• Proceedings of the Korea Information Processing Society Conference
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• 2014.11a
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• pp.215-217
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• 2014

In Software Defined Network (SDN), data plane and control plane are decoupled. Dummy switches on the data plane simply forward packet based on the flow entries that are stored in its flow table. The flow entries are generated by a centralized controller that acts as a brain of the network. However, the size of flow table is limited and it can conduct a security issue related to Distributed Denial of Service (DDoS). Especially, it related to resource attack that consumes all flow table resource and consumes controller resources. In this paper, we will analyze the impact of flow table limitation to the controller. Then we propose an approach that is called Flow Table Management to handle flow table limitation.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.46 no.2
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• pp.95-105
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• 2018

Increasing cruise speed is an important issue for the development of the next generation rotorcraft. Among several concepts proposed by previous research, the rotorcraft with ducted fan demonstrates its possibility of high-speed flight. In this study, numerical simulations are conducted to investigate the aerodynamic and flow characteristics of multi-ducted fan in forward flight. The aerodynamic efficiency around front ducted fan is determined by interaction between free-stream velocity and the induced velocity. While flow characteristics of rear ducted fan are dominantly influenced by the front ducted fan. Separation in the front ducted fan occurs faster than that of rear ducted fan, and the separation at duct inlet induces an increase of fan thrust. As a result of interaction effect between each ducted fan, relatively aligned inflow enters to the rear ducted fan. Therefore, thrust of the rear fan is decreased steadily before separation, and sudden changes of thrust in fans occur simultaneously. Due to the pressure decrease on lower surface, the normal force of rotorcraft is reduced with forward speed.

• International Journal of Ocean System Engineering
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• v.2 no.2
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• pp.116-138
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• 2012

As in the other fields of mechanics, analytical methods represent an important analysis tool in marine hydrodynamics. The analytical approach is interesting for different reasons : it gives reference results for numerical codes verification, it gives physical insight into some complicated problems, it can be used as a simplified predesign tool, etc. This approach is of course limited to some simplified geometries (cylinders, spheres, ...), and only the case of one or more cylinders, truncated or not, will be considered here. Presented methods are basically eigenfunction expansions whose complexity depends on the boundary conditions. The hydrodynamic boundary value problem (BVP) is formulated within the usual assumptions of potential flow and is additionally simplified by the perturbation method. By using this approach, the highly nonlinear problem decomposes into its linear part and the higher order (second, third, ...) corrections. Also, periodicity is assumed so that the time dependence can be factorized i.e. the frequency domain formulation is adopted. As far as free surface flows are concerned, only cases without or with small forward speed are sufficiently simple to be solved semi-analytically. The problem of the floating body advancing in waves with arbitrary forward speed is far more complicated. These remarks are also valid for the general numerical methods where the case of arbitrary forward speed, even linearized, is still too difficult from numerical point of view, and "it is fair to say that there exists at present no general practical numerical method for the wave resistance problem" [9], and even less for the general seakeeping problem. We note also that, in the case of bluff bodies like cylinders, the assumptions of the potential flow are justified only if the forward speed is less than the product of wave amplitude with wave frequency.

• Journal of Korean Society of Transportation
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• v.11 no.1
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• pp.87-103
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• 1993

The subject this paper is the signal control strategy under oversaturated conditions. The nature of traffic control for oversaturation is essentially different from the standard control modes. While under non-saturated situation traffic control is needed for the sake of safety and efficiency, the throughput is essential under oversaturated conditions. Therefore berth objective and strategies differ. For an oversaturated stream the cycle time and the signal offset are thought to be of rather secondary importance. For this case the green split may well be the most important control variable to serve the excessive demand. Up to now, however, most efforts have concentrated on the strategy with the concept which lies just on the extension of Webster's. "Green-split Coordination Strategy for Over-Saturated Networks", presents newly contrived three types of strategies named Forward-coordination, Backward-coordination and Network-coordination respectively and describes the algorithms with the evaluations. The forward coordination strategy treats the forward wave of flow between two signals. The aim is to prevent the outbreak of queue due to the accumulation of temporary excess of demand in near-saturation or saturation flow. The backward coordination strategy treats the backward rave of flow between two signals. The goal is to prevent the waste of green time caused by the exit block at the upstream signal. for this purpose a feedback regulation is provided of the upstream green-split so that the inflow-outflow balance is kept zero. The resultant surplus of green time is alloted to other signal stages. Also here the examination is made of the appropriate value of the feedback control parameter. The network coordination strategy is operated to maximize the network throughput in a specific direction applying a bang-bang control at the bottleneck intersection. This is a type of intervenient control for policy reasons. For this strategy the green-split coordinations, particuarly the backward coordination, are essential as the tactical elements. In order to evaluate the preposed strategies those are compared with the latest existing strategy called saturation-degree-ratio control by the simulation experiments in an assumed 4$\times$4 grid network. The results are satisfactory showing a 10-15% reduction in delays and a 15% increase in network capacity.

• The Journal of the Korea Contents Association
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• v.20 no.12
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• pp.241-249
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• 2020

The citation information of academic resources contains the knowledge flow from previous research, so it is possible to connect fragmented research in relational aspects. The citation information can grasp the overall flow of research, so it can promote convergence research such as developing existing research or deriving related fields. Therefore, in this study, the citation information of academic literature, which was previously provided at the level of simple disclosure, was reconstructed based on the citation relationship. Through this, backward and forward citation analysis were conducted based on time series, and the research flow was analyzed by setting the citation stage. Finally, we developed an academic information service that visualizes the main research contents of backward and forward citation based on time series. This accesses academic resources through the meaning contained in the citation information.

• Journal of Power System Engineering
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• v.20 no.1
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• pp.5-10
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• 2016

Swing check valve is opened when the flow direction is forward, when the flow is reversed, the valve is automatically closed by back pressure. In this study, the internal flow field analysis of the valve was conducted by Fluent. The working fluid used in the study, using liquefied methane $-165^{\circ}C$ (CH4) and velocity field, pressure field, pressure drop coefficient were simulated by varying separately the opening divergence into four intervals from 0 to 100%. The approximate research result are as follow : When the opening divergence is smaller, it appears high pressure on the upstream side, this value is relaxed when the opening divergence is large. Flow rate coefficient of the valve shows a larger value as the degree of opening becomes larger, confirming that the check valve used in the study is in the effective flow rate counting range.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2000.04a
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• pp.155-158
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• 2000

Flow defect of a piston-pin for automobile parts is investigated in this study. In cold forging of piston-pin Lapping defect a kind of flow defect appears by the dead metal zone. This appearance evidently happens in products with a thin piercing thickness for the dimension accuracy and the decrease of material loss. The best method that can prevent flow defect is removing dead metal zone. The finite element simulations are applied to analyze the flow defect. This study proposed processes for preventing flow defect by removing dead metal zone. Then the results are compared with the experiments for verification. These FE simulation results are in good agreement with the experimental ones.

• International Journal of Fluid Machinery and Systems
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• v.3 no.4
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• pp.369-378
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• 2010

The creation of the hydraulic turbine flow factor map will undoubtedly benefit its design by decreasing both the design cycle time and product cost. In this paper, the geometry and flow variables, which effectively affect the flow factor, are proposed, analyzed and determined. These flow variables are further used to create the operating condition maps by using different model approaches categorized into Response Surface Method (RSM) and Artificial Neural Network (ANN). The accuracies of models created by different approaches are compared and the performances of model approaches are analyzed. The influences of chosen variables and the combination of Principle Component Analysis (PCA) and model approaches are also studied. The comparison results between predicted and actual flow factors suggest that two-hidden-layer Feed-forward Neural Network (FFNN), and one.hidden-layer FFNN with PCA has the best performance on forming this mapping, and are accurate sufficiently for hydraulic turbine design.

• Journal of the Korean Society for Precision Engineering
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• v.8 no.3
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• pp.93-104
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• 1991

Limit analysis is based on the duality theorem which equates the least upper bound to the greatest lower bound. In this study, limit analysis of axisymmetric forming problem with workhardening materials is formulated by minimizing the upper bound functional and finite element program is developed for forward estrusion. Limit loads, velocity and flow line fields are directly obtained under various process conditions and deformation characteristics such as strains, strain rates and grid distortion are obtained from the optimum velocity components by numerical calculation. The experimental observation was carried out for extrusion and compared with computed results. The good agreement between theoretical and experimental results is shown that the developed programming is very effective for the analysis of axisymmetric extrusion.

• Proceedings of the IEEK Conference
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• summer
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• pp.259-262
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• 2004

In most of the character recognition systems, the method of template matching or statistical method using hidden Markov model is used to extract and recognize feature shapes. In this paper, we used modified chain-code which has 8-directions but 4-codes, and made the chain-code of hand-written character, after that, converted it into transition chain-code by applying to HMM(Hidden Markov Model). The transition chain code by HMM is analyzed as signal flow graph by Mason's theory which is generally used to calculate forward gain at automatic control system. If the specific forward gain and feedback gain is properly set, the forward gain of transition chain-code using Mason's theory can be distinguished depending on each object for recognition. This data of the gain is reorganized as tree structure, hence making it possible to distinguish different hand-written characters. With this method, $91\%$ recognition rate was acquired.