction to efficiently obtain accurate results.

• Clean Technology
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• v.10 no.1
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• pp.9-17
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• 2004

Direct methanol fuel cell(DMFC) with proton exchange membrane (PEM) has advantages over the conventional power source (e.g. vehicle). DMFC, however, has a problem to be solved such as methanol crossover, high anodic overpotential and limiting current density, etc. The physicochemical phenomena in DMFC can be described by coupled PDEs (partial differential equations), which can be solved by a PDE solver. In this paper, we utilized a commercial software FEMLAB to solve the PDEs. The FEMLAB is one of the software programs available which are developed as a solver for building physics problems based on PDEs and is designed to simulate systems of coupled PDEs which may be 1D, 2D, 3D, non-liner and time dependent. We performed simulation using the Tafel equation as an electrochemical reaction model to analyze methanol concentration profile in DMFC system. We confirm that the rapid decrease of methanol concentration at anodic catalyst layer with the increase of the current density is a main reason of the low performance in DMFC through simulation results.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.33 no.3
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• pp.1131-1142
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• 2013

This study aims to verify the applicability of TRANSIMS (TRansportion ANalysis SIMulation System) in interrupted traffic flow through calibration and validation process based on observation data; such as headway, traffic volume, speed, and travel time from Dalguberl Boulevard in Dae-gu metropolitan city. On this study, several micro-simulation parameters are derived from the calibration and validation process through performing a headway comparison and applying an ID back tracking methodology. As a result, it is figured out that actual circumstances of Korean roadway; for example, traffic volume per lane, speed, and travel time, can be applied on the TRANSIMS. Especially, it was possible to find out the influence of cell size parameter to traffic flow characteristic of simulation. However, it is hard to conclude that TRANSIMS is applicable to Korean roadway environment with studying particular target area. Therefore, additional studies; such as more case studies with various types of road, signal, and land use, will be required to localize TRANSIMS to Korea.

• Journal of Power Electronics
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• v.15 no.6
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• pp.1609-1618
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• 2015

DC microgrids are considered as prospective systems because of their easy connection of distributed energy resources (DERs) and electric vehicles (EVs), reduction of conversion loss between dc output sources and loads, lack of reactive power issues, etc. These features make them very suitable for future industrial and commercial buildings' power systems. In addition, the bipolar-type dc system structure is more popular, because it provides two voltage levels for different power converters and loads. To keep voltage balanced in such a dc system, a bidirectional dual buck-boost voltage balancer with direct coupling is introduced based on P-cell and N-cell concepts. This results in greatly enhanced system reliability thanks to no shoot-through problems and lower switching losses with the help of power MOSFETs. In order to increase system efficiency and reliability, a novel burst-mode control strategy is proposed for the dual buck-boost voltage balancer. The basic operating principle, the current relations, and a small-signal model of the voltage balancer are analyzed under the burst-mode control scheme in detail. Finally, simulation experiments are performed and a laboratory unit with a 5kW unbalanced ability is constructed to verify the viability of the bidirectional dual buck-boost voltage balancer under the proposed burst-mode control scheme in low-voltage bipolar-type dc microgrids.

• Journal of the Institute of Convergence Signal Processing
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• v.17 no.1
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• pp.46-52
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• 2016

LTE-Advanced system consisting of the number of cells in the cellular environment because it is built to allow efficient use of limited frequency resources of adjacent cell interference avoidance should be considered. Transition services in accordance with the development of the mobile communication technology, wireless multimedia content from voice-centric mobile communications services and causing a lot of mobile data traffic, such as smart phones and tablet terminals spread of a data-driven surge in mobile data traffic base stations in urban areas by increasing became a reality that can not be prevented. In this paper, we propose a new Hybrid resource allocation technique for improving the performance of the cell boundary and analyzed the performance of the proposed new techniques to perform the simulation using LTE-Advanced system level simulator based on 19cell of cellular system model.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.43 no.1
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• pp.139-147
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• 2023

As heavy precipitation rates have increased due to climate change, the risk of landslides has also become greater. Studies in the field of disaster risk assessment predominantly focus on evaluating intrinsic importance represented by the use or role of facilities. This work, however, focused on evaluating risks according to the external conditions of facilities, which were presented via debris flow simulation. A random walk model (RWM) was partially improved and used for the debris flow simulation. The existing RWM algorithm contained the problem of the simulation results being overly concentrated on the maximum slope line. To improve the model, the center cell height was adjusted and the inertia application method was modified. Facility information was collected from a digital topographic map layer. The risk level of each object was evaluated by combining the simulation result and the digital topographic map layer. A risk assessment technique suitable for the polygon and polyline layers was applied, respectively. Finally, by combining the evaluated risk with the attribute table of the layer, a system was prepared that could create a list of objects expected to be damaged, derive various statistics, and express the risk of each facility on a map. In short, we used an easy-to-understand simulation algorithm and proposed a technique to express detailed risk information on a map. This work will aid in the user-friendly development of a debris flow risk assessment system.