• Journal of computational fluids engineering
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• v.15 no.2
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• pp.14-20
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• 2010

Shock-focusing concave reflectors can have parabolic, circular or elliptic cross-sections. They produce effectively a very high pressure at the focusing point. In the past, many optical images have been obtained on shock focusing via experiments. Measurement of field variables is, however, difficult in the experiment. Using the wave propagation algorithm and the Cartesian embedded boundary method, we have successfully obtained numerical Schlieren images that appear very much like the experimental results. In addition, we obtained the detailed field variables such as pressure, velocity, density and vorticity in the unsteady domain. The present numerical results have made it possible to understand the shock focusing phenomenon in more detail than before.

• Journal of the Korean Society of Industry Convergence
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• v.2 no.1
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• pp.121-130
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• 1999

It is necessary to measure pressure, temperature, chemical equilibrium and the shape of flame in order to understand the combustion process in a combustion chamber. In particular, the flame formulation and combustion process of divided combustion chamber are different from those of a single chamber, And the variable diameter of a jet hole can effect not only physical properties like ejection velocity, temperature and time of combustion but also a chemical property like the reaction mechanism. Accordingly temperature is one of the most important factors which influence the combustion mechanism. This paper observed shape of flame by using the schlieren photographs and measured the pressure in a combustion chamber and the reaching time of the flame by ion probe By doing these, we investigation the formulation of the flame and the process of propagation. These measurement methods can be advanced in understanding the combustion process and process and propagation of flame.

• Journal of the Semiconductor & Display Technology
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• v.19 no.2
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• pp.37-44
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• 2020

In improving laser cutting of optical plastic films for mass production of optoelectronics display units, it is important to understand particle contamination over optical film surface due to fume particle generation and dispersion. This numerical study investigates the effects of downward and upward air flow motions on fume particle dispersion around laser cut line. The simulations employ random particle sampling of up to one million fume particles by probabilistic distributions of particle size, ejection velocity and angle, and fume particle dispersion and surface landing are predicted using Basset-Boussinesq-Oseen model of low Reynolds number flows. The numerical results show that downward air flow scatters fume particles of a certain size range farther away from laser cut line and aggravate surface contamination. However, upward air flow pushes fume particles of this size range back toward laser cut line or sucks them up with rising air motion, thus significantly alleviating surface contamination.

• Journal of the Semiconductor & Display Technology
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• v.18 no.4
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• pp.62-68
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• 2019

The optoelectronic display units such as TFT-LCD or OLED require many thin optical plastic films and their mass manufacturing processes employ CO2 laser cutting of those thin films in a large quantity. However, laser film cutting could generate fume particles through melt shearing, vaporization, and chemical degradation and those particles could be of great concern for film surface contamination. In order to appreciate the fume particle dispersion behaviors in laser film cutting, this study relies on random particle simulations by probabilistic distributions of particle size, ejection velocity and angles coupled with Basset-Boussinesq-Oseen model of particle trajectory in low Reynolds number flows. Here, up to one million particles of random sampling have been tested to effectively show fume particles dispersed on the film surface. The computational results could show that particular range of fume particle size could easily disperse into the pixel region of processed optical films.

• Journal of Yeungnam Medical Science
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• v.16 no.2
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• pp.309-317
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• 1999

Background: Left ventricular diastolic filling is an important determinant for maintenance of cardiac output during hemodialysis. Few investigators have studied the influence of hemodialysis on diastolic function. To evaluate the change of left ventricular systolic and diastolic function. we performed M-mode and Doppler echocardiographic studies before and after hemodialysis. Methods: The study population consisted of 30 patients(15 patients were male, mean age $45{\pm}10$ years) with CRF on maintenance hemodialysis. They have normal left ventricular systolic function(Fractional shortening>30%) and no evidence of valvular heart disease or regional wall motion abnormalities. The ejection fraction (EF) was measured using M-mode echocardiography and Doppler indices such as peak E velocity, peak A velocity, isovolumetric relaxation time(IVRT), deceleration time(DT). and left ventricular ejection time(LVET) obtained from Doppler echocardiography. The index of myocardial performance (IMP) was calculated from each of the Doppler velocity indices. Results: The weight reduction after hemodialysis was $2.1{\pm}1.0kg$(p<0.0001), After hemodialysis, there was some decrease in blood pressure(p<0.05), but no significant change in heart rate, EF and fractional shortening, mean VCF, peak A velocity, and DT. And significant reduction in peak E velocity, E/A ratio(p<0.0001. p<0.001), and significant increase in IVRT and IMP(p<0.05, p<0.0001) were noted. Conclusion: In conclusion, preload reduction is the main mechanism that accounts for changes in Doppler diastolic indices after hemodialysis. And an increased IMP suggests that diastolic function may be aggravated after hemodialysis, and that implies impaired left ventricular filling and disturbed left ventricular compliance.

• Korean Circulation Journal
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• v.54 no.6
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• pp.311-322
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• 2024

Background and Objectives: Early diastolic mitral annular tissue (e') velocity is a commonly used marker of left ventricular (LV) diastolic function. This study aimed to investigate the prognostic implications of e' velocity in patients with mitral regurgitation (MR). Methods: This retrospective cohort study included 1,536 consecutive patients aged <65 years with moderate or severe chronic primary MR diagnosed between 2009 and 2018. The primary and secondary outcomes were all-cause and cardiovascular mortality, respectively. According to the current guidelines, the cut-off value of e' velocity was defined as 7 cm/s. Results: A total of 404 individuals were enrolled (median age, 51.0 years; 64.1% male; 47.8% severe MR). During a median 6.0-year follow-up, there were 40 all-cause mortality and 16 cardiovascular deaths. Multivariate analysis revealed a significant association between e' velocity and all-cause death (adjusted hazard ratio [aHR], 0.770; 95% confidence interval [CI], 0.634-0.935; p=0.008) and cardiovascular death (aHR, 0.690; 95% CI, 0.477-0.998; p=0.049). Abnormal e' velocity (≤7 cm/s) independently predicted all-cause death (aHR, 2.467; 95% CI, 1.170-5.200; p=0.018) and cardiovascular death (aHR, 5.021; 95% CI, 1.189-21.211; p=0.028), regardless of symptoms, LV dimension and ejection fraction. Subgroup analysis according to sex, MR severity, mitral valve replacement/repair, and symptoms, showed no significant interactions. Including e' velocity in the 10-year risk score improved reclassification for mortality (net reclassification improvement [NRI], 0.154; 95% CI, 0.308-0.910; p<0.001) and cardiovascular death (NRI, 1.018; 95% CI, 0.680-1.356; p<0.001). Conclusions: In patients aged <65 years with primary MR, e' velocity served as an independent predictor of all-cause and cardiovascular deaths.

• Clinical and Experimental Pediatrics
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• v.61 no.2
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• pp.59-63
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• 2018

Purpose: Cardiomyopathy is becoming the leading cause of death in patients with Duchenne muscular dystrophy because mechanically assisted lung ventilation and assisted coughing have helped resolve respiratory complications. To clarify cardiopulmonary function, we compared cardiac function between the home ventilator-assisted and non-ventilator-assisted groups. Methods: We retrospectively reviewed patients with Duchenne muscular dystrophy from January 2010 to March 2016 at Gangnam Severance Hospital. Demographic characteristics, pulmonary function, and echocardiography data were investigated. Results: Fifty-four patients with Duchenne muscular dystrophy were divided into 2 groups: home ventilator-assisted and non-ventilator-assisted. The patients in the home ventilator group were older ($16.25{\pm}1.85years$) than those in the nonventilator group ($14.73{\pm}1.36years$) (P=0.001). Height, weight, and body surface area did not differ significantly between groups. The home ventilator group had a lower seated functional vital capacity ($1,038{\pm}620.41mL$) than the nonventilator group ($1,455{\pm}603.2mL$). Mean left ventricular ejection fraction and fractional shortening were greater in the home ventilator group, but the data did not show any statistical difference. The early ventricular filling velocity/late ventricular filling velocity ratio ($1.7{\pm}0.44$) was lower in the home ventilator group than in the nonventilator group ($2.02{\pm}0.62$. The mitral valve annular systolic velocity was higher in the home ventilator group (estimated ${\beta}$, 1.06; standard error, 0.48). Patients with Duchenne muscular dystrophy on a ventilator may have better systolic and diastolic cardiac functions. Conclusion: Noninvasive ventilator assistance can help preserve cardiac function. Therefore, early utilization of noninvasive ventilation or oxygen may positively influence cardiac function in patients with Duchenne muscular dystrophy.

• Journal of Korean Tunnelling and Underground Space Association
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• v.24 no.4
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• pp.305-316
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• 2022

Hydrogen energy is emerging as an alternative to the depletion of fossil fuels and environmental problems, and the use of hydrogen vehicles is increasing in the automobile industry as well. However, since hydrogen has a wide flammability limit of 4 to 75%, there is a high concern about safety in case of a hydrogen car accident. In particular, in semi-enclosed spaces such as tunnels and underground parking lots, a fire or explosion accompanied by hydrogen leakage is highly likely to cause a major accident. Therefore, it is necessary to review hydrogen safety through analysis of flammability areas caused by hydrogen leakage. Therefore, in this study, the effect of the air velocity in the tunnel on the flammability area was investigated by analyzing the hydrogen concentration according to the hydrogen leakage conditions of hydrogen vehicles and the air velocity in the tunnel in a road tunnel with standard section. Hydrogen leakage conditions were set as one tank leaking and three tanks leaking through the TPRD at the same time and a condition in which a large crack occurred and leaked. And the air velocity in the tunnel were considered 0, 1, 2.5, and 4.0 m/s. As a result of the analysis of the flammability area, it is shown that when the air velocity of 1 m/s or more exists, it is reduced by up to 25% compared to the case of air velocity of 0 m/s. But there is little effect of reducing the flammability area according to the increase of the wind speed. In particular, when a large crack occurs and completely leaks in about 2.5 seconds, the flammability area slightly increases as the air velocity increases. It was found that in the case of downward ejection, hydrogen gas remains under the vehicle for a considerably long time.

• Journal of the Korea Society for Simulation
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• v.29 no.1
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• pp.1-9
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• 2020

The ballistic missile threat continues to increase with the proliferation of missile technology. In response to this threat, many kinds of interceptors have been emphasized over the years. For development of interceptor, systematic flight tests are essential. Flight tests provide valuable data that can be used to verify performance and confirm the technological progress of ballistic missile defense system including interceptor. However, during flight tests, civilians near the test region could be risk due to a lot of intercept debris. For this reason, reliable estimate of safety area for the flight tests should be preceded. In this study, prediction of safety area is performed through modeling and simulation. Firstly, behaviors of ballistic missile and interceptor are simulated for those entire phase including interception to obtain the relative intercept velocity and the relative impact angle. By using obtained data of kinetic energy, the fragment ejection velocity is calculated and fragment trajectories are simulated by considering drag, gravity and wind effects. Based on the debris field formation and hazard evaluation of debris, final safety area is calculated.

• Fire Science and Engineering
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• v.27 no.6
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• pp.64-69
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• 2013

The Numerical simulation was performed on the flow field around the two-dimensional rectangular bluff body in order to simulate an engine nacelle fire and to complement the previous experimental results of the bluff body stabilized flames. Fire Dynamic Simulator (FDS) based on the Direct Numerical Simulation (DNS) was employed to clarify the characteristics of reacting flow around bluff body. The overall reaction was considered and the constant for reaction was determined from flame extinction limits of experimental results. The air used atmosphere and the fuel used methane. For both fuel ejection configurations against an oxidizer stream, the flame stability and flame mode were affected mainly by vortex structure near bluff body. In the coflow configuration, air velocity at the flame extinction limit are increased with fuel velocity, which is comparable to the experiment results. Comparing with the isothermal flow field, the reacting flow produces a weak and small recirculation zone, which is result in the reductions of density and momentum due to temperature increase by reaction in the wake zone.