• Publications of The Korean Astronomical Society
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• v.11 no.1
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• pp.27-47
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• 1996

We have performed the high resolution computer simulation with 1D spherical hydrodynamic code in order to study the dynamical evolution of supernova ejecta interacting with a pre-existing fast wind structure. The fast wind structure has been calculated with $M_{in}=3{\times}10^{-6}M_{\odot}yr^{-1}$ and ${\upsilon}_{in}=1000km/sec$, which velocity is higher than the critical velocity relating to the initial radiative cooling. The fast wind becomes initially adiabatic. After a shell formation time of ${\sim}4000yrs$, the wind becomes radiative cooling at the shell zone, forming a thin dense radiative shell and an adiabatic wind bubble afterward. When supernova explodes in the wind center at 20,000yrs after the wind evolves, the supernova ejecta, which has a dense distribution of ${\rho}{\propto}r^{-n}$(here we have n = 9), interacts initially with, the understood wind zone, producing forward and reverse shocks. The reverse shock heats the supernova ejecta and its temperature increases. In this study, as the mass of the supernova ejecta is larger than that of the wind shell ($M_{ej}=5M_{\odot}$, $M_{sw}=2M_{\odot}$), we can conform two shell structures: an outer shell by the supernova ejecta and a secondarily shocked wind shell by it. The secondarily shocked wind shell should accelerates in this case to be R-T unstable, consequently producing the knots.

• New & Renewable Energy
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• v.18 no.3
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• pp.60-71
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• 2022

A wind tunnel provides artificial airflow around a model throughout the test section for investigating aerodynamic loads. It has various applications, which include demonstration of aerodynamic loads in the building, automobile, wind energy, and aircraft industries. However, owing to the high equipment costs and space-requirements of wind tunnels, it is challenging for numerous studies to utilize a wind tunnel. Therefore, a digital wind tunnel can be utilized as an alternative for experimental research because it occupies a significantly smaller space and is easily operable. In this study, we performed airfoil testing based on velocity field measurements utilizing a digital wind tunnel. This wind tunnel can potentially be utilized to test the full-range aerodynamic characteristics of airfoils.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.22 no.2
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• pp.145-152
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• 2009

In this paper, dynamic instability of plane membrane structures under wind action has been studied. The key to solving the governing equations of membrane structures under wind action is how to obtain the air pressure on membrane. Based on Bernoulli's theorem, fluid pressure has a certain relationship with velocity potential. Velocity potential could be solved according to thin aerofoil theory, where air around the membrane is regarded as a sheet of vortices. In this paper, we take advantage of the most commonly used three-node triangular membrane element and weighted residual-Galerkin method to obtain the determining equation for stability evaluation. Square and rectangular membrane structures are studied. The influence of initial prestressing force and wind direction towards critical wind velocity are also analyzed in this paper.

• Wind and Structures
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• v.20 no.2
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• pp.275-292
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• 2015

The presence of traffic on a slender long-span bridge deck will modify the cross-section profile of the bridge, which may influence the flutter derivatives and in turn, the critical flutter wind velocity of the bridge. Studies on the influence of vehicles on the flutter derivatives and the critical flutter wind velocity of bridges are rather rare as compared to the investigations on the coupled buffeting vibration of the wind-vehicle-bridge system. A typical streamlined cross-section for long-span bridges is adopted for both experimental and analytical studies. The scaled bridge section model with vehicle models distributed on the bridge deck considering different traffic flow scenarios has been tested in the wind tunnel. The flutter derivatives of the modified bridge cross section have been identified using forced vibration method and the results suggest that the influence of vehicles on the flutter derivatives of the typical streamlined cross-section cannot be ignored. Based on the identified flutter derivatives, the influence of vehicles on the flutter stability of the bridge is investigated. The results show that the effect of vehicles on the flutter wind velocity is obvious.

• Journal of the Korean Society of Visualization
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• v.15 no.1
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• pp.19-24
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• 2017

In this study, we conduct a numerical experiment of the single 5MW NREL wind turbine and compare the performance of various vortex core identification for the wake behind the wind turbine. In the kinetic analysis of wind turbine, 20% velocity deficit at 200 s is observed, showing wake which contains tip vortex near blade tip and rotor vortex at the center of the wind turbine. Time series of velocity and turbulent intensity show numerical simulation converge to a quasi-steady state near 200 s. In the comparison between methods for vortex identification, ${\lambda}_2$-method has good performance in terms of tip vortex, rotor vortex, vortex during its cascade process compared to vorticity magnitude criteria, ${\Delta}$-method. We conclude that ${\lambda}_2$-method is suitable for vortex identification method for wake visualization.

• Wind and Structures
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• v.35 no.2
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• pp.131-146
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• 2022

Tornadic wind flow is inherently turbulent. A turbulent wind flow is characterized by fluctuation of the velocity in the flow field with time, and it is a dynamic process that consists of eddy formation, eddy transportation, and eddy dissipation due to viscosity. Properly modeling turbulence significantly increases the accuracy of numerical simulations. The lack of a clear and detailed comparison between turbulence models used in tornadic wind flows and their effects on tornado induced pressure demonstrates a significant research gap. To bridge this research gap, in this study, two representative turbulence modeling approaches are applied in simulating real-world tornadoes to investigate how the selection of turbulence models affects the simulated tornadic wind flow and the induced pressure on structural surface. To be specific, LES with Smagorinsky-Lilly Subgrid and k-ω are chosen to simulate the 3D full-scale tornado and the tornado-structure interaction with a building present in the computational domain. To investigate the influence of turbulence modeling, comparisons are made of velocity field and pressure field of the simulated wind field and of the pressure distribution on building surface between the cases with different turbulence modeling.

• Journal of the korean society of oceanography
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• v.38 no.3
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• pp.135-142
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• 2003

Gas flux across the air-sea interface is often determined by the product of gas transfer velocity k) and the difference of concentrations in water and air. k is primarily controlled by wind stress on the air-sea interface, thus all parameterizations ofk involve wind speed, a rough indicator of wind stress, as one of the independent variables. We attempted to explore the spatial and temporal variations of k in the East (Japan) Sea using a database from Naet al. (1992). Three different parameterizations were employed: those of Liss and Merlivat (1986), Wanninkhof(1992), and Wanninkhofand McGillis (1999). The strong non-linear dependence of k on wind speed in all parameterizations leads us to examine the effect of time resolution, in which the binned wind speeds are averaged, on the estimation ofk. Two time resolutions of 12 hours (short-term) and one month (long-term) were chosen. The mean wind speeds were fed into the given parameterizations, resulting in six different transfer velocities of $CO_2$ ranging from 12 to 32 cm/h. In addition to the threefold difference depending on the choice of parameterization, the long-term average of wind speed results in a value ofk up to 20% higher than the short-term (12 hours) average of wind speed due to the non-Rayleigh wind distribution in the East (Japan) Sea. While it is not known which parameterization is more reliable, this study proposes that the time-averaged wind speed should not be used in areas where non-Ralyleigh wind distribution prevails such as the East (Japan) Sea. The net annual $CO_2$ flux was estimated using the value of k described above and the monthly ${\Delta}fCO_2$ of Oh et al. (1999); this ranges from 0.034 to 0.11 Gt-C/yr.

• Structural Engineering and Mechanics
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• v.70 no.4
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• pp.479-497
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• 2019

In the current code design, the use of a uniform internal pressure coefficient of cooling towers as internal suction cannot reflect the 3D characteristics of flow field inside the tower body with different ventilation rate of shutters. Moreover, extreme weather such as heavy rain also has a direct impact on aerodynamic force on the internal surface and changes the turbulence effect of pulsating wind. In this study, the world's tallest cooling tower under construction, which stands 210m, is taken as the research object. The algorithm for two-way coupling between wind and rain is adopted. Simulation of wind field and raindrops is performed iteratively using continuous phase and discrete phase models, respectively, under the general principles of computational fluid dynamics (CFD). Firstly, the rule of influence of 9 combinations of wind speed and rainfall intensity on the volume of wind-driven rain, additional action force of raindrops and equivalent internal pressure coefficient of the tower body is analyzed. The combination of wind velocity and rainfall intensity that is most unfavorable to the cooling tower in terms of distribution of internal pressure coefficient is identified. On this basis, the wind/rain loads, distribution of aerodynamic force and working mechanism of internal pressures of the cooling tower under the most unfavorable working condition are compared between the four ventilation rates of shutters (0%, 15%, 30% and 100%). The results show that the amount of raindrops captured by the internal surface of the tower decreases as the wind velocity increases, and increases along with the rainfall intensity and ventilation rate of the shutters. The maximum value of rain-induced pressure coefficient is 0.013. The research findings lay the basis for determining the precise values of internal surface loads of cooling tower under extreme weather conditions.

• Journal of the Korean Institute of Illuminating and Electrical Installation Engineers
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• v.26 no.12
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• pp.87-92
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• 2012

In this paper, EHD(electrohydrodynamics) characteristics of AC corona discharge for the various frequency was investigated. Ionic wind velocity is controlled by the frequency of applied ac high voltage, and maximum velocity of the ionic wind is obtained at 1.2kHz. Maximum velocity are 1.90 m/s by metal corona electrode and 2.72m/s by wet porous corona electrode, These attain 91~99% of the maximum velocity in the DC corona discharge by adjusting the frequency through the experiments. In this paper, wet porous corona electrode has high possibility of cooling methode because a AC corona discharge using wet porous corona electrode is able to eject more water droplets than DC corona discharge.

• Journal of the Korea Institute of Military Science and Technology
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• v.22 no.3
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• pp.353-359
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• 2019

In this study, a aerodynamic loads prediction to design a deploying device of folded fin was introduced. In general, resultant flow conditions around the fin are used to obtain deploying moments and required energy. However, when it comes to the air vehicles launched from a mobile platform, more specific flow conditions can be provided. With the conditions, the design criteria can be calculated more realistically. In this study, therefore, aerodynamic moments induced by aerodynamic loads and energy required in deployment were calculated using wind-over-deck(WOD) velocity, combination of a platform velocity and a wind velocity. For the calculation, wind tunnel test was conducted on various angle of attack, side slip angles, and folding angles. It was found that the aerodynamic moments and the energy required in deployment using the non-uniform flow due to the velocity components were less than those using the uniform flow without the components.