• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2017.05a
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• pp.942-944
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• 2017

High-speed vehicles are subjected to complex loads, such as acoustic pressure from the engine at launch and aerodynamic heating and aerodynamic pressure during flight. A thermal protection system panel is required to protect internal systems such as the fuel tank of the vehicle from the external environment. This study defines analytical models for heat transfer and thermal structure characteristics of the thermal protection system panel. Furthermore, the study performed parameters analysis to achieve the thermal structural integrity and to make it lighter.

• Journal of the Korean Geotechnical Society
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• v.35 no.11
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• pp.63-74
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• 2019

Numerical liquefaction model and response history analysis procedure are verified based on dynamic centrifuge test results. The test was a part of the Liquefaction Experiments Analysis Project (LEAP). The model ground was formed inside of rigid box by using the submerged Ottawa F65 sand with a relative density of 55% and 5° of surface inclination. A tapered sinusoidal wave with a frequency of 1 Hz was applied to the base of the model box. Numerical analyses were performed by two dimensional finite difference method in prototype scale. The soil is modeled to show hysteretic behavior before shear failure, and Mohr-Coulomb model is applied for shear failure criterion. Byrne's liquefaction model was applied to track the changes in pore pressure due to cyclic loading after static equilibrium. In order to find an appropriate flow condition for the liquefaction analysis, numerical analyses were performed both in drained and undrained condition. The numerical analyses performed under the undrained condition showed good agreement with the centrifuge test results.

• Journal of the Korean Geotechnical Society
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• v.38 no.11
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• pp.31-42
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• 2022

This study verified numerical analysis of the liquefaction phenomenon using LEAP-2017 international round-robin centrifuge test results. Dynamic centrifuge test is performed by applying a 1 Hz tapered sine wave to the model soil deposit, which was formed under a water table in a surface slope of 5° using Ottawa F-65 sand. A numerical model was made on a prototype scale and analyzed using the finite difference method in 2D and 3D conditions. The analyses were verified for acceleration and pore-water pressure histories with depth and residual displacement. Verification results revealed that all numerical liquefaction models agree reasonably with the test result for acceleration histories but not for pre-water pressure histories. Numerical analyses showed much smaller residual displacement than the centrifuge test. Thus, it is necessary to compare the results of numerical analysis with the centrifuge test performed by other institutes in the future.

• Proceedings of the Korean Society of Broadcast Engineers Conference
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• 2009.11a
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• pp.299-302
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• 2009

본 논문에서는 프레임 안에서의 인접픽셀간의 복잡도를 이용하여 할당비트 조절과 장면 전환을 추정하는 H.264/AVC 비트율 제어 기법을 제안한다. Quadratic 율-왜곡 모델은 프레임 할당 비트와 선형 모델을 통해 예측된 MAD정보를 필요로 한다. 이때 장면전환과 같은 급격한 화질 변화는 예측된 MAD정보만으로는 적합한 양자화파라미터를 결정하지 못하고 이후 비트율 제어를 어렵게 한다. 제안하는 방법은 프레임 안에서 인접픽셀간의 차분 값들의 절대 합을 구하여 이전프레임과 현제프레임의 비로 나타낸 후 이 값의 일정 범위를 벗어나면 장면전환이 이루어진 것으로 간주하여 이에 알맞은 양자화 파라미터 결정기법을 적용하도록 하였다.

• Transactions of the Korean Society of Mechanical Engineers
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• v.13 no.6
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• pp.1282-1291
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• 1989

본 연구에서는 이러한 연구의 시작단계로 노즐내의 열 경계층 방정식을 정리하고 압축성 와점도모델을 도입하여 유한 차분법을 이용한 전산프로그램을 작성 하였다. 한편 실제로 추진모테에서는 고온 고속의 열기류가 분출되기 때문에 추진 하고 있는 동안 노즐벽을 형성하고 있는 내열재가 심하게 손상되어 표면의 상태가 매우 거칠게 된다. 더구나 경우에 따라서는 내열재가 용발(ablation)하게 된다. 이러한 상태를 감안하여 마찰계수와 열전달 계수를 합리적으로 추정해야만 노즐의 설계와 주변장치를 합리적으로 수행할 수 있다. 따라서 본 연구에는 주로 경계층 내의 압력구배, 압축성의 효과, 물성치의 변화를 고려한 기존 난류모델에 근거하여 프로그램을 작성하고, 이것을 토대로 노즐표면 조도의 영향 및 분출(blowing)의 영 향을 중점적으로 고려하여 그 특성을 연구하였다. 노즐벽에서 분출을 고려한 이유는 표면이 용발할 때 표면의 온도가 거의 일정하게 유지된 상태로 노즐표면이 화학작용을 수반하면서 가스화됨을 초보적으로 고려해보기 위함이다.

• 한국지구물리탐사학회:학술대회논문집
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• 2006.06a
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• pp.265-270
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• 2006

High-frequency electromagnetic (EM) wave propagation phenomena associated with borehole ground-penetrating radar (GPR) surveys are complex. To improve the understanding of governing physical processes, we present a finite-difference time-domain solution of Maxwell's equations in cylindrical coordinates. This approach allows us to model the full EM wavefield associated with borehole GPR surveys. The algorithm can be easily implemented perfectly matched layers for absorbing boundaries, frequency-dependent media, and finite-length transmitter antenna.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.12 no.2
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• pp.70-80
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• 2000

Princeton Ocean Model (POM) is modified to construct a three-dimensional, semi-implicit hydro¬dynamic model with a wetting-and-drying scheme. The model employs semi-implicit treatment of the barotropic pressure gradient terms and the vertical mixing terms in the momentum equations, and the velocity divergence term in the vertically-integrated continuity equation. Such treatment removes the external mode and thus the mode splitting scheme in POM, allowing the semi-implicit model to use a larger time step. Applied to hypothetical systems, both the semi-implicit model and POM give nearly the same results. The semi-implicit model, however, runs approximately 4.4 times faster than POM showing its improved computational efficiency. Applied to a hypothetical system with intertidal flats, POM employing the mode splitting scheme produces noises at the intertidal flats, that propagate into the main channel resulting in unstable current velocities. Despite its larger time step, the semi-implicit model gives stable current velocities both at the intertidal flats and main channel. The semi-implicit model when applied to Kyeonggi Bay gives a good reproduction of the observed tides and tidal currents throughout the modeling domain, demonstrating its prototype applicability.

• Journal of the Society of Naval Architects of Korea
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• v.37 no.2
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• pp.70-76
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• 2000

A numerical study on the viscous flow around a 2-dimensional circulation control foil is carried out for application on the field of naval architecture and ocean engineering. The governing equations are the RANS and the continuity equations. The equations are discretized by finite difference method and MAC method and the pressure poisson equation is calculate by a SOR method and an O-type non-staggered boundary fitted coordinate system which is overlapped near the slot is used to improve the numerical accuracy. Turbulence is approximated by a modified Baldwin-Lomax turbulence model. In the present paper, the Coanda effect on a 2-dimensional foil of a 20% thickness ellipse with modified rounded trailing edge has been numerically studied. The change in drag and lift of the foil with various jet momentums are calculated and compared to the experimental results to show good agreements.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.13 no.1
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• pp.1-8
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• 2001

Wave-induced currents drive nearshore transport processes, and hence an accurate understanding of wave-current interaction is required for proper management of coastal zone. This paper presents details of an adaptive quadtree grid based numerical model of the coupled wave climate and depth-averaged current field. The model accounts for wave breaking, shoaling, refraction, diffraction, wave-current interaction, set-up and set-down, mixing processes, bottom friction effects, and movement of land-water interface at the shoreline. The wave period- and depth-averaged governing equations arc discrctized explicitly by means of an Adarns¬Bashforth second-order finite difference technique on adaptive hierarchical staggered quadtree grids. Results from the numerical model are in reasonable agreement with the laboratory data of longshore current generated by oblique waves on a plane beach (Visser 1980, 1991).

• Journal of the Korean Society of Propulsion Engineers
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• v.4 no.4
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• pp.59-69
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• 2000

A numerical analysis based on two-dimensional, incompressible and compressible Navier-Stokes equations was carried out for double circular arc compressor cascade and the results are compared with available experimental data. The incompressible code based on SIMPLE algorithm adopts pressure weighted method and hybrid scheme for the convective terms. The compressible code with preconditioning method involves a upwind-biased scheme for the convective terms and LU-SGS scheme for temporal integration. Several turbulence models are evaluated by computing the turbulent viscous flows; Baldwin-Lomax, standard $\kappa$ -$\varepsilon$, $\kappa$ -$\varepsilon$ Lam. Bremhorst, standard $\kappa$-$\omega$, $\kappa$ -$\omega$ SST model.