• Proceedings of the KWS Conference
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• 2002.10a
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• pp.600-605
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• 2002

The keyhole formed by high energy density laser-material interaction periodically collapses due to surface tension of the molten metal in partial penetration welds. The collapse sometimes traps a void at the bottom of the keyhole, and it remains as welding defects. This phenomenon is seen as one cause of the instability of the keyhole during laser beam welding. Thus, it seems likely that improving the stability of the keyhole can reduce voids and uniform the penetration depth. The goal of this work is to develop techniques for controlling laser weld keyhole dynamics to reduce weld defects such as voids and inconsistent penetration. Statistical analysis of the penetration depth signals in glycerin determined that keyhole dynamics are chaotic. The chaotic nature of keyhole fluctuations and the ability of laser power modulation to control them have been demonstrated by high-speed video images of laser welds in glycerin. Additionally, an incident leading beam angle is applied to enhance the stability of the keyhole. The quasi-sinusoidal laser beam power of 400Hz frequency and 15$^{\circ}$ incident leading beam angle were determined to be the optimum parameters for the reduction of voids. Finally, chaos analyses of uncontrolled signals and controlled signals were done to show the effectiveness of modulation on the keyhole dynamics. Three-dimensional phase plots for uncontrolled system and controlled system are produced to demonstrate that the chaotic keyhole dynamics is converted to regular periodic behavior by control methods: power modulation and incident leading beam angle.

• The Journal of the Acoustical Society of Korea
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• v.25 no.1E
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• pp.32-35
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• 2006

The screech tone of underexpanded jet is numerically calculated without any specific modeling for the screech tone itself. Fourth-order optimized compact scheme and fourth-order Runge-Kutta method are used to solve the 2D axisymmetric Euler equation. Adaptive nonlinear artificial dissipation model and generalized characteristic boundary condition are also used. The screech tone, generated by a closed loop between instability waves and quasi-periodic shock cells at the near field, is reasonably analyzed with present numerical methods for the underexpanded jet having Mach number 1.13. First of all, the centerline mean pressure distribution is calculated and compared with experimental and other numerical results. The instantaneous density contour plot shows Mach waves due to mixing layer convecting supersonically, which propagate downstream. The pressure signal and its Fourier transform at upstream and downstream shows the directivity pattern of screech tone very clearly. Most of all, we can simulate the axisymmetric mode change of screech tone very precisely with present method. It can be concluded that the basic phenomenon of screech tone including the frequency can be calculated by using high-order and high-resolution schemes without any specific numerical modeling for screech tone feedback loop.

• The Bulletin of The Korean Astronomical Society
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• v.41 no.2
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• pp.41.2-41.2
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• 2016

We perform three statistical studies on the physical properties and oscillations in the confined plasma such as a photospheric sunspot and confined coronal loop. From the statistical studies on the sunspot umbra and its oscillation, we find that (1) the total magnetic flux inside the umbra for the three groups increases proportionally with the powers of the umbral area and the power indices in the three groups significantly differ from each other; (2) the three groups have different characteristics in their umbral area, intensity, magnetic field strength, and Doppler velocity as well as their relationships; (3) the mean frequency of the umbral oscillations increases with the umbral mean magnetic field strength and height; (4) the time delay of the core intensity of Fe I absorption line relative to the continuum which are de-convolved with the frequency range higher than 3.5 mHz is mostly positive, implying that the photospheric umbral oscillations are likely upwardly propagating; (5) the umbral mean plasma beta ranges approximately 0.6-1.1 and does not vary significantly from pores to mature sunspots. From the comparative study on the quasi-periodic pulsations (QPPs) in the solar and stellar flares, (6) we find that the power index of the periods scaling the damping times observed in the stellar QPPs is consistent with that observed in the solar QPPs, suggesting that physical mechanisms responsible for the stellar QPPs are likely the magneto-hydrodynamic oscillation of solar coronal loops.

• The Bulletin of The Korean Astronomical Society
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• v.42 no.2
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• pp.62.2-62.2
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• 2017

We have made an investigation on the radial and azimuthal wave modes of full halo coronal mass ejections (HCMEs). For this, we consider 24 HCMEs which are simultaneously observed by SOHO and STEREO A & B from August 2010 to August 2012 when they were roughly in quadrature. Using the SOHO/LASCO C3 and STEREO COR2 A & B running difference images, we estimate the instantaneous apparent speeds of the HCMEs at 24 different position angles. Major results from this study are as follows. First, there are quasi-periodic variations of the instantaneous radial velocity with the periods ranging from 24 to 48 mins. Second, the amplitudes of instant speed variations are about a third of the projected speeds. Third, the amplitudes are found to have a weak anti-correlation with period. Our preliminary identification from SOHO observations shows that there are several distinct radial and azimuthal wave modes: m=0 (radial) for five events, m=1 for eleven events, m=2 for three events, and unclear for the other events. In addition, we are making a statistical investigation on the oscillation of 733 CMEs to understand their physical origins.

• The Bulletin of The Korean Astronomical Society
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• v.42 no.2
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• pp.61.4-62
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• 2017

In this study we have made a statistical investigation on the kinematic classification of coronal mass ejections (CMEs) using about 4,000 SOHO/LASCO CMEs from 1996 to 2015. For this we use their SOHO/LASCO C3 data and exclude all poor events. Using the constant acceleration model, we classify these CMEs into three groups: Acceleration group, Constant Velocity group, and Deceleration group. For classification we adopt four different methods: Acceleration method, Velocity Variation method, Height Contribution method, and Visual Inspection method. Our major results are as follows. First, the fractions of three groups depend on the method used. Second, the results of the Height Contribution method are most consistent with those of the Visual Inspection method, which is thought to be most promising. Third, the fractions of different kinematic groups for the Height contribution method are: Acceleration (35%), Constant speed (47%), and Deceleration (18%). Fourth, the fraction strongly depend on CME speed; the fraction of Acceleration decreases from 0.6 to 0.05 with CME speed; the fraction of Constant increases from 0.3 to 0.7; the fraction of Deceleration increases from 0.1 to 0.3. Finally we present dozens of CMEs with non-constant accelerations. It is found that about 40 % of these CMEs show quasi-periodic oscillations.

• 한국전산유체공학회:학술대회논문집
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• 2007.04a
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• pp.120-124
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• 2007

Helicopters and rotary-wing vehicles encounter a wide variety of complex aerodynamic phenomena and these phenomena present substantial challenges for computational fluid dynamics(CFD) models. This investigation presents the rotor aerodynamic analysis items for the helicopter development and variety aerodynamic analysis methods to provide the better solution to researchers and helicopter developers between aerodynamic problems and numerical aerodynamic analysis methods. The numerical methods to make an analysis of helicopter rotor are as below - CFD Modelling : actuator disk model, BET model, fully rotor model,... - Grid : sliding mesh, chimera mesh / structure mesh, unstructure mesh,... - etc. : panel method periodic boundary, quasi-steady simulation, incompressible,... The choice of CFD methodology and the numerical resolution for the overall problem have been driven mostly by available computer speed and memory at any point in time. The combination of the knowledge of aerodynamic analysis items, available computing power and choice of CFD methods now allows the solution of a number of important rotorcraft aerodynamics design problems.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.25 no.12
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• pp.1861-1868
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• 2001

Vortical structures in the near-wake region of a cubic obstacle are studied using numerical simulation. We consider flow between two parallel plates with a cube mounted on one of the plates. In the turbulent near-wake region of the flow, coherent structures such as hairpin vortices are found. Quasi-periodic behavior of the hairpin vortices is noticed; its dimensionless frequency at Re=1,000 is about 0.82 which is consistent with the result of Elavarasan of et al.'s experiment [Fluid Dyn. Research, 2000, 27] although their geometry is somewhat different from on. In the case of Re=3,500, the dimensionless frequency of the hairpin vortex is about 1.60. It is observed that the translating speed of the head of the hairpin vortex is lower than the streamwise mean velocity at that location. In the vicinity of the lower plate downstream of the cube, various length scales are identified thus less coherent. However, it is noticed that the vortical structures become gradually elongated downstream of the new reattachment.

• International Journal of Naval Architecture and Ocean Engineering
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• v.3 no.1
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• pp.53-64
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• 2011

The present study considers multi-level approach for the analysis of parametric roll phenomena. Three kinds of computation method, GM variation, impulse response function (IRF), and Rankine panel method, are applied for the multi-level approach. IRF and Rankine panel method are based on the weakly nonlinear formulation which includes nonlinear Froude-Krylov and restoring forces. In the computation result of parametric roll occurrence test in regular waves, IRF and Rankine panel method show similar tendency. Although the GM variation approach predicts the occurrence of parametric roll at twice roll natural frequency, its frequency criteria shows a little difference. Nonlinear roll motion in bichromatic wave is also considered in this study. To prove the unstable roll motion in bichromatic waves, theoretical and numerical approaches are applied. The occurrence of parametric roll is theoretically examined by introducing the quasi-periodic Mathieu equation. Instability criteria are well predicted from stability analysis in theoretical approach. From the Fourier analysis, it has been verified that difference-frequency effects create the unstable roll motion. The occurrence of unstable roll motion in bichromatic wave is also observed in the experiment.

• Journal of Korean Biological Nursing Science
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• v.22 no.3
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• pp.184-191
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• 2020

Purpose: The purpose of this study was to validate the effects that the structured pamphlet and education through tablets regarding the daily life management and disease nursing education program has towards pneumonia patients in compliance, and self-efficacy. Methods: This study used the quasi-experimental study design based upon the nonequivalent control group pretest-posttest design. A total of 100 patients-50 patients who were hospitalized February 2020 as the control group and 50 patients who were hospitalized March 2020 as the experimental group - were used as material for the statistical analysis. These data were analyzed with a significance level of p< .05 using the SPSS WIN 21.0 program. Results: The patients' compliance, and self-efficacy had no significance difference between the two previous scores, meaning homogeneity in the two groups. Through verifying the experimental group who had disease and daily-life managing nursing education will have higher scores in compliance (t= 20.95, p< .001), and self-efficacy (t= 17.24, p< .001) than the control group who had not received those education, were statistically significantly different leading to all hypothesis being supported. Conclusion: For improving pneumonia patients' compliance, the methods should be simple, easy to understand, effective in numerous clinical situations, require constant education and reinforcement, and periodic nursing education program.

• The Bulletin of The Korean Astronomical Society
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• v.41 no.1
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• pp.46.2-46.2
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• 2016

We explore the similarity and difference of the quasi-periodic pulsations (QPPs) observed during the solar and stellar X-ray flares. For this, we identified 59 solar QPPs in the X-ray observed by the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) and 52 stellar QPPs from X-ray Multi Mirror Newton observatory (XMM-Newton). The Empirical Mode Decomposition (EMD) method and least-square-fit with the damped sine function are applied to obtain the periods and damping times of the QPPs. We found that (1) the periods and damping times of the stellar QPPs are 7.80 and 13.80 min, which are comparable with those of the solar QPPs 0.55 and 0.97 min. (2) The ratio of the damping times to the periods observed in the stellar QPPs are found to be statistically identical to the solar QPPs, (3) The damping times are well describe by the power law. The power indices of the solar and stellar QPPs are $0.891{\pm}0.172$ and $0.953{\pm}0.198$, which are consistent with the previous results. Thus, we conclude that the underlying mechanism responsible for the stellar QPPs are the natural oscillations of the flaring or adjacent coronal loops as in the Sun.