• 대한기계학회논문집B
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• 제26권3호
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• pp.394-401
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• 2002

The instantaneous flow characteristics of a round jet issuing normally into a crossflow has been studied using a flow visualization technique and particle image velocimetry. The effects of parameters such as jet inflow profile and turbulence intensity of the jet are evaluated for various Reynolds numbers in range between 735 and 3150, which are based on the crossflow velocity and jet-pipe diameter. The jet-to-crossflow velocity ratio is fixed at the value of 3.3. Instantaneous later tomographic images of the symmetry plane of the crossflow jet show that there exist very different natures in the flow structures of the near-field of the jet even though the velocity ratio is same. It is found that when the turbulence intensity of jet is elevated, the shear layer becomes much thicker due to the strong entrainment of the ambient fluid by turbulent interaction between the jet and crossflow. The detailed characteristics of instantaneous velocity and vorticity fields are presented to illustrate the effects of the above parameters on the vertical structures of the crossflow jet.

• 대한기계학회논문집B
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• 제31권11호
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• pp.895-903
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• 2007

CMBT(Curved Moving Boundary Treatment) is a newly developed scheme for the treatment of a no slip condition on the curved solid wall of moving obstacle in a flow field. In our research CMBT was used to perform LBM simulation of a flow over a moving circular cylinder to determine the flow feature and aerodynamics characteristic of the cylinder. To ascertain the applicability of CMBT on the complex shape of the obstacle, it was first simulated for the case of the flow over a fixed circular cylinder in a channel and the results were compared against the solution of Navier-Stokes equation with deforming mesh technique. The simulations were performed in a moderate range of reynolds number at each moving cylinder to identify the flow feature and aerodynamic characteristics of circular cylinder in a channel. The drag coefficients of the cylinder were calculated from the simulation results. We have numerically confirmed that the critical reynolds number for vortex shedding is ar Re=250 and the result is the same as the case of fixed cylinder. As the cylinder approaching to one wall, the 2nd vortex is developed by interacting with the wall boundary-layer vorticity. As the velocity ratio increase the third vortex are generated by interacting with the 2nd vortexes developed on the upper and lower wall boundary layer. The resultant $C_d$ decrease as reynolds number increasing and the Cd approached to a value when Re>1000.

• 한국수자원학회논문집
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• 제38권5호
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• pp.365-377
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• 2005

본 연구는 수력발전시설에서 물고기의 생존과 상해를 유도하는 흐름현상을 파악하기 위한 진보된 수치해석기법의 개발을 다루고 있다. 원형 젤의 LES를 실시하여 난류젤의 전단지역에 물고기를 방류하는 실험의 결과를 수치적으로 해석하였다. 이 연구에서는 순간 LES 흐름장이 유속, 압력 그리고 와도의 강한 변동으로 특성지울 수 있으며, 이것이 물고기에게 시간평균 정상류보다 상당히 큰 추진력과 모멘트를 발휘함을 보여준다. 이 연구는 아울러 수력터빈 드래프트관에서의 부정류를 RANS/LES의 혼성모형 즉 DES를 이용하여 해석하였으며, 물고기가 드래프트관내에서 방향감각을 상실하거나 과도하게 지체하도록 할 수 있는 난류가 발생함을 보여준다.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2004년도 춘계학술대회
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• pp.1672-1677
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• 2004

Leading edge extension(LEX) in a highly swept shape applied to a delta wing features the modem air-fighters. The LEX vortices generated upon the upper surface of the wing at high angle of attack enhance the lift force of the delta wing by way of increased negative suction pressure over the surfaces. The present 3-D stereo PIV includes the Identification of 2-D cross-correlation equation, stereo matching of 2-D velocity vectors of two cameras, accurate calculation of 3-D velocity vectors by homogeneous coordinate system, removal of error vectors by a statistical method followed by a continuity equation criterion and so on. A delta wing model with or without LEX was immersed in a circulating water channel. Two high-resolution, high-speed digital cameras($1280pixel{\times}1024pixel$) were used to allow the time-resolved animation work. The present dynamic stereo PIV represents the complicated vortex behavior, especially, in terms of time-dependent characteristics of the vortices at given measuring sections. Quantities such as three velocity vector components, vorticity and other flow information can be easily visualized via the 3D time-resolved post-processing to make the easy understanding of the LEX effect or vortex emerging and collapse which are important phenomena occurring in the field of delta wing aerodynamics.

• 한국항해항만학회:학술대회논문집
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• 한국항해항만학회 2009년도 추계학술대회
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• pp.243-244
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• 2009

선박의 표면은 소형선박에서부터 대형까지 매끄럽지 않고 어느 정도의 표면조도(surface roughness)를 가지고 있다. 표면조도는 표면저항과 열전달을 증가시키기 때문에 선박의 설계시 고려해야 될 중요한 설계인자 중 하나이다. 때문에 표면조도에 따른 주위유동에 관한 연구와 조도변화에 따른 유동 및 난류에 대한 연구가 지속적으로 이루어지고 있다. 선박의 표면조도는 선박에서 뿐만 아니라 기계나 항공까지 광범위하게 적용이 가능하며 가용 분야 또한 매우 넓다. 본 연구에서는 레저선박의 표면조도 간격변화에 따른 표면유동에 어떠한 영향을 끼치며 표면조도 영역에 따른 경계층에 대한 실험적 연구를 수행하였다.

• 대기
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• 제31권1호
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• pp.45-60
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• 2021

The synoptic structures and precipitation impact of extratropical cyclones (ETCs) influencing on the three adjacent megacities in East Asia, i.e., Beijing (Beijing ETCs), Seoul (Seoul ETCs) and Tokyo (Tokyo ETCs), are analyzed using ERA-interim reanalysis data from 1979 to 2018. Individual ETC tracks are identified with the automated tracking algorithm applied to 850-hPa relative vorticity field. Among four seasons, ETCs are the most frequent in spring. In this season, Beijing ETCs are mainly generated at the leeside of Altai-Sayan Mountains and primarily develop through interaction between the upper-level trough and lower-level cyclonic circulation. For Seoul ETCs, the leesides of Altai-Sayan Mountains (Seoul-N ETCs) and Tibetan Plateau (Seoul-S ETCs) are main genesis regions and the features of ETCs are different according to the genesis regions. While Seoul-N ETCs mainly develope by the same mechanism of Beijing ETCs, strong diabatic heating due to vapor transport is responsible for the genesis of Seoul-S ETCs. Tokyo ETCs are originated from the leesides of Tibetan Plateau and Kuroshio-Oyashio Extension regions, and strong diabatic heating as well as interaction between upper and lower levels determines the genesis of these ETCs. The precipitation impact resulting from ETCs become strong in the order of Beijing ETCs, Seoul-N ETCs, Seoul-S ETCs, and Tokyo ETCs and accounts for up to 40%, 27%, 52%, and 70% of regional precipitation, respectively.

• 대한토목학회논문집
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• 제28권1B호
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• pp.115-123
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• 2008

본 연구에서는 파 잠제 해빈의 상호간섭에 의한 파동장 및 흐름장의 변화를 검토하기 위하여, 2기의 잠제와 모래해빈을 설치한 경우를 대상으로 허와 이(2007)에 의해 개발된 모델(LES-WASS-3D)을 이용하여 잠제 주변의 3차원적 흐름특성과 파고분포에 대해 고찰하였다. 그 결과, 잠제단부를 중심으로 회전하는 순환류를 확인할 수 있었으며, 여러 단면의 평균류 및 평균와도를 검토함으로서 잠제주변의 3차원 흐름특성을 고찰하였다. 아울러 잠제 천단상의 쇄파점분포, 잠제주변의 파고분포 및 wave set-up에 대해서도 논의하였다.

• 한국해안·해양공학회논문집
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• 제28권3호
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• pp.146-159
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• 2016

본 연구에서는 정상흐름 하에서 스포일러가 부착된 해저파이프라인의 자가매설 기구를 분석하기 위하여 유체역학적 특성을 고정도로 해석할 수 있는 Navier-Stokes Solver(LES-WASS-2D)를 이용하였다. 본 논문에서 적용하는 수치모형의 타당성 및 유효성을 확보하기 위하여 기존의 스포일러 유무에 따른 파이프라인 주변의 흐름특성을 나타낸 수리모형실험결과와 비교 분석하였다. 그리고 입사유속, 스포일러의 제원 및 배치에 따른 파이프라인 주변의 수리특성(유동, 와동, 압력)과 작용력 특성을 수치적으로 분석하였다. 그 결과 1차적으로 해저파이프에 스포일러가 부착된 경우에 투영면적이 증가함으로 인하여 배후로 빠져나가는 유속이 커지고, 동시에 배후에서 발생하는 후류에 기인한 강한 와동이 발생한다. 그리고 2차적으로는 스포일러의 영향으로 상하 비대칭적인 유동 및 와동장이 발생하고, 이로 인해 비대칭적인 압력장이 형성된다. 이것은 파이프에 작용하는 힘의 비대칭성을 증가시켜 하향의 유체력을 크게 발달시킨다. 이와 같은 두 가지의 큰 원인으로 인하여 스포일러 부착형 해저파이프라인이 자가매설 되는 것으로 이해된다.

• 한국대기환경학회지
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• 제26권5호
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• pp.475-489
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• 2010

In order to verify the enhancement of ozone and carbon monoxide (CO) during springtime in East Asia, we investigated weather conditions and data from remote sensors, air quality models, and air quality monitors. These include the geopotential height archived from the final (FNL) meteorological field, the potential vorticity and the wind velocity simulated by the Meteorological Mesoscale Model 5 (MM5), the back trajectory estimated by the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model, the total column amount of ozone and the aerosol index retrieved from the Total Ozone Mapping Spectrometer (TOMS), the total column density of CO retrieved from the Measurement of Pollution in the Troposphere (MOPITT), and the concentration of ozone and CO simulated by the Model for Ozone and Related Chemical Tracers (MOZART). In particular, the total column density of CO, which mightoriginate from the combustion of fossil fuels and the burning of biomass in China, increased in East Asia during spring 2000. In addition, the enhancement of total column amounts of ozone and CO appeared to be associated with both the upper cut-off low near 500 hPa and the frontogenesis of a surface cyclone during a weak Asian dust event. At the same time, high concentrations of ozone and CO on the Earth's surface were shown at the Seoul air quality monitoring site, located at the surface frontogenesis in Korea. It was clear that the ozone was invaded by the downward stretched vortex anomalies, which included the ozone-rich airflow, during movement and development of the cut-off low, and then there was the catalytic photochemical reaction of ozone precursors on the Earth's surface during the day. In addition, air pollutants such as CO and aerosol were tracked along both the cyclone vortex and the strong westerly as shown at the back trajectory in Seoul and Busan, respectively. Consequently, the maxima of ozone and CO between the two areas showed up differently because of the time lag between those gases, including their catalytic photochemical reactions together with the invasion from the upper troposphere, as well as the path of their transport from China during the weak Asian dust event.

• 대기
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• 제22권1호
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• pp.29-45
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• 2012

An unusual autumn storm developed rapidly in the western part of the East sea on the early morning of 23 October 2006. This storm produced a record-breaking heavy rain and strong wind in the northern and middle part of the Yeong-dong region; 24-h rainfall of 304 mm over Gangneung and wind speed exceeding 63.7 m $s^{-1}$ over Sokcho. In this study, MTSAT-1R (Multi-fuctional Transport Satellite) water vapor and infrared channel imagery are examined to find out some features which are dynamically associated with the development of the storm. These features may be the precursor signals of the rapidly developing storm and can be employed for very short range forecast and nowcasting of severe storm. The satellite features are summarized: 1) MTSAT-1R Water Vapor imagery exhibited that distinct dark region develops over the Yellow sea at about 12 hours before the occurrence of maximum rainfall about 1100 KST on 23 October 2006. After then, it changes gradually into dry intrusion. This dark region in the water vapor image is closely related with the positive anomaly in 500 hPa Potential Vorticity field. 2) In the Infrared imagery, low stratus (brightness temperature: $0{\sim}5^{\circ}C$) develops from near Bo-Hai bay and Shanfung peninsula and then dissipates partially on the western coast of Korean peninsula. These features are found at 10~12 hours before the maximum rainfall occurrence, which are associated with the cold and warm advection in the lower troposphere. 3) The IR imagery reveals that two convective cloud cells (brightness temperature below $-50^{\circ}C$) merge each other and after merging it grows up rapidly over the western part of East sea at about 5 hours before the maximum rainfall occurrence. These features remind that there must be the upward flow in the upper troposphere and the low-layer convergence over the same region of East sea. The time of maximum growth of the convective cloud agrees well with the time of the maximum rainfall.