• The Bulletin of The Korean Astronomical Society
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• v.37 no.2
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• pp.132.2-132.2
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• 2012

We have investigated the characteristics of magnetic cloud (MC) and ejecta (EJ) associated coronal mass ejections (CMEs) based on the assumption that all CMEs have a flux rope structure. For this, we used 54 CMEs and their interplanetary counter parts (interplanetary CMEs: ICMEs) that constitute the list of events used by the NASA/LWS Coordinated Data Analysis Workshop (CDAW) on CME flux ropes. We considered the location, angular width, and speed as well as the direction parameter, D. The direction parameter quantifies the degree of asymmetry of the CME shape, and shows how closely the CME propagation is directed to Earth. For the 54 CDAW events, we found several properties of the CMEs as follows: (1) the average value of D for the 23 MCs (0.62) is larger than that for the 31 EJs (0.49), which indicates that the MC-associated CMEs propagate more directly to the Earth than the EJ-associated CMEs; (2) comparison between the direction parameter and the source location shows that the majority of the MC-associated CMEs are ejected along the radial direction, while many of the EJ-associated CMEs are ejected non-radially; (3) the mean speed of MC-associated CMEs (946 km/s) is faster than that of EJ-associated CMEs (771 km/s). For seven very fast CMEs (>1500 km/s), all CMEs with large D (>0.4) are associated with MCs and the CMEs with small D are associated with EJs. From the statistical analysis of CME parameters, we found the superiority of the direction parameter. Based on these results, we suggest that the CME trajectory essentially decides the observed ICME structure.

• Journal of the Korean Society of Propulsion Engineers
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• v.16 no.3
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• pp.1-8
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• 2012

A water-flow test was carried out for the nonimpinging-type injector to be equipped on 70 N-class liquid-rocket engine under development. Breakup patterns of injector-spray transit from a smooth jet to wavy one as the injection angle increases, whereas spray-breakup lengths are inversely proportional to the injection pressure. It is confirmed that there exist ruffles on the surface of liquid column, which could be caught through the instantaneous spray images captured by high-speed camera. A phenomenon of spray shedding amplified at the specific pressure level of 0.93 MPa was an unexpected behavior of the injected stream and it is to be investigated further.

• ETRI Journal
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• v.34 no.6
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• pp.911-921
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• 2012

In high-speed wireless communications, an analysis of the propagation characteristics is an important process. Information on the propagation characteristics suitable for each environment significantly helps in the design of mobile communications. This paper presents the analysis results of radio propagation characteristics in outdoor environments for a new mobile wireless system at 781 MHz. To avoid the interference of Korean DTV broadcasting, we measure the channel characteristics in urban, suburban, and rural areas on Jeju Island, Republic of Korea, using a channel sounder and $4{\times}4$ antenna. The path loss (PL) measurement results differ from those of existing propagation models by more than 10 dB. To analyze the frequency characteristics for Korean propagation environments, we derive various propagation characteristic parameters: PL, delay spread, angular spread, and K-factor. Finally, we verify the validity of the measurement results by comparing them with the actual measurement results and 3D ray-tracing simulation results.

• Proceedings of the KIEE Conference
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• 1996.07a
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• pp.139-141
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• 1996

The switched reluctance motors(SRM) are simple and robust in structure. Because the wide range of power and speed, their application field is increasing. In order to design the motors and to evaluate the performance of them properly, an accurate study about the analysis of motor characteristics is required. In this paper, for the analysis of SRM characteristics, the finite element method which is based on the solution of combined equations both the electromagnetic field equations and the circuit equations of stator is adopted. The analysis model is to he assumed two-dimensional and the nonlinear property of magnetic materials is considered by Newton-Raphson method. To verify the usefulness of the proposed algorithm, commercial SRM is chosen and simulated. The computed torques obtained by Maxwell Stress Tensor are compared with the experimental data and it is found that they are in good agreement. By applying the proposed algorithm to two cases, currents of stator and torques at every angular positions of rotor are obtained step by step. Comparing them, one can recognize that torque ripple of SRM can he improved by controlling the switching sequences of driving circuits.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.22 no.1
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• pp.33-43
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• 1998

High cutting speeds and feeds are essential requirements of a machine tool structure to accomplish its basic function which is to produce a workpiece of the required geometric form with an acceptable surface finish at as high a rate of production as is economically possible. Since the bearings in high speed spindle units are the main heat source of the total cutting system, in this work, the thermal characteristics of the spindle bearing system with a tilting axis were investigated using finite element methods to improve the performance of the spindle bearing system. Based on the theoretical results, a specially designed prototype spindle bearing system has been manufactured. Using the manufactured spindle bearing system, the thermal characteristics were measured. From the comparison of the experimental results with the theoretical ones, it was found that the finite element method predicted well the thermal characteristics of the spindle bearing system.

• The Bulletin of The Korean Astronomical Society
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• v.38 no.2
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• pp.39.2-39.2
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• 2013

We use grid-based hydrodynamic simulations to study star formation history in nuclear rings of barred-spiral galaxies. In our previous study, we concentrated on bar-only galaxies without spirals, finding that the star formation rate (SFR) in a nuclear ring exhibits a strong primary burst at early time before decreasing to below 1 $M_{\odot}/yr$ at late time. The rapid decline is caused by the paucity of the gas in the bar region, due to early massive gas inflows to the nuclear ring. Since star formation in nuclear rings is observed to be sustained for about 1-2 Gyr, this requires mechanisms to supply the gas to the bar regions. In this work, we study the effect of spiral arms on the radial gas inflows and related star formation in the nuclear rings. We show that spiral arms are efficient to remove angular momentum of the gas to cause significant gas inflows to the bar region, provided the patten speed of the arms is much smaller than that of the bar. The inflowing gas is added to a nuclear ring, making the ring SFR episodic over a long period of time. The time interval of multiple bursts of star formation is a few tens to hundred million years, with the mean peak SFR of ${\sim}5M_{\odot}/yr$, consistent with observations of M100.

• Journal of the Korean Society of Manufacturing Process Engineers
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• v.15 no.1
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• pp.14-19
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• 2016

Transmission error (TE) is the most important cause of gear noise and vibration because TEs affect the changes of the force and the speed of gears. TE is usually expressed as an angular deviation, or a linear deviation measured at the pitch point and calculated at successive positions of the pinion as it goes through the meshing cycle. Accurate measurement of TE for gear transmission will provide a reasonable basis for gear design, manufacturing processes and quality control. Therefore, in order to study the accuracy of the gear transmission, stability, TE, vibration and noise after gear micro-geometry modification, a gear transmission test rig is proposed in this paper, which is based on the existing technical conditions, by using reasonable testing methods, hardware and a signal processing method. All of the details and the experience can be taken into consideration in the next upgraded test rig.

• Korean Journal of Applied Biomechanics
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• v.22 no.3
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• pp.277-284
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• 2012

The purpose of this study was to find out the changes in beginners' batting movements after taking undergraduate baseball lectures, by comparing and analyzing the speed of bat, the angle of body and its segments, the angular velocity and so on. For this purpose, the author picked up five undergraduates who had not taken baseball lectures, and conducted three-dimension computerized tomography twice on them, that is, before and after taking baseball classes, with eight infrared cameras and two force platforms. The conclusions are, first, the time required for swing was shortened after taking the classes. Second, the maximum velocity of the bat-end was increased. Third, at the time of impact, while the rotation angles of the pelvis and the tip of left foot were increased, the left elbow had more flexion after the classes. Fourth, the size of ground reaction force was increased at the point of swing where the ground reaction force of left foot became maximized. As we can infer from these conclusions, beginners' hitting movements before taking baseball classes tended to swing only with the arms, without waist rotation and weight shift of the lower body, but after the classes, their movements were proved to be corrected into the swing using the lower body. For the future studies, the author expects a research in a close to real environment by using pitching machine.

• Journal of ILASS-Korea
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• v.16 no.1
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• pp.7-14
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• 2011

High pressure LPG fuel spray with a conventional swirl injector was visualized and the impact of the injection pressure was also investigated using a DISI (direct injection spark ignition) LPG single cylinder engine. Engine performance and emission characteristics were evaluated over three different injection pressure and engine loads at an engine speed of 1500 rpm. The fuel spray pattern appeared to notably have longer penetration length and narrower spray angle than those of gasoline due to its lower angular momentum and rapid vaporization. Fuel injection pressure did not affect combustion behaviors but for high injection pressure and low load condition ($P_{inj}$=120 bar and 2 bar IMEP), which was expected weak flow field configuration and low pressure inside the cylinder. In terms of nano particle formation the positions of peak values in particle size distributions were not also changed regardless of the injection pressure, and its number densities were dramatically reduced compared to those of gasoline.

• Transactions of the Korean Society of Mechanical Engineers
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• v.18 no.12
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• pp.3386-3394
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• 1994

An experimental study has been performed on the flow over a rotating disk, where the diameter of the disk is 500 mm and the maximum vertical deviation of the upper surface is $50 \mu{m}$ for the whole range of the angular velocity up to 3400 rpm. The flow visualization experiment for the wall jet flow induced by impinging circular jet is carried out using schlieren system and measurements are made by 3-hole and 5-hole pitot tubes. Schlieren photographs show that as the rotating speed increases the wall jet flow becomes more stable and the size of the largest eddies becomes smaller. Measurements for impinging jet flow on the stationary disk verify the accuracy of the present experiment, and those for free rotating disk flow display the existence of transition region from laminar to turbulent flows. Measurements for impinging jet flow on the rotating disk exhibit the interaction between the wall jet and the viscous pumping effect, which explains the decay in size of turbulent eddies illustrated by the schlieren photographs.