• 천문학회지
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• 제34권4호
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• pp.321-323
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• 2001

Compressible, magnetohydrodynamic (MHD) turbulence in two dimension is studied through high-resolution, numerical simulations with the isothermal equation of state. First, hydrodynamic turbulence with Mach number $(M)_{rms}\;\~$1 is generated by enforcing a random force. Next, initial, uniform magnetic field of various strengths with Alfvenic Mach number Ma $\gg$ 1 is added. Then, the simulations are followed until MHD turbulence is fully developed. Such turbulence is expected to exist in a variety of astrophysical environments including clusters of galaxies. Although no dissipation is included explicitly in our simulations, truncation errors produce dissipation which induces numerical resistivity. It mimics a hyper-resistivity in our second-order accurate code. After saturation, the resulting flows are categorized as SF (strong field), WF (weak field), and VWF (very weak field) classes respectively, depending on the average magnetic field strength described with Alfvenic Mach number, $(Ma)_{rms}{\ge}1$, $(Ma)_{rms}{\~}1$, and $(Ma)_{rms}{\gg}1$. The characteristics of each class are discussed.

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2008년 영문 학술대회
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• pp.480-485
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• 2008

In this study, two types of ramjet intake were designed for the flight condition of Mach number 2 and 5 and numerical analysis was performed. In order to widen the flight envelope range(Mach number $2{\sim}6$), movable intake concept was applied. The central body was designed so that the capture area ratio which is one of most important factors of ramjet intake design could be adjusted. And various types of cowl and movable insert part of shell were designed in order to control throat area which could increase total pressure recovery. The numerical results showed that the designed ramjet intake could be applied in various flights Mach number.

• 천문학회지
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• 제36권1호
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• pp.1-12
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• 2003

Nonthermal particles can be produced due to incomplete thermalization at collisionless shocks and further accelerated to very high energies via diffusive shock acceleration. In a previous study we explored the cosmic ray (CR) acceleration at cosmic shocks through numerical simulations of CR modified, quasi-parallel shocks in 1D plane-parallel geometry with the physical parameters relevant for the shocks emerging in the large scale structure formation of the universe (Kang & Jones 2002). Specifically we considered pancake shocks driven by accretion flows with $U_o = 1500 km\;s^{-l}$ and the preshock gas temperature of $T_o = 10^4 - 10^8K$. In order to consider the CR acceleration at shocks with a broader range of physical properties, in this contribution we present additional simulations with accretion flows with $U_o = 75 - 1500 km\;s^{-l}$ and $T_o = 10^4K$. We also compare the new simulation results with those reported in the previous study. For a given Mach number, shocks with higher speeds accelerate CRs faster with a greater number of particles, since the acceleration time scale is $t_{acc}\;{\propto}\;U_o^{-2}$. However, two shocks with a same Mach number but with different shock speeds evolve qualitatively similarly when the results are presented in terms of diffusion length and time scales. Therefore, the time asymptotic value for the fraction of shock kinetic energy transferred to CRs is mainly controlled by shock Mach number rather than shock speed. Although the CR acceleration efficiency depends weakly on a well-constrained injection parameter, $\epsilon$, and on shock speed for low shock Mach numbers, the dependence disappears for high shock Mach numbers. We present the 'CR energy ratio', ${\phi}(M_s)$, for a wide range of shock parameters and for $\epsilon$ = 0.2 - 0.3 at terminal time of our simulations. We suggest that these values can be considered as time-asymptotic values for the CR acceleration efficiency, since the time-dependent evolution of CR modified shocks has become approximately self-similar before the terminal time.

• 한국항공우주학회지
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• 제37권1호
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• pp.17-27
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• 2009

압축성 영역에서 개발된 2상 유동 RoeM과 AUSMPW+를 전 마하수 영역 해석을 위해 예조건화 하였다. 압축성 영역에서 개발된 풍상차분 수치기법들은 수치점성에서 음속을 포함하는 항들을 갖고 있는데, 이는 비압축성 영역에서 관련 수치점성 크기를 지나치게 크거나 작게 만들어 해의 정확도를 떨어뜨리거나 수치 불안정 현상을 일으킨다. 따라서 예조건화된 시스템 고유값을 수치점성에 고려하여 전 마하수 영역에서 해의 정확성을 확보하였고, 점근 분석을 통해 개발된 수치기법이 비압축 극한 영역에서 보존형 지배방정식과 일관됨을 확인하였다. 수치 예제들을 통해 예조건화된 2상 유동 RoeM과 AUSMPW+가전 마하수 영역에서 합리적인 해를 도출함을 알 수 있었다.

• 한국가시화정보학회:학술대회논문집
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• 한국가시화정보학회 2002년도 추계학술대회 논문집
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• pp.113-116
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• 2002

The shock wave discharged from an annular duct leads to very complicated flow features, such as Mach stem, spherical waves, and vortex rings. In the current study, the merging phenomenon and propagation characteristics of the shock wave are numerically investigated using a CFD method. The Harten-Yee's total variation diminishing (TVD) scheme is used to the unsteady, axisymmetric, two-dimensional, compressible Euler equations. The Mach number of incident shock wave $M_s$ is varied in the range below 2.0. The computational results are visualized to observe the major features of the annular shock waves discharged from the tube. On the symmetric axis, the peak pressure produced by the shock wave and its location depend upon strongly the radius of the annular tubes. A Mach stem is generated along the symmetric axis of the annular tubes.

• 전자공학회논문지A
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• 제33A권2호
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• pp.140-147
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• 1996

The output of the mach-zehnder optical fiber interferometer varies with birefringences existing in the figer arms. New equations experessing for the visibility of the mach-zehnder interferometer were derived in terms of a number of parameters related to the birefringences, the input SOP and the optical losses. Based on the equations the visibility of the interferometer was simulated in different cases. Some maximum visibility conditons were also presented.

• 대한기계학회논문집B
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• 제40권12호
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• pp.785-792
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• 2016

본 연구에서는 NACA0012 천음속 익형 유동에 있어 비평형 응축이 항력 발산 마하수 $M_D$에 미치는 영향을 TVD 유한 차분법을 사용하여 연구하였다. 받음각 ${\alpha}$가 동일한 경우, 정체점 상대습도 ${\phi}_0$가 높을수록 충격파 직전의 최대 마하수 $M_{max}$는 작게 되고 초음속 영역의 크기도 적게 된다. 주류 마하수 $M_{\infty}$, 정체점 상대습도 ${\phi}_0$ 및 정체온도 $T_0$가 동일한 경우 받음각 ${\alpha}$가 클수록 비평형 응축영역 길이 ${\Delta}_z$은 짧게 된다. 한편, 주류 마하수 $M_{\infty}$와 받음각 ${\alpha}$가 동일한 경우 정체점 상대습도 ${\phi}_0$가 높을수록 조파저항의 감소 때문에 항력계수 $C_D$는 적어진다. $M_D$는 ${\alpha}$가 동일한 경우 ${\phi}_0$가 클수록 크게 되며, ${\phi}_0$가 동일한 경우는 ${\alpha}$가 클수록 $M_D$는 적게 된다.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2007년도 춘계학술대회B
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• pp.3529-3533
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• 2007

The advantages of the SITVC(secondary injection thrust vector control) technique over mechanical thrust vector systems include a reduction in both the nozzle weight and complexity due to the elimination of the mechanical actuators that are used in conventional vectoring. Computational study is performed to understand the fluidic thrust vectoring control of an axisymmetric nozzle, in which secondary gas injection is made in the divergent section of the nozzle. The nozzle has a design mach number 3. The effect of injection hole number and shape of secondary jet on the mach number distribution of SITVC were investigated. The standard ${\kappa}$ - ${\epsilon}$ turbulence model solved the complex three-dimensional nozzle flows perturbed by the secondary gas jet. The numerical code was validated by experiment. The results showed that the mach number distribution of circular and square nozzle are similar each other. As number of second injection hole increasing, a effect of deflection was decreased.

• 대한기계학회논문집B
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• 제21권4호
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• pp.509-517
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• 1997

Results of flat plate compressible boundary layer calculation, based on discrete formulation of DSMC method, are presented in low Mach number and low Knudsen number range. The free stream is a uniform flow of pure nitrogen at various Mach numbers in low pressures (i.e. rarefied gas). Complete thermal accommodation and diffuse molecular reflections are used as the wall boundary condition, replacing unreal no-slip condition used in continuum calculations. In the discrete formulation of DSMC method, there is no need to use ad hoc assumptions on transport properties like viscosity and thermal conductivity, instead viscosity is calculated from values of other field variables (velocity and shear stress). Also the results are compared with existing self-similar continuum solutions. In all Mach number cases computed, velocity slip is most pronounced in regions near the leading edge where continuum formulation renders the solution singular. As the boundary layer develops further downstream, velocity slips asymptote to values that are between 10 to 20% of the magnitude of free stream velocity. When the free stream number density is reduced, so the gas more rarefied, the velocity slip increases as expected.

• 천문학회보
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• 제43권2호
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• pp.42.2-43
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• 2018

When galaxy clusters interact, the intergalactic gas collides, forming shocks that are characterized by a low sonic Mach number (~3) but a comparatively high Alfvenic Mach number (~30). Such shocks behave differently from the more common astrophysical shocks, which tend to have higher sonic Mach numbers. We wish to determine whether these shocks, despite their low sonic Mach number, are capable of accelerating particles and thereby contributing to the cosmic ray spectrum. Using the PIC-MHD method, which separates the gas into a thermal and a non-thermal component to increase computational efficiency, and relying on existing PIC simulations to determine the rate at which non-thermal particles are injected in the shock, we investigate the evolution of galaxy cluster shocks and their ability to accelerate particles. Depending on the chosen injection fraction of non-thermal particles into the shock, we find that even low-Mach shocks are capable of accelerating particles. However, the interaction between supra-thermal particles and the local magnetic field triggers instabilities and turbulence in the magnetic field. This causes the shock to weaken, which in turn reduces the effectiveness of the supra-thermal particle injection. We investigate how this influences the shock evolution by reducing the particle injection rate and energy and find that a reduction of the particle injection fraction at this stage causes an immediate reduction of both upstream and downstream instabilities. This inhibits particle acceleration. Over time, as the instabilities fade, the shock surface straightens, allowing the shock to recover. Eventually, we would expect this to increase the efficiency of the particle injection and acceleration to previous levels, starting the same series of events in an ongoing cycle of increasing and decreasing particle acceleration.