• Journal of the Korean Society for Aeronautical & Space Sciences
• /
• v.45 no.3
• /
• pp.233-240
• /
• 2017

The flowfield around transonic wing-body configuration was simulated using in-house CFD code and compared with the experimental data to understand the influence of several features of CFD(Computational Fluid Dynamics) ; grid dependency, turbulence models, spatial discretization, and viscosity. The wing-body configuration consists of a simple planform RAE Wing 'A' with an RAE 101 airfoil section and an axisymmetric body. The in-house CFD code is a compressible Euler/Navier-Stokes solver based on unstructured grid. For the turbulence model, the $k-{\omega}$ model, the Spalart-Allmaras model, and the $k-{\omega}$ SST model were applied. For the spatial discretization method, the central differencing scheme with Jameson's artificial viscosity and Roe's upwind differencing scheme were applied. The results calculated were generally in good agreement with experimental data. However, it was shown that the pressure distribution and shock-wave position were slightly affected by the turbulence models and the spatial discretization methods. It was known that the turbulent viscous effect should be considered in order to predict the accurate shock wave position.

• Journal of the Computational Structural Engineering Institute of Korea
• /
• v.27 no.6
• /
• pp.615-624
• /
• 2014

It has been well known the sloshing pressure has complex shape and various patterns. The pattern of sloshing pressure is variously characterized by the pressure amplitude, duration time and skewness. The structural response induced by the sloshing pressure is also affected by the pattern of sloshing pressure and the type of structural members. In order to understand the structural response by the perspective view of categorized pattern, it is more efficient to make simple sloshing pressure pattern than to reflect the complex pressure history. In this study, the sloshing pressures obtained by the small scale model test are simplified with respect to their duration and skewness. Dynamic analyses of Mark-III LNG CCS are then parametrically performed with the consideration of various types of sloshing impact. Meanwhile, the failure pressures given the duration and skewness are investigated after parametric calculations are conducted to investigate the effect of pressure parameters on the structural response.

• Journal of the Korean Geotechnical Society
• /
• v.18 no.4
• /
• pp.271-283
• /
• 2002

P-wave velocity of concrete is a crucial parameter in determining the thickness of concrete lining, the location of cracks or other defects in Impact-Echo(IE) method. This study introduces an IE-SASW method that may determine the P-wave velocity on a surface of each testing area using the Spectral Analysis of Surface Wave (SASW) method. In numerical studies(Part I), it was verified that P-wave velocities could be obtained from SASW. In this paper(Part II), experimental studies were made in slab type concrete model specimens in which voids and waterproof sheet were included at the known locations. Accordingly, the feasibility of the proposed method was evaluated. The IE-SASW method was also performed in the precast model tunnel on ground and open-cut tunnel in ground. SASW tests were performed to determine the P-wave velocity of the concrete and then IE tests were carried at regularly spaced points along the testing lines to determine the thickness of structures. The nondestructive testing method which combined SASW and IE tests showed the great potential in the field applications.

• Journal of Korea Water Resources Association
• /
• v.47 no.11
• /
• pp.1077-1086
• /
• 2014

The flood wave generated due to dam break is affected by initial depth upstream since it is related with hydraulic characteristics propagating downstream, and flow resistance stress has influence on the celerity, travel distance, and approaching depth of shock wave in implementing numerical simulation. In this study, a shallow water flow model employing SU/PG scheme was developed and verified by analytic solutions; propagation characteristics of dam break according to flow resistance and initial depth were analyzed. When bottom frictional stress was applied, the flow depth was relatively higher while the travel distance of shock wave was shorter. In the case of Coulomb stress, the flow velocity behind the location of dam break became lower compared with other cases, and showed values between no stress and turbulent stress at the reach of shock wave. The value of Froude number obtained by no frictional stress at the discontinuous boundary was the closest to 1.0 regardless of initial depth. The adaption of Coulomb stress gave more appropriate results compared with turbulent stress at low initial depth. However, as the initial depth became increased, the dominance of flow resistance terms was weakened and the opposite result was observed.

• Proceedings of the Korea Water Resources Association Conference
• /
• 2012.05a
• /
• pp.147-147
• /
• 2012

댐 및 제방 등의 수공구조물 붕괴에 의한 극한홍수 전파를 해석하기 위한 2차원 홍수 해석기법에는 현재까지 다양한 기법들이 개발되어 왔고 다양한 모형들이 상용화 또는 범용화 되고 있다. 그 중 흐름의 전파양상을 정확하게 반영할 수 있는 상류이송기법인 Godunov형태의 유한체적기법은 충격파와 같은 불연속적인 해를 가지는 문제를 정확히 해석할 수 있고, 비구조적 격자 사용의 용이성 등의 장점 때문에 2차원 홍수파 전파 해석에 있어서 최근 십수년간 가장 활발하게 연구되어왔다. 하지만 이러한 기법은 양해법을 근간으로 하는 해석 기법으로써, 계산거리의 간격이나 계산시간의 간격, 격자망의 구성 등 엄격한 제한이 필요하다. 특히 방대한 계산시간을 요구하는 기법의 약점은 홍수예 경보 등을 위한 실시간 모형의 구동에 있어서 큰 제약이 되어 왔다. CPU의 성능이 지속적으로 발전하면서 이러한 문제는 점차 극복되어 왔으나, 발열 등의 문제와 이를 극복하기 위한 멀티코어의 등장으로 인해 단일 코어의 성능개발은 매우 더딘 것이 사실이다. 현재까지 연구되고 개발되어 온 모형들은 특별한 처리 없이는 단일 코어만을 사용하여 계산할 수 밖에 없기 때문에 멀티코어의 장점을 전혀 이용할 수 없다. 이러한 점을 극복하기 위해 프로그램을 병렬화하여 단일 문제에 대해 멀티코어를 사용할 수 있다면 계산시간 단축에 큰 효과를 거둘 수 있을 것이다. 현재까지 IT분야에서 다양한 병렬프로그래밍 기법들이 개발되고 소개되어 왔다. 본 연구에서는 병렬프로그래밍 기법중 가장 널리 사용되고 있는 MPI(Message Passing Interface)기법을 적용함으로써 기 개발된 고정확도 유한체적모형을 병렬화 하여 계산시간을 단축하고자 하였다. 개발된 모형을 장애물이 존재하는 실험하도의 댐 붕괴 문제와 2002년 태풍 루사 시 큰 피해를 입은 강릉시 일원의 섬석천 유역에 위치한 장현저수지와 동막저수지의 붕괴사례에 대해 적용하였다. 모형을 코어 개수별로 다양하게 모의함으로써 기존모형과의 결과에 대한 일치성을 확인하였고, 기존 모형 대비 계산시간 단축의 효과를 입증할 수 있었다. 개발된 본 모형을 실시간 홍수범람해석을 위한 시스템으로 구축할 수 있다면, 실시간 홍수예 경보에 있어 주요지점에서의 수위해석 뿐만이 아닌 제내지 범람 예보 분야까지 확대 적용할 수 있을 것으로 사료된다.

• Magazine of the Korea Concrete Institute
• /
• v.10 no.4
• /
• pp.171-182
• /
• 1998

Vibration-controlled concrete has been developed by using various concrete mixtures, such as latex, rubber powders, plastic resins and polystyrene(styrofoam). As part of the recycling research of obsolete aged tires and plastic materials, various vibration-reducing mixtures are used for 10 concrete panels having above 200 kg/cm$^2$ in uniaxial compressive strength. Plywood box with sand uniformly saturated by the raining device has been used for the analysis of the impact wave, of which data have been transfered by the FFT technique to comparatively investigate damping ratios of 10 concrete panels.According to wave propagation analysis on vibration-controlled concrete for this research, it can be concluded that Latex concrete has relatively larger damping ratios than those for noncontrolled normal concrete in a similar compressive strength

• Journal of Korea Water Resources Association
• /
• v.40 no.4
• /
• pp.325-334
• /
• 2007

In this study, a long-term unsteady simulation model has been developed using rigid water column theory which is more accurate than Extended-period model and more efficient comparing with water-hammer simulation model. The developed model is applied to 24-hours unsteady simulation considering daily water-demand and water-hammer analysis caused by closing a valve. For the case of 24-hours daily simulation, the pressure of each node decreases as the water demand increase, and when the water demand decrease, the pressure increases. During the simulation, the amplitudes of flow and pressure variation are different in each node and the pattern of flow variation as well as water demand is quite different than that of KYPIPE2. Such discrepancy necessitates the development of unsteady flow analysis model in water distribution network system. When the model is applied to water-hammer analysis, the pressure and flow variation occurred simultaneously through the entire network system by neglecting the compressibility of water. Although water-hammer model shows the lag of travel time due to fluid elasticity, in the aspect of pressure and flow fluctuation, the trend of overall variation and quantity of the result are similar to that of water-hammer model. This model is expected for the analysis of gradual long-term unsteady flow variations providing computational accuracy and efficiency as well as identifying pollutant dispersion, pressure control, leakage reduction corresponding to flow-demand pattern, and management of long-term pipeline net work systems related with flowrate and pressure variation in pipeline network systems

• Proceedings of the Korea Water Resources Association Conference
• /
• 2007.05a
• /
• pp.1332-1336
• /
• 2007

댐 제방 붕괴파는 갑작스러운 유량의 증가가 발생하여 불연속적인 흐름특성을 가지는 충격파(shock wave)가 전파되며, 갈수기 저수기에는 중소하천의 상류, 여울과 소에서의 흐름 또는 낙차공이나 보, 댐 여수로 등의 수공구조물에서 부분적인 사류 흐름이 발생된다. 이 때 흐름은 한계수위를 통과하게 되므로 기존 수치해법의 적용에 어려움이 존재한다. 본 연구에서는 실제하천에 적용될 수 있는 1차원 HLL, Roe Riemann 근사해법들을 간단히 소개하고, 시간공간적으로 2차의 고정확도 기법으로 확장하는 방법에 대하여 소개하였다. 각 기법을 정확해가 존재하는 댐붕괴 및 마른하도 전파의 경우에 적용하여 각 기법의 적용성 및 정확성을 비교하였다. 그리고 기존 Lax-Friedrichs 기법과 Lax-Wendroff 기법의 적용결과를 비교하였다. 적용결과 Lax-Friedrichs 기법을 제외한 모든 기법이 정확해와 잘 일치하였으며 특히 HLL 기법을 2차 정확도로 확장한 WAF 기법이 가장 높은 정확도로 계산되었다. 그러나 비선형 생성항이 존재하는 실제하천에 있어서 MUSCL 기법을 이용한 2차 정확도 기법이 합리적일 것으로 판단된다.

• Journal of Ocean Engineering and Technology
• /
• v.7 no.1
• /
• pp.33-55
• /
• 1993

선수충격파의 문제를 푸는데 있어서 Boundary Integral Method(BIM)의 여러가지 수치 해석방법이 검토되었으며, 특히 여러가지 Time stepping scheme, Green function, far-field 조건등에 따른 수치해석안정성과 정확성의 상관관계가 연구되었다. von Neumann 안정성해석과 matrix 안정성해석 등을 이용한 선형 안정성해석을 기초로하여, 수치해석방법의 안정성 여부를 체계적으로 조사할 수 있는 parameter(Free Surface Stability number)를 설정하고, 이 parameter의 변화에 따른 비선형 운동해석을 연구하였다. 그 결과 비선형성이 심하지 않은 기진파의 경우에서는 비선형 운동해석의 수치해석 안정성의 선형 수치해석 안정성과 큰 차이가 없음을 알 수 있게 된다.

• KSCE Journal of Civil and Environmental Engineering Research
• /
• v.29 no.5B
• /
• pp.429-439
• /
• 2009

When Catastrophic extreme flood occurs due to dam break, the response time for flood warning is much shorter than for natural floods. Numerical models can be powerful tools to predict behaviors in flood wave propagation and to provide the information about the flooded area, wave front arrival time and water depth and so on. But flood wave propagation due to dam break can be a process of difficult mathematical characterization since the flood wave includes discontinuous flow and dry bed propagation. Nevertheless, a lot of numerical models using finite volume method have been recently developed to simulate flood inundation due to dam break. As Finite volume methods are based on the integral form of the conservation equations, finite volume model can easily capture discontinuous flows and shock wave. In this study the numerical model using Riemann approximate solvers and finite volume method applied to the conservative form for two-dimensional shallow water equation was developed. The MUSCL scheme with surface gradient method for reconstruction of conservation variables in continuity and momentum equations is used in the predictor-corrector procedure and the scheme is second order accurate both in space and time. The developed finite volume model is applied to 2D partial dam break flows and dam break flows with triangular bump and validated by comparing numerical solution with laboratory measurements data and other researcher's data.