• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.45 no.5
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• pp.367-375
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• 2017

RANS computational analysis was performed on the head of the launch vehicle including the hammerhead nose pairing in the supersonic regime. The two-dimensional axisymmetric analysis was performed by using laminar, fully turbulent and transition models and compared with the experimental data. It was observed that different flow phenomena occurred depending on the Reynolds number. Under the high Reynolds number condition, the boundary layer becomes turbulent, which is not separated from the surface of the launch vehicle. With the low Reynolds number condition, laminar separation bubble was produced due to the separation and reattachment of the boundary layer on the expansion-compression edge of the hammerhead type nose fairing. The three-dimensional computations with the angle of attack showed a fully detached vortical structure due to the laminar separation bubble. It is proved that the turbulent transition should be considered to predict the separation bubble with the Reynolds number.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.49 no.11
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• pp.901-907
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• 2021

The present study introduces an artificial neural network (ANN) that can predict the missile aerodynamic coefficients for various missile nose shapes and flow conditions such as Mach number and angle of attack. A semi-empirical missile aerodynamics code is utilized to generate a dataset comprised of the geometric description of the nose section of the missiles, flow conditions, and aerodynamic coefficients. Data normalization is performed during the data preprocessing step to improve the performance of the ANN. Dropout is used during the training phase to prevent overfitting. For the missile nose shape and flow conditions not included in the training dataset, the aerodynamic coefficients are predicted through ANN to verify the performance of the ANN. The result shows that not only the ANN predictions are very similar to the aerodynamic coefficients produced by the semi-empirical missile aerodynamics code, but also ANN can predict missile aerodynamic coefficients for the untrained nose section of the missile and flow conditions.

• Journal of the Korean Society of Fisheries and Ocean Technology
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• v.40 no.3
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• pp.214-224
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• 2004

A large cage system for the purpose of fishes farming in the open sea was influenced by various forces from the ocean environment. The deformation of the cage by these forces affects the safety of the cage itself, as well as that of the cultivated creatures. In this research, theoretical model was established to analyzing dynamic movement influenced by current for cage. Also, to increase the accuracy of calculations, the reduction ratio of flow speed acquired using the flume tank experiment. Applying the reduction ratio of flow speed to the numerical calculation, the calculation values were compared with the measured values in the flume tank experiment using cage model. The results were as follows ; 1. When the flow speed of the flume tank is fixed, the decrease of the velocity of flow which is passed the upper panel side is proportion to the increase of porosity ratio of netting. 2. When the porosity ratio is fixed, the increase of the velocity of flow which is passed the upper panel side is proportion to the increase of velocity of flow. 3. When the porosity ratio and the flow speed of the flume tank are fixed, the decrease of the velocity of flow which is passed the upper panel side is proportion to the increase of attack angle. 4. As a result of comparison between the underwater shape by simulation which is applying the reduction ratio of flow speed from the experiment using plane netting and that by model experiment, it was found out that the result of the simulation was very close to that of model gear within ${\pm}$ 5 % error range.

• Korean Journal of Adult Nursing
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• v.21 no.5
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• pp.477-488
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• 2009

Purpose: This study was conducted to identify the clinical characteristics and risk factors on the occurrence of variant angina, and to examine the predicting factors on the vascular endothelial dysfunction of the patients with variant angina. Methods: A total of 134 patients diagnosed with variant angina were recruited from 2006 to 2008. The degrees of endothelial dysfunction were measured and recorded by the researcher using the values of flow-mediated vasodilation of their brachial arteries and Nitroglycerine-mediated dilation. Subjects' demographic data and risk factors were gathered after obtaining informed consent, and their electronic medical records were reviewed to collect laboratory data. Results: The mean age was $54.2{\pm}9.6$ years and 52% was male patients. More than 50% of the male patients were cigarette smokers and had hypercholesterolemia. 84% of the male patients and 70% of the female patients had more than one risk factor of cardiovascular disease. A stepwise multiple regression analysis showed that smoking and hypercholesterolemia predicted the decrease of flow-mediated vasodilation (Adjusted $R^2$ = .204, p < .001). Conclusion: Tailored educational interventions for smoking cessation and cholesterol management are needed to prevent recurrence of angina attack for patients with variant angina and to prevent cardiovascular disease for middle-aged workers.

• International Journal of Aeronautical and Space Sciences
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• v.17 no.3
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• pp.285-295
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• 2016

A contact strip shape of a high speed train pantograph system was optimized with CFD to increase the aerodynamic performance and stability of contact force, and the results were validated by a wind tunnel test. For design of the optimal contact strip shape, a Kriging model and genetic algorithm were used to ensure the global search of the optimal point and reduce the computational cost. To enhance the performance and robustness of the contact strip for high speed pantograph, the drag coefficient and the fluctuation of the lift coefficient along the angle of attack were selected as design objectives. Aerodynamic forces were measured by a load cell and HWA (Hot Wire Anemometer) was used to measure the Strouhal number of wake flow. PIV (Particle Image Velocimetry) was adopted to visualize the flow fields. The optimized contact strip shape was shown a lower drag with smaller fluctuation of vertical lift force than the general shaped contact strip. And the acoustic noise source strength of the optimized contact strip was also reduced. Finally, the reduction amount of drag and noise was assessed when the optimized contact strip was applied to three dimensional pantograph system.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• v.y2005m4
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• pp.394-398
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• 2005

Ramjet engine have been usually operated on Mach $1.5\sim3$ as the vehicle of supersonic cruising engine and studied about the higher performance above Mach 4. The research of Duel mode Scramjet engine which have duel operating mode of ramjet/Scramjet are in progress actively nowadays. This paper suggests the effect the flow characteristics and the effects of back pressure, angle of attack, angle of yow on the supersonic air intake on mach 4 through the Schlieren/Oil flow visualization, and pressure measurement on experimental model.

• KSII Transactions on Internet and Information Systems (TIIS)
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• v.10 no.9
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• pp.4529-4548
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• 2016

DDoS attacks have had a devastating effect on the Internet, which can cause millions of dollars of damage within hours or even minutes. In this paper we propose a practical dynamic defense approach that overcomes the shortage of static defense mechanisms. Our approach employs a group of SDN-based proxy switches to relay data flow between users and servers. By substituting backup proxy switches for attacked ones and reassigning suspect users onto the new proxy switches, innocent users are isolated and saved from malicious attackers through a sequence of remapping process. In order to improve the speed of attacker segregation, we have designed and implemented an efficient greedy algorithm which has been demonstrated to have little influence on legitimate traffic. Simulations, which were then performed with the open source controller Ryu, show that our approach is effective in alleviating DDoS attacks and quarantining the attackers by numerable remapping process. The simulations also demonstrate that our dynamic defense imposes little effect on legitimate users, and the overhead introduced by remapping procedure is acceptable.

• Journal of Advanced Marine Engineering and Technology
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• v.31 no.7
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• pp.842-851
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• 2007

Oscillating foil propulsion, the engineering application of fish-like movement of a hydrofoil, has received in recent decades as a possible competitor for propellers. The oscillating foil produces an effective angle of attack, resulting in a normal force vector with thrust and lift components, and it can be expected to be a new highly effective propulsion system. We have explored propulsion hydrodynamics as a concept in wake flow pattern. The present study has been examined various conditions such as oscillating frequencies and amplitudes in NACA0010 profile. Flow visualizations showed that high thrust was associated with the generation of moderately strong vortices, which subsequently combine with trailing-edge vorticity leading to the formation of a reverse $K\acute{a}rm\acute{a}n$ vortex street. Vortex generation was inherent to jet production and playeda fundamental role in the wake dynamics. And it was shown that the strong thrust coefficient obtained as the Strouhal number was larger.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.34 no.10
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• pp.7-15
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• 2006

The effects of strake incidence-angle on the vortex characteristics and the wing-surface pressure distribution for a double-delta wing with strake were investigated experimentally. The strake incidence-angle of negative sign(strake is pitched down from the main-wing upper-surface) increased the suction pressure of the wing-upper surface, which was the same effect of increase of angle of attack. This change of the suction pressure was caused by the closer movement of the vortex cores to the wing upper surface rather than the increase of the vortex strength.

• Journal of the Korean Society of Systems Engineering
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• v.12 no.2
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• pp.101-114
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• 2016

A model-based systems engineering (MBSE) approach to implementing hardware-based network cybersecurity controls for APR1400 non-safety data network is presented in this work. The proposed design was developed by implementing packet filtering and deep packet inspection functions to control the unauthorized traffic and malicious contents. Denial-of-Service (DoS) attack was considered as a potential cybersecurity issue that may threaten the data availability and integrity of DCS gateway servers. Logical design architecture was developed to simulate the behavior of functions flow. HDL-based physical architecture was modelled and simulated using Xilinx ISE software to verify the design functionality. For effective modelling process, enhanced function flow block diagrams (EFFBDs) and schematic design based on FPGA technology were together developed and simulated to verify the performance and functional requirements of network security controls. Both logical and physical design architectures verified that hardware-based cybersecurity controls are capable to maintain the data availability and integrity. Further works focus on implementing the schematic design to an FPGA platform to accomplish the design verification and validation processes.