• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.41 no.7
• /
• pp.481-487
• /
• 2017

The interaction between a shock wave and a boundary layer causes boundary layer separation, shock train, and in some cases, strong unsteadiness in the flow field. Such a situation is also observed in a shock tube, where the reflected shock wave interacts with the unsteady boundary layer. However, only a few studies have been conducted to investigate the shock train phenomenon in a shock tube. In the present study, numerical studies were conducted using the two-dimensional axisymmetric domain of a shock tube, and compressible Navier-Stokes equations were solved to clarify the flow characteristics of shock train phenomenon inside a shock tube. A detailed wave diagram was developed based on the present computational results, which were validated with existing experimental data.

• The Journal of Engineering Geology
• /
• v.11 no.2
• /
• pp.131-139
• /
• 2001

Until recently, limestone yield production is mainly depend on geological investigation and boring. In the study seismic and electrical method are applied to calculate the basic data of limestone yield production. the result of geophysical prospecting, the depth of bed rock is approximately 17m. And there is a slightly difference between the limestone layer boundary which is drawn by electrical prospecting method and that of geological investigation.

• Proceedings of the Korean Society of Coastal and Ocean Engineers Conference
• /
• 1993.07a
• /
• pp.29-33
• /
• 1993

이제까지 천해역에서의 파랑변형을 계산하는 여러가지 수치모형이 제안되어 있다. 그 가운데 Berkhoff(1972)가 유도한 완경사방정식을 수치계산이 쉽고, 쇄파감쇠 및 반사의 고려가 용이한 형태로 개량한 환산·경도(1985)의 시간의존 쌍곡선형 완경사방정식은 널리 이용되고 있다. 계산대상영역에 파가 비스듬하게 입사하는 경우, 외해측 경계뿐만 아니라, 파가 입사하는 측의 측방경계도 입사경계가 될 수 있다. (중략)

• Proceedings of the Optical Society of Korea Conference
• /
• 2006.07a
• /
• pp.83-84
• /
• 2006

• The Journal of the Acoustical Society of Korea
• /
• v.25 no.5
• /
• pp.229-238
• /
• 2006

This paper shows the analytic error caused by the inconsistency of the approximation order between the non local boundary condition (NLBC) and the parabolic governing equation. To obtain the analytic error, we first transform the NLBC to the half space domain using plane wave analysis. Then, the analytic error is derived on the boundary between the true numerical domain and the half space domain equivalent to the NLBC. The derived analytic error is physically expressed as the artificial reflection. We examine the characteristic of the analytic error for the grazing angle, the approximation order of the PE or the NLBC. Our main contribution is to present the analytic method of error estimation and the application limit for the high order parabolic equation and the NLBC.

• Proceedings of the Korean Society of Coastal and Ocean Engineers Conference
• /
• 1991.07a
• /
• pp.30-35
• /
• 1991

해저가 완만하게 변하는 해역 위를 지나는 파랑의 반사는 무시할 수 있으며 이에 대한 파랑계산 방법으로는 굴절 모델 (서 등, 1989)과 포물형 근사식 모델(서, 1990b)을 들 수 있다. 그러나 해저의 변화가 심해 상당량의 파랑에너지가 반사되는 경우에는 파랑 운동을 지배하는 원시 방정식인 Laplace을 사용하여야 한다.(중략)

• Journal of Korean Society for Geospatial Information Science
• /
• v.22 no.4
• /
• pp.91-97
• /
• 2014

In this research, we would try to evaluate the applicability in cadastral resurvey surveying by analysing the accuracy of building boundary surveying using the reflective sheet total station surveying. When we analyse it, we refer the reflective sheet which can supplement not only the difficulties of total station surveying and GPS surveying caused by the diversity of the building structure but also the errors of non prism total station caused by material of the object. Each reflected angles $90^{\circ}$ and $60^{\circ}$, $30^{\circ}$ of RMSE results were analyzed by RMSE between 1.2mm~2.8mm and 2.2mm~4.0mm, 2.5mm~4.4mm for each distance. The result of X RMSE was analyzed to be 0.0043m in a boundary surveying for existing building between prism surveying and reflective sheet surveying, and also Y RMSE was 0.038m. The source of error is estimated that the body of the prism can not be exactly attached to the edge of a building. Therefore, it will be very helpful to use a reflective sheet surveying with a prism in both the limit of collimation and error reductions as a building boundary measurement in cadastral resurvey surveying.

• Geophysics and Geophysical Exploration
• /
• v.19 no.2
• /
• pp.76-83
• /
• 2016

Migration image can be distorted due to reflected waves in the source and receiver wavefields when discontinuities of input velocity model exist in seismic imaging. To remove reflected waves coming from layer interfaces, it is a common practice to smooth the velocity model for migration. If the velocity model is smoothed, however, the subsurface image can be distorted because the velocity changes around interfaces. In this paper, we attempt to minimize the distortion by reducing reflection energy in the source and receiver wavefields through acoustic impedance homogenization. To make acoustic impedance constant, we define fake density model and use it for migration. When the acoustic impedance is constant over all layers, the reflection coefficient at normal incidence becomes zero and the minimized reflection energy results in the improvement of migration result. To verify our algorithm, we implement the reverse-time migration using cell-based finite-difference method. Through numerical examples, we can note that the migration image is improved at the layer interfaces with high velocity contrast, and it shows the marked improvement particularly in the shallow part.

• 한국지구물리탐사학회:학술대회논문집
• /
• 2008.10a
• /
• pp.57-62
• /
• 2008

Finite difference method using not general SSG(standard staggered grid) but RSG(rotated staggered grid) was applied to simulation of elastic wave propagation. Special free surface boundary condition such as imaging method is needed in finite difference method using SSG in elastic wave propagation but free surface boundary condition in finite difference method using RSG is easily solved with adding air layer. Recently PML(Perfectly Matched layer) is widely used to eliminate artificial reflection waves from finite boundary because of its' greate efficiency. Absorbing ability of CPML(convolutional Perfectly Matched Layer) that is more efficient than that of PML was applied to FDM using RSG in this study. The results of CPML eliminated artificial boundary waves very effectively in FDM using RSG in being compared with that of Cerjan's absorbing method.

• Korean Journal of Optics and Photonics
• /
• v.31 no.1
• /
• pp.20-25
• /
• 2020

To design a wide-field two-mirror system, the system must feature an off-axial field, to avoid ray obstruction and field screening by the secondary mirror. The off-axial aberrations of the system cannot be corrected sufficiently, though, because there are only a few design parameters. The present study designs an improved off-axial-field two-mirror system using a decentered aperture stop.