• 대한기계학회논문집
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• 제18권2호
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• pp.446-455
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• 1994

An experimental study is conducted for the turbulent recirculating flow behind a backward-facing step when the oscillating jet is issued sinusoidally through a thin slit at the separation edge. Two key parameters are dealt with in the present experiment, i.e., the amplitude of forcing and the forcing frequency. The Reynolds number based on the step height is varied from 25,000 to 35,000. In order to investigate the effect of local forcing, turbulent structures are scrutinized for both the flow of forcing and the flow of no forcing. The growth of shear layer with a local forcing is larger than that of no forcing. The influence of a local forcing brings forth the decrease of reattachment length and the particular frequency gives a minimum reattachment length. The most effective frequency depends on the non-dimensional frequency, St/sub .theta./, based on the momentum thickness at the separation point. A comparative study leads to the conclusion that the large-scale vortical structure is strongly associated with the forcing frequency and the natural flow instability.

• International Journal of Aeronautical and Space Sciences
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• 제16권3호
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• pp.325-338
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• 2015

Experimental and numerical investigations were conducted to identify the wake characteristics downstream of two vane-type vortex generators over laminar flat plate boundary layer. Experimental study was carried out by using the stereoscopic particle image velocimetry. To describe the flow field around the vortex generator in detail, numerical study was performed. We considered two different planform shapes of vortex generator: triangular and rectangular shape. The height of the generator was chosen to be about the boundary layer thickness at the position of its installation. Two different lengths of the generator were chosen: two and five times the height. Wake measurements were carried out at three angles of attack for each configuration. Wake characteristics for each case such as overall vortical structure, vorticity distribution, and location of vortex center with downstream distance were obtained from the PIV data. Wake characteristics, as expected, were found to vary strongly with the geometry and angle of attack so that no general tendency could be deduced. Causes of this irregular tendency were explained by using the results of the numerical simulation.

• 한국소음진동공학회논문집
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• 제14권10호
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• pp.923-929
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• 2004

In this research, Forced Vibration Test(FVT) on a cable stayed bridge was conducted to examine the validity of the frequency domain pattern recognition method using signal anomaly index and artificial neuralnetwork. 7he considering structure, Samchunpo Bridge, located in Sachun-Shi, Kyungsangnam-Do, is a cable stayed bridge with the 436 meter span. The excitation force was induced by a sudden braking of a fully loaded truck. and vortical acceleration signals were acquired at 14 points. The initial 2-dimensional FE-model was developed from the design documents to prepare the training sets for the artificial neural network, and then the model calibration was performed with the field test data. As a result of the model calibration, we obtained the FFT spectrums from the model simulation, which was similar to those from the vibration test. These tests and the simulation data will be used for the structural identification using arbitrarily added masses to the bridge.

• 대한조선학회논문집
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• 제39권3호
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• pp.41-47
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• 2002

선박용 프로펠러 후류의 유동 특성을 적응형 하이브리드 2-frame PTV(Particle Tracking Velocimetry)기법을 적용하여 실험적으로 해석하였다. 프로펠러 위상각에 대해 위상평균하여 하류로 나아감에 따른 후류 유동의 발달과정을 연구하였으며, 주 유동 방향으로 날개의 뒷날로부터 프로펠러 직경만큼의 거리까지를 측정하였다. 하나의 날개에 대해 4개의 다른 위상각 각각에서 얻은 위상평균 속도장 결과는 프로펠러 날개의 압력 차이로 인해 발생하는 주기적인 날개끝 보오텍스가 하류로 이동해 나감을 보여주고 있다. 또한, 프로펠러 날개 표면을 따라 발달하는 경계층에 기인한 점성 후류는 축방향 속도성분의 결손을 가진다. 프로펠러 날개 뒷날에서 발생하는 후연 보오텍스는 하류로 나아감에 따라 수축되며 점성 소산으로 인해 그 세기 및 크기는 점차 작아졌다.

• 한국해안해양공학회지
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• 제5권3호
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• pp.198-203
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• 1993

유공방파제의 반사특성을 규명하기 위하여 파고 및 주기가 다른 여러 규칙파를 대상으로 유수실의 폭과 유공벽의 형태를 변화시키며 수리실험을 수행하였다. 실험결과, 유공방파제의 반사특성은 유수실 폭과 유수실내 파장의 비(B/L$_{c}$)에 민감하게 반응하며, B/L$_{c}$가 0.2인 부근에서 최소의 반사율을 보임을 알 수 있었다. 이러한 현상은 주로 유수실내에서의 공진현상에 기인한 것으로 판단된다. 유공벽형태(종슬리트, 횡슬리트 또는 원형 구멍)의 변화는 반사율면에 있어서 약간의 변화를 초래할 수는 있어도, 전반적인 반사특성 변화에는 영향을 주지 못함을 확인하였다.하였다.

• 한국입자에어로졸학회지
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• 제13권4호
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• pp.173-182
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• 2017

An Eulerian-Lagrangin approach is used to compute particle dispersion from a power plant chimney. For air flow, three-dimensional incompressible filtered Navier-Stokes equations are solved with a subgrid-scale model by integrating the Newton's equation, while the dispersed phase is solved in a Lagrangian framework. The velocity ratios between crossflow and a jet of 0.455 and 0.727 are considered. Flow fields and particle distribution of both cases are evaluated and compared. When the velocity ratio is 0.455, it demonstrates a Kelvin-Helmholtz vortex structure above the chimney caused by the interaction between crossflow and a jet, whereas the other case shows flow structures at the top of the chimney collapsed by fast crossflow. Also, complex wake structures cause different particle distributions behind the chimney. The case with the velocity ratio of 0.727 demonstrates strong particle concentration at the vortical region, whereas the case with the velocity ratio of 0.455 shows more dispersive particle distribution. The simulation result shows similar tendency to the experimental result.

• 대한조선학회논문집
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• 제45권6호
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• pp.620-630
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• 2008

The characteristics of complicated propeller wake influenced by hull wake are investigated by using a two-frame PIV (Particle Image Velocimetry) technique. As the propeller is significantly affected by the hull wake in a real marine vessel, the measurements of propeller wake under the hull wake would be certainly necessary for more reliable validation and the prediction of numerical simulation with wake modeling. Velocity field measurements have been conducted in a medium-size cavitation tunnel with a hull wake. Generally, the hull wake generated by the boundary layer of ship's hull produces the different loading distribution on the propeller blade in both upper and lower propeller planes. The difference of the propeller wake behaviors caused by the hull wake is discussed in terms of axial velocity, vorticity and turbulence kinetic energy distribution in the present study.

• 항공우주기술
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• 제12권1호
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• pp.207-212
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• 2013

스월 제트 전단층 제어에 관한 인공적 방법의 가능성을 연구하기 위해 전단 자극장치가 제안되었다. 주요 구성품으로는 두개의 동심 아음속 노즐 그리고 유동 자극 생성을 위한 내부 돌기 등이다. 돌기의 숫자는 방위각 방향의 다양한 모드를 유도 할 수 있도록 변화가 가능하다. 획득된 속도 결과는 평면 contour plot과 공간 입자 궤적으로 표현하였다. 다양한 자극 모드에서의 결과 값들이 기준 값(무자극)과 비교되었다. 결과적으로, 새로 고안된 기법이 스월 유동 와류 구조 제어에 있어 효과적인 방법임이 입증되었다.

• 한국해양공학회:학술대회논문집
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• 한국해양공학회 2003년도 춘계학술대회 논문집
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• pp.49-55
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• 2003

When a body with slant angle after its shoulder is moving at high speed, the turbulent motion around the afterbody is generally associated with the flaw separation and determines the normal component of the drag. By changing the slant angle of afterbody, there exists a critical angle at which the drag coefficients change drastically. Understanding and control of the turbulent separated flows are of significant importance for the design of optimal configuration of the moving bodies. In the present paper, a new Large Eddy Simulation technique has been developed to investigate turbulent vortical motions around the afterbodies with slant angle. By basis of understanding the structure of turbulent flaw around the body, the new configuration of afterbodies are designed to reduce the drag of body and the nonlinear effects due to the interaction between the body configuration and the turbulent separated flows are investigated by use of the developed LES technique.

• Journal of Advanced Marine Engineering and Technology
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• 제29권4호
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• pp.399-406
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• 2005

Alloy 625 is used widely in industrial applications such as aeronautical aerospace, chemical, petrochemical and marine applications. Because of a good combination of yield strength. tensile strength, creep strength, excellent fabricability, weldability and good resistance to high temperature corrosion on prolonged exposure to aggressive environments. High qualify weldments for this material are readily produced by commonly used processes. But all of processes are not applicable to this material by reason of unavailability of matching, position or suitable welding filler metals and fluxes may limit the choice of welding processes. Recently, the flux cored wire is developed and applied for the better productivity in several welding position including the vortical position. In this study. the weldability and weldment characteristics of Alloy 625 are evaluated in FCAW weld associated with the several shielding gases($80\%Ar+20\%\;CO_2,\;50\%Ar+50\%\;CO_2.\;100\%\;CO_2$) in viewpoint of welding productivity. The results of the experimental study on corrosive characteristics of Alloy 625 are as follows; There is no remarkable difference among shielding gases. however they has a striking difference among corrosive solutions by results of distinguished density and time of corrosive solution. Generally, the shielding gases($80\%Ar+20\%\;CO_2$) was superior to the other gases on high temperature tensile and a low temperature impact. but all of the shield gases were making satisfactory results on corrosion test.