• Journal of Astronomy and Space Sciences
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• v.22 no.2
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• pp.147-174
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• 2005

To understand the physical processes that control the high-latitude lower thermospheric dynamics, we quantify the forces that are mainly responsible for maintaining the high-latitude lower thermospheric wind system with the aid of the National Center for Atmospheric Research Thermosphere-Ionosphere Electrodynamics General Circulation Model (NCAR-TIEGCM). Momentum forcing is statistically analyzed in magnetic coordinates, and its behavior with respect to the magnitude and orientation of the interplanetary magnetic field (IMF) is further examined. By subtracting the values with zero IMF from those with non-zero IMF, we obtained the difference winds and forces in the high-latitude 1ower thermosphere(<180 km). They show a simple structure over the polar cap and auroral regions for positive($B_y$ > 0.8|$\overline{B}_z$ |) or negative($B_y$ < -0.8|$\overline{B}_z$|) IMF-$\overline{B}_y$ conditions, with maximum values appearing around -80$^{\circ}$ magnetic latitude. Difference winds and difference forces for negative and positive $\overline{B}_y$ have an opposite sign and similar strength each other. For positive($B_z$ > 0.3125|$\overline{B}_y$|) or negative($B_z$ < -0.3125|$\overline{B}_y$|) IMF-$\overline{B}_z$ conditions the difference winds and difference forces are noted to subauroral latitudes. Difference winds and difference forces for negative $\overline{B}_z$ have an opposite sign to positive $\overline{B}_z$ condition. Those for negative $\overline{B}_z$ are stronger than those for positive indicating that negative $\overline{B}_z$ has a stronger effect on the winds and momentum forces than does positive $\overline{B}_z$ At higher altitudes(>125 km) the primary forces that determine the variations of tile neutral winds are the pressure gradient, Coriolis and rotational Pedersen ion drag forces; however, at various locations and times significant contributions can be made by the horizontal advection force. On the other hand, at lower altitudes(108-125 km) the pressure gradient, Coriolis and non-rotational Hall ion drag forces determine the variations of the neutral winds. At lower altitudes(<108 km) it tends to generate a geostrophic motion with the balance between the pressure gradient and Coriolis forces. The northward component of IMF By-dependent average momentum forces act more significantly on the neutral motion except for the ion drag. At lower altitudes(108-425 km) for negative IMF-$\overline{B}_y$ condition the ion drag force tends to generate a warm clockwise circulation with downward vertical motion associated with the adiabatic compress heating in the polar cap region. For positive IMF-$\overline{B}_y$ condition it tends to generate a cold anticlockwise circulation with upward vertical motion associated with the adiabatic expansion cooling in the polar cap region. For negative IMF-$\overline{B}_z$ the ion drag force tends to generate a cold anticlockwise circulation with upward vertical motion in the dawn sector. For positive IMF-$\overline{B}_z$ it tends to generate a warm clockwise circulation with downward vertical motion in the dawn sector.

• Korean Journal of Agricultural Science
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• v.41 no.4
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• pp.473-480
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• 2014

Upon setting up a dedicated plastic greenhouse for tomato cultivation developed by the Rural Development Administration on the Gyehwa reclaimed land, this study was aimed at analyzing the problems can be occurred in the installation of plastic greenhouse on reclaimed lands as well as finding out solutions for improvement. A relatively cheaper wooden pile was used in the installation in order to supplement the soft ground conditions. Based on the results of ground investigation of the installation site, both the allowable bearing capacity and pulling resistance of the wooden pile with a diameter of 150 mm and a length of 10 m were computed and came out to be 30.645 kN. It was determined that the values were enough to withstand the maximum compressive force (17.206 kN) and the pullout force (20.435 kN) that are generally applied to the greenhouse footing. There are three problems aroused in the process of greenhouse installation, and the corresponding countermeasures are as follow. First, due to the slightly bent shape of the wooden pile, there were phenomenon such as deviation, torsion, and fracture when driving the pile. This could be prevented by the use of the backhoe (0.2) rotating tongs, which are holding the pile, to drive the pile while pushing to the direction of the driving and fixing it until 5 m below ground and applying a soft vibrating pressure until the first 2 m. Second, there exists a concrete independent footing between the column of the greenhouse and the wooden pile driven to the underground water level. Since it is difficult to accurately drive the pile on this independent footing, the problem of footing baseplate used to fix the column being off the independent footing was occurred. In order to handle with this matter, the diameter of the independent footing was changed from 200 mm to 300 mm. Last, after films were covered in the condition that the reinforcing frame and bracing are not installed, there was a phenomenon of columns being pushed away by the strong wind to the maximum of $11m{\cdot}s^{-1}$. It is encouraged to avoid constructions in winter, and the film covering jobs always to be done after the frame construction is completely over. The height of the independent footing was measured for 9 months after the completion of the greenhouse installation, and it was found to be within the margin of error meaning that there was no subsidence. The extent to the framework distortion and the value of inclinometers as well showed not much alteration. In other words, the wooden pile was designed to have a sufficient bearing capacity.

• Journal of the Korean Society of Marine Environment & Safety
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• v.24 no.6
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• pp.646-653
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• 2018

Marine accidents caused by dragging anchors occur constantly due to enlargement of ships' size and unusual weather conditions. Nevertheless, vessel operators rely on their experience because the calculations of actual holding power and external forces are complex and inconvenient. The purpose of this study was to propose a program for the anchor dragging risk assessment in order to provide crew and VTSO with the information to determine easily the danger of dragging and take appropriate action. The input data in this program were composed of the ship's basic particulars, anchoring condition, and external environment etc. on calculating for the wind pressure, frictional force, drift force, and holding power. Three dragging anchor accidents were applied to the program's data input at the time of the day, then the result was assessed by 'warning', which was verified with a high confidence. As a result, the risk of dragging anchors can be predicted in advance through this program. In further studies, it is necessary to simplify the input data and improve user convenience through automatic input from various equipment.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.43 no.9
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• pp.765-772
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• 2015

Aerodynamic force measurements and phase-locked PIV study were carried out to check the bio-mimetic MAV applicability of a swift flight. Two-rotational DOF robotic wing model and blowing-type wind tunnel were employed. The amplitude of twist angle were ${\pm}0$, ${\pm}5$, ${\pm}10$, and ${\pm}20$ deg. and stroke angles were manipulated by simple harmonic function with out-of-phase in regards to the stroke motion. It is acknowledged that the time-varying lift coefficients in accordance with the change of the twist angle did not result in any noticeable differences, just the small decrease and delay. However, the drag exhibited that the small change of the twist angle can produce large thrust. These findings imply why a swift uses small twist angle during flight. The PIV results displayed that the delay of aerodynamic forces is highly associated with the vortical structures around the wing. It is therefore indicated that a process of designing a swift-based Micro Air Vehicle should take the twist angle into consideration, as the essential parameter.

• Journal of Korean Tunnelling and Underground Space Association
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• v.19 no.5
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• pp.685-703
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• 2017

Recently, as the number of high-rise building and earthquake occurrence are increasing, it is more important to consider horizontal load such as wind and seismic loads, earth pressure, for the pile foundation. Also, development of underground space in urban areas is more demanded to meet various problem induced by growing population. Many studies on pile subjected to horizontal load have been conducted by many researchers. However, research regarding interactive behavior on pile subjected to horizontal load with tunnel are rare, so far. In this study, therefore, study on the behaviors of ground and horizontal and vertical loads applied to single pile was carried out using laboratory model test and numerical analysis. The pile axial force and ground deformation were investigated according to offset between pile and tunnel (0.0D, 1.0D, 2.0D: D = tunnel diameter). At the same time, close range photogrammetry was used to measure displacement of underground due to tunnelling during laboratory model test. The results from numerical analysis were compared to that from laboratory model test.

• Journal of the Korean Society of Marine Environment & Safety
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• v.26 no.7
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• pp.931-941
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• 2020

There is a growing interest this paper for ocean sensing where autonomous vehicles can play an essential role in assisting engineers, researchers, and scientists with environmental monitoring and collecting oceanographic data. This study was conducted to develop a rigid sail for the autonomous sailing drone. Our study aims to numerically analyze the aerodynamic characteristics of curvy twin sail and compare it with wing sail. Because racing regulations limit the sail shape, only the two-dimensional geometry (2D) was open for an optimization. Therefore, the first objective was to identify the aerodynamic performance of such curvy twin sails. The secondary objective was to estimate the effect of the sail's spacing and shapes. A viscous Navier-Stokes flow solver was used for the numerical aerodynamic analysis. The 2D aerodynamic investigation is a preliminary evaluation. The results indicated that the curvy twin sail designs have improved lift, drag, and driving force coefficient compared to the wing sails. The spacing between the port and starboard sails of curvy twin sail was an important parameter. The spacing is 0.035 L, 0.07 L, and 0.14 L shows the lift coefficient reduction because of dramatically stall effect, while flow separation is improved with spacing is 0.21 L, 0.28 L, and 0.35 L. Significantly, the spacing 0.28 L shows the maximum high pressure at the lower area and the small low pressure area at leading edges. Therefore, the highest lift was generated.

• Journal of Korea Water Resources Association
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• v.45 no.4
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• pp.349-360
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• 2012

A horizontally curvilinear non-hydrostatic free surface model that was applicable to three-dimensional viscous flows was developed. The proposed model employed a top-layer equation to close kinematic free-surface boundary condition, and an isotropic k-${\varepsilon}$ model to close turbulence viscosity in the Reynolds averaged Navier-Stokes equation. The model solved the governing equations with a fractional step method, which solved intermediate velocities in the advection-diffusion step, and corrects these provisional velocities by accounting for source terms including pressure gradient and gravity acceleration. Numerical applications were implemented to the wind-driven currents in a two-dimensional closed basin, the flow in a steep-sided trench, and the flow in a strongly-curved channel accounting for secondary current by the centrifugal force. Through the numerical simulations, the model showed its capability that were in good agreement with experimental data with respect to free surface elevation, velocity, and turbulence characteristics.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.15 no.1
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• pp.47-54
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• 2014

Endurance of ship hull can be estimated by existing regulations and fatigue estimation methods. These can be applied to the hull materials that are recommended by ship regulations but can't be applied to new materials. In this study, structural force in the worst sailing condition is obtained by the acceleration measurement test of small polyethylene boat and the endurance of small polyethylene hull is estimated by rainflow cycle counting method and linear cumulative damage rule. Maximum Von-Mises stress on the polyethylene boat is 1.8MPa and much lower than the fatigue strength of at least 5.9MPa for the fatigue life of $1{\times}10^9$ cycles. Fatigue life of the polyethylene boat hull is estimated to be 6,229 years.

• Journal of Korean Society for Atmospheric Environment
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• v.28 no.5
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• pp.495-505
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• 2012

Modeling the effects of high-rise buildings on thermo-dynamic conditions and meteorological fields over a coastal urban area was conducted using the modified meso-urban meteorological model (Urbanized MM5; uMM5) with the urban canopy parameterization (UCP) and the high-resolution inputs (urban morphology, land-use/land-cover sub-grid distribution, and high-quality digital elevation model data sets). Sensitivity simulations was performed during a typical sea-breeze episode (4~8 August 2006). Comparison between simulations with real urban morphology and changed urban morphology (i.e. high-rise buildings to low residential houses) showed that high-rise buildings could play an important role in urban heat island and land-sea breeze circulation. The major changes in urban meteorologic conditions are followings: significant increase in daytime temperature nearly by $1.0^{\circ}C$ due to sensible heat flux emitted from high density residential houses, decrease in nighttime temperature nearly by $1.0^{\circ}C$ because of the reduction in the storage heat flux emitted from high-rise buildings, and large increase in wind speed (maximum 2 m $s^{-1}$) during the daytime due to lessen drag-force or increased gradient temperature over coastal area.

• Journal of the Korean Society for Railway
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• v.15 no.6
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• pp.531-536
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• 2012

This study presents a result on aero-acoustic characteristics of pantograph panheads. To analyze the fluid flow around the panhead and resulting sound radiation, simple models of panhead were used in the numerical simulations called Lattice-Boltzmann method. The simulation results were verified using the wind tunnel test. The main aerodynamic noise was generated from the vortex shedding which is characterized by the Strouhal number, flow speed and geometry. The reduction in the radiated noise with simultaneously achieving increased lifting force was implemented for the simple rectangular geometry used in this study. Also, it was shown that the radiated sound power was significantly reduced by minimizing vortex shedding using through-holes or streamline shapes.