• 천문학회지
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• 제37권4호
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• pp.225-232
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• 2004

Shocks are ubiquitous in astrophysical environments and cosmic-rays (CRs) are known to be accelerated at collisionless shocks via diffusive shock acceleration. It is believed that the CR pressure is important in the evolution of the interstellar medium of our galaxy and most of galactic CRs with energies up to ${\~}\;10^{15}$ eV are accelerated by supernova remnant shocks. In this contribution we have studied the CR acceleration at shocks through numerical simulation of 1D, quasi-parallel shocks for a wide range of shock Mach numbers and shock speeds. We show that CR modified shocks evolve to time-asymptotic states by the time injected particles are accelerated to moderately relativistic energies, and that two shocks with the same Mach number, but with different shock speeds, evolve qualitatively similarly when the results are presented in terms of a characteristic diffusion length and diffusion time. We find that $10^{-4} - 10^{-3}$ of the particles passed through the shock are accelerated to form the CR population, and the injection rate is higher for shocks with higher Mach number. The time asymptotic value for the CR acceleration efficiency is controlled mainly by shock Mach number, and high Mach number shocks all evolve towards efficiencies ${\~}50\%$, regardless of the injection rate and upstream CR pressure. We conclude that the injection rates in strong quasi-parallel shocks are sufficient to lead to significant nonlinear modifications to the shock structures, implying the importance of the CR acceleration at astrophysical shocks.

• 한국가시화정보학회지
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• 제20권3호
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• pp.49-57
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• 2022

The interaction of planar shock wave with rectangular water column is investigated numerically. The flow phenomenon like reflection, transmission, cavitation, recirculation of shock wave, and large negative pressure due to expansion waves was discussed qualitatively and quantitatively. The numerical simulation was performed in a shock tube with a water column, and planar shock was initiated with a pressure ratio of 10. Three cases of the water column with different thicknesses, namely 0.5D, 1D, and 2D, were installed and studied. Water naturally has a higher acoustic impedance than air and mitigates the shock wave considerably. The numerical simulations were modelled using Eulerian and Volume of fluids multiphase models. The Eulerian model assumes the water as a finite structure and can visualize the shockwave propagation inside the water column. Through the volume of fluids model, the stages of breakup of the water column and mitigation effects of water were addressed. The numerical model was validated against the experimental results. The computational results show that the installation of a water column significantly impacts the mitigation of shock wave.

• 한국초전도저온공학회:학술대회논문집
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• 한국초전도저온공학회 2002년도 학술대회 논문집
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• pp.23-26
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• 2002

Two modes of shock waves propagating in He II (superfluid helium), this is a compression and a thermal shock waves, were studied experimentally by using superconductive temperature sensors, piezo pressure transducers and Schlieren visualization method with an ultra-high-speed video camera (40,500 pictures/sec). The shock waves are induced by a gas dynamic shock wave impingement upon a He II free surface. It is found that the shock Mach number of a transmitted compression shock wave is up to 1.16, and the shock Mach number of a thermal shock wave coincides well with the second sound velocity under each compressed He II state condition. The temperature rise ratio of an induced thermal shock wave to that of an incident gas dynamic shock wave was found to be very small, as small as 0.003 at 1.80K.

• 한국음향학회지
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• 제24권5호
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• pp.271-281
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• 2005

솔레노이드 코일을 이용하여 체외 충격파 치료술에 적합한 전자기식 충격파 발생기를 구성하였다. 충격파 발생기의 충격파의 특성은 바늘형 하이드로폰을 이용하여 평가하였다 충격파 발생기 방전 전압이 8에서 18 kV로 증가할 때 측정된 충격파의 최대 양압 (P+)은 $10\~77\;MPa$사이를 비선형적으로 증가하는 것으로 나타났다. 반면, 충격파 최대 음압 (P-)은 $-3.2\~-6.8\;MPa$ 에서 변화하고 있으며, 방전 전압이 14 kV에서 -6.9 MPa로 가장 낮은 값을 보였다. 동일한 설정에서 반복 측정된 충격파의 크기 P+는 평균값의 $5\;\%$ 이내에서 변화하며, 전기 수력학적 방식 충격파 발생기 경우의 $50\;\%$ 정도와 비교하여, 매우 작은 것으로 나타났다. 시간 축에서 1 ms 동안 측정한 하이드로폰 신호로부터 충격파에 의해 야기된 음향 공동 현상, 즉, 기포의 파열 현상으로 발생된 다수의 순차적인 음향 임펄스를 관찰할 수 있었다. 웨이블렛 변환 기법을 이용하여, 충격파 강도와 밀접한 관련이 있는 것으로 알려진, 첫 번째와 두 번째 기포 파열 시간 지연을 정확히 측정하였다. 충격파 크기 P+가 10 에서 77 MPa로 증가할 때 측정된 기포 파열 지연 시간은 120부터 $700\;{\mu}s$ 로 거의 선형적으로 증가함을 관찰할 수 있었다.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 춘계학술대회논문집E
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• pp.355-360
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• 2001

This study addresses a computational work of the impulsive wave which is discharged from the open end of a pipe. An initial compression wave inside the pipe is assumed to propagate toward atmosphere. The over pressure and wave-length of the initial compression wave are changed to investigate the characteristic values of the impulsive wave. The second order total variation diminishing (TVD) scheme is employed to solve the axisymmetric, compressible, unsteady Euler equations. The relationship between the initial compression wave form and impulsive wave is characterized in terms of the peak pressure of the impulsive wave and its directivity. The results obtained show that for the initial compression wave of a large wave-length the peak pressure of the impulsive wave does not depend on the over pressure of the initial compression wave, but for the initial compression wave of a very short wave-length, like a shock wave, the peak pressure of the impulsive wave is increased with an increase in the over pressure of the initial compression wave. The directivity of the impulsive wave to the pipe axis becomes significant with a decrease in the wave-length of the initial compression wave.

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2004년도 제22회 춘계학술대회논문집
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• pp.75-78
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• 2004

Various jet engines (Turbine engine family and RAM Jet engine) have been developed for high speed aircrafts. but their application to hypersonic flight is restricted by principle problems such as increase of total pressure loss and thermal stress. Therefore, the development of next generation propulsion system for hypersonic aircraft is a very important subject in the aerospace engineering field, SCRAM Jet engine based on a key technology, Supersonic Combustion. is supposed as the best choice for the hypersonic flight. Since Supersonic Combustion requires both rapid ignition and stable flame holding within supersonic air stream, much attention have to be given on the mixing state between air stream and fuel flow. However. the wider diffusion of fuel is expected with less total pressure loss in the supersonic air stream. So. in this study the direction of fuel injection is inclined 30 degree to downstream and the total pressure of jet is controlled for lower penetration height than thickness of boundary layer. Under these flow configuration both streams, fuel and supersonic air stream, would not mix enough. To spread fuel wider into supersonic air an aerodynamic force, baroclinic torque, is adopted. Baroclinic torque is generated by a spatial misalignment between pressure gradient (shock wave plane) and density gradient (mixing layer). A wedge is installed in downstream of injector orifice to induce an oblique shock. The schlieren optical visualization from side transparent wall and the total pressure measurement at exit cross section of combustor estimate how mixing is enhanced by the incidence of shock wave into supersonic boundary layer composed by fuel and air. In this study non-combustionable helium gas is injected with total pressure 0.66㎫ instead of flammable fuel to clarify mixing process. Mach number 1.8. total pressure O.5㎫, total temperature 288K are set up for supersonic air stream.

• 한국추진공학회지
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• 제13권1호
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• pp.27-33
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• 2009

충격파관 유동의 측정결과는 격막의 유한 파막 시간에 의하여 영향을 받게 된다. 그러나 지금까지 이에 관한 구체적인 연구사례는 많지 않다. 본 연구에서는 저압력비에서 작동하는 충격파관의 유동을 조사하기 위하여, 실험 및 수치해석을 수행하였다. 수치해석에서는 2차원 비정상 압축성 Navier-Stokes 방정식에 TVD MUSCL 유한차분법을 적용하였다. 본 수치해석 결과는 충격파관의 실험결과를 잘 예측하였으며, 충격파관의 파막 과정을 시간의 함수로 적절히 모사할 수 있었다. 본 연구에서는 유한의 파막 시간으로 인하여 발생하는 Non-centered 팽창파의 특성을 정량화하기 위하여 가상중심의 개념을 적용하였다. 본 연구로부터 충격파관의 압력비가 증가할수록 파막 시간은 감소하였으며, 충격파관 유동에 미치는 파막 시간의 영향은 저압력비에서 현저하게 나타나게 됨을 알았다.

• Journal of Trauma and Injury
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• 제25권1호
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• pp.17-24
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• 2012

Purpose: This study was performed to investigate whether therapeutic hypercapnia could attenuate systemic inflammatory responses in hemorrhagic shock in rats. Methods: Male Sprague-Dawley rats were mechanically ventilated and underwent pressure-controlled (mean arterial pressure: $38{\pm}1$ mmHg) hemorrhagic shock. At 10 minutes after the induction of hemorrhagic shock, the rats were divided into the normocapnia ($PaCO_2$=35-45 mmHg, n=10) and the hypercapnia ($PaCO_2$=60-70 mmHg) groups. The $PaCO_2$ concentration was adjusted by using the concentration of inhaled $CO_2$ gas. After 90 minutes of hemorrhagic shock, rats were resuscitated with shed blood for 10 minutes and were observed for 2 hours. The mean arterial pressure (MAP) and the heart rate were monitored continuously, and the results of arterial blood gas analyses, as well as the plasma concentrations of interleukin (IL)-6, IL-10, and nitrite/nitrate were compared between the normocapnia and the hypercapnia groups. Results: The MAP and the heart rate were not different between the two groups. The plasma concentration of IL-6 was significantly lower in the hypercapnia group than in the normocapnia group (p<0.05). The IL-10 concentration was not different and the IL-6 to IL-10 ratio was significantly lower in the hypercapnia group compared to the normocapnia group. The plasma nitrite/nitrate concentration of the hypercapnia group was lower than that of the normocapnia group. Conclusion: Therapeutic hypercapnia attenuates systemic inflammatory responses in hemorrhagic shock.

• 한국전기전자재료학회:학술대회논문집
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• 한국전기전자재료학회 2004년도 하계학술대회 논문집 Vol.5 No.2
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• pp.1109-1112
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• 2004

This study reports on the pressure relief design and braided composite of surge arrester. Surge arresters with porcelain housing must not have explosive breakage of the housing to minimize damage to other equipment when subjected to internal high short circuit current. As a solution, this study describes pressure relief design performance of arresters with braided composite module. In general, braided composite has Potential for improved impact and delamination resistance. Manufacturing processes of the braided composite could also be automated and could potentially lead to lower costs. Therefore, in consideration of characteristics of pressure relief for polymer arrester, the fabric pattern of braided composite was decided. And Polymer arrester module was manufactured with braid. The mechanisms of pressure occurrence and relief were investigated basically by analyzing arc energy and the correlation between thermal shock and indoor pressure in pressure relief test.

• 한국가시화정보학회지
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• 제10권1호
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• pp.40-46
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• 2012

Recently micro shock tube is extensively being used in many diverse fields of engineering applications but the detailed flow physics involved in it is hardly known due to high Knudsen number and strong compressibility effects. Unlike the macro shock tube, the surface area to volume ratio for a micro shock tube is very large. This unique effect brings many complexities into the flow physics that makes the micro shock tube different compared with the macro shock tube. In micro shock tube, the inter- molecular forces of working gas can play an important role in specifying the flow characteristics of the unsteady shock wave flow which is essentially generated in all kinds of shock tubes. In the present study, a CFD method was used to predict and visualize the unsteady shock wave flows using the unsteady compressible Navier-Stokes equations, furnished with the no-slip and slip wall boundary conditions. Maxwell's slip equations were used to mathematically model the shock movement at high Knudsen number. The present CFD results show that the propagation speed of the shock wave is directly proportional to the initial pressure and diameter of micro shock tube.