• KSCE Journal of Civil and Environmental Engineering Research
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• v.33 no.6
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• pp.2267-2275
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• 2013

Existing conventional model for analysis of shallow water flow just assumed the internal boundary condition as free-slip, which resulted in the wrong prediction about the velocity, vorticity, water level, shear stress distribution, and time variation of drag and lift force around a structure. In this study, a finite element model that can predict flow characteristics around the structure accurately was developed and internal boundary conditions were generalized as partial slip condition using slip length concept. Laminar flow characteristics behind circular cylinder were analyzed by varying the internal boundary conditions. The simulation results of (1) time variations of longitudinal and transverse velocities, and vorticity; (2) wake length; (3) vortex shedding phenomena by slip length; (4) and mass conservation showed that the vortex shedding had never observed and laminar flow like creeping motion was occurred under free-slip condition. Assignment of partial slip condition changed the velocity distribution on the cylinder surface and influenced the magnitude of the shear stress and the occurrence of vorticity so that the period of vortex shedding was reduced compared with the case of no slip condition. The maximum mass conservation error occurred in the case of no slip condition, which had the value of 0.73%, and there was 0.21 % reduction in the maximum mass conservation error by changing the internal boundary condition from no slip to partial slip condition.

• Journal of Advanced Marine Engineering and Technology
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• v.34 no.1
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• pp.76-83
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• 2010

The Characteristics of the flow field through cylinders with in-line and staggered arrangements were calculated by vortex method. Vortex distributions and velocity profiles around the cylinders with in-line and staggered arrangements were simulated at the pitch ratio of Pt/D=1.25~2.0 and Reynolds number of Re=$4.0{\times}10^1{\sim}4.0{\times}10^4$. As the results the vortices of clockwise at the upper separation point cylinder and the vortices of anticlockwise at the lower separation point of each cylinder were generated at both in-line and staggered arrangements. The generation of the reverse flow in the rear region of the cylinders was caused by the pitch ratio and Reynolds number, the boundary region was at the pitch ratio of Pt/D=1.5 and Reynolds number of Re=$4.0{\times}10^2{\sim}4.0{\times}10^3$ in case of in-line arrangement and was at the pitch ratio of Pt/D=1.4 and Reynolds number of Re=$4.0{\times}10^1{\sim}4.0{\times}10^2$ in case of staggered arrangement.

• Wind and Structures
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• v.22 no.2
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• pp.253-272
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• 2016

The paper concerns with the method and results of wind tunnel investigations of the Strouhal number (St) of a stationary iced cable model of cable-supported bridges with respect to different angles of wind attack. The investigations were conducted in the Climatic Wind Tunnel Laboratory of the Czech Academy of Sciences in $Tel{\check{c}}$. The methodology leading to the experimental icing of the inclined cable model was prepared in a climatic section of the laboratory. The shape of the ice on the cable was registered by a photogrammetry method. A section of an iced cable model with a smaller scale was reproduced with a 3D printing procedure for subsequent aerodynamic investigations. The St values were determined within the range of the Reynolds number (Re) between $2.4{\cdot}10^4$ and $16.5{\cdot}10^4$, based on the dominant vortex shedding frequencies measured in the wake of the model. The model was oriented at three principal angles of wind attack for each of selected Re values. The flow regimes were distinguished for each model configuration. In order to recognize the tunnel blockage effect the St of a circular smooth cylinder was also tested. Good agreement with the reported values in the subcritical Re range of a circular cylinder was obtained. The knowledge of the flow regimes of the airflow around an iced cable and the associated St values could constitute a basis to formulate a mathematical description of the vortex-induced force acting on the iced cable of a cable-supported bridge and could allow predicting the cable response due to the vortex excitation phenomenon.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.33 no.3
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• pp.1-9
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• 2005

The interaction between hypersonic free stream and side jet flow at high altitudes is investigated by using the direct simulation Monte Carlo (DSMC) method. In order to alleviate the difficulty associated with the large density difference between the free stream and the side jet flow and to simulate the two flows simultaneously, a weighting factor technique is applied. For validation, the corner flow over a pair of plates perpendicularly attached is calculated with and without a side jet, and the results are compared with experiment. For a more realistic configuration, the flow past a blunted cone cylinder shape is solved. The leeward or windward jet is injected into the free stream and the effect on the aerodynamic force and moment is observed at various flow angles. The lambda shock effect and the wake structure are studied in terms of the surface pressure differential. A higher interaction between the free stream and the side jet flow is observed when the side jet is injected in the windward direction.

• 한국전산유체공학회:학술대회논문집
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• 2004.03a
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• pp.126-131
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• 2004

The interaction between the hypersonic free stream and the side jet flow at high altitudes is investigated by direct simulation Monte Carlo(DSMC) method. Since there is a great difference in density between the free stream and the side jet flow, the weighting factor technique which could control the number of simulation particles, is applied to calculate these two flows simultaneously. Chemical reactions are not considered in the calculation. For validation, the corner flow passing between a pair of plates that are perpendicularly attached is solved. The side jet flow is then injected into this comer flow and solution is found for the merged flow. Results are compared with the experiments. For a more realistic rocket model, the flow past a blunted cone cylinder shape is solved. The leeward or windward jet injection is merged with this flow. The effect on the rocket surface is observed at various flow angles. The lambda effect and the wake structure are found like low attitudes. High interaction between the free stream and the side jet flow is observed when the side jet is injected in the windward direction.

• Transactions of the Korean Society of Mechanical Engineers
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• v.19 no.5
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• pp.1300-1307
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• 1995

The effect of thermal stratification on the turbulent dispersion from a fine cylindrical heat source was experimentally examined in a wind tunnel with and without a strong temperature gradient. A 0.5 mm dia. nichrome wire was used as a line heat source. Turbulent intensities, r.m.s. value of temperature and convective heat fluxes were measured by using a hot-wire and cold-wire combination probe. The results show that the peack value and the spread of the vertical turbulent intensity for the stratified case are far lower than those in the neutral case, which indicates that the stable temperature gradient suppresses the vertical velocity component. All of the third order moments including heat fluxes measured in the stable condition have very small values than those of the neutral case. This nature suggests that the decrease of scalar fluctuations in the stably stratified flow is mainly due to the suppression ofthe turbulent diffusion processes by the stable stratification. A simple gradient model with a composite timescale which has a simple weighted algebraic mean between dynamic and thermal time scale yields reasonably good numerical values in comparison with the experimental data.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.22 no.9
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• pp.1335-1348
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• 1998

Flow characteristics in transitional boundary layers on a flat plate were experimentally investigated under three different freestream conditions i. e. uniform flow with 0.1 % and 3.7% freestream turbulent intensity and cylinder-wake with 3.7% maximum turbulent intensity. Instantaneous streamwise velocities in laminar, transitional and turbulent boundary layers were measured by I-type hot-wire probe. For estimation of wall shear stresses on the flat plate, measured mean velocities near the wall were applied to the principle of Computational Preston Tube Method (CPM). Distributions of skin friction coefficients were reasonably predicted in all developed boundary layers. Intermittency profiles, which were estimated using Conditional Sampling Technique in transitional boundary layers, were also consistent with previously published data. It was predicted that the incoming turbulent intensity had more influence on transition onset point and transition process than freestream turbulent intensity existed just over the transition region. It was also confirmed that non-turbulent and turbulent profiles in transitional boundary layers could not be simply treated as Blasius and fully turbulent profiles.

• Proceedings of the Korea Water Resources Association Conference
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• 2022.05a
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• pp.256-256
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• 2022

레이놀즈 분해법은 유속을 비롯한 변수를 평균 성분과 변동 성분으로 분해하는 분석 방법으로, 난류 분석의 기본이 되는 방법이다. 그러나 유체 내에 장애물이 존재할 경우, 흐름에 큰 와류가 존재하여 난류 변동 성분과 구분되는 고유 구조가 형성되는데, 이러한 경우에 레이놀즈 분해법을 적용하면 고유 구조의 변동 성분이 난류로 처리되어 난류 강도가 과다하게 책정될 수 있다는 한계점이 있다. 이에 대한 대안으로 제안된 것이, 변수를 평균 성분, 파동 성분, 변동 성분으로 분해하는 삼중 분해법이다. 삼중 분해법은 흐름 내의 고유 구조를 추출하는 것을 가능하게 하여 다양한 연구에서 사용되어왔다. 삼중 분해법을 구현하기 위해 이용되는 방법론 중 하나로, 공분산 행렬을 이용하여 유속장을 분해하는 방법인 적합 직교 분해법이 많이 사용된다. 본 연구에서는 원기둥 후류에 적합 직교 분해법을 사용하여 삼중 분해법을 시행하고, 후류의 흐름 구조를 분석하는 것을 목표로 하였다. 영상 유속계를 사용하여 실험을 통해 원기둥 후류의 수평 유속장을 측정하였고, 측정 자료에 적합 직교 분해법을 적용한 결과, 첫 두 모드에서 큰 규모의 와류가 파동 형태로 전파되는 것이 관찰되어 고유 구조의 존재를 확인할 수 있었다. 해당 성분을 삼중 분해법의 파동 성분으로 상정하였고, 푸리에 분석을 적용한 결과에서도 원기둥 후류의 고유 진동수가 뚜렷하게 나타나는 것을 확인하였다. 또한, 원기둥 후류의 에너지 전달 구조를 확인하기 위하여 에너지 방정식에 삼중 분해법을 적용하여 식을 유도하고, 실험 자료로부터 각 항을 계산하여 비교해보았다.

• Journal of the Korean Society of Marine Environment & Safety
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• v.26 no.4
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• pp.419-428
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• 2020

The flow past a wall mounted square cylinder, a typical and basic shape of building, bridge or offshore structure, was simulated using URANS computation through adoption of three turbulence models, namely, the k-ε model, k-ω model, and the v²-f model. It is well known that this flow is naturally unstable due to the Karman vortex shedding and exhibits a complex flow structure in the wake region. The mean flow field including velocity profiles and the dominant frequency of flow oscillation that was from the simulations discussed earlier were compared with the experimental data observed by Wang et al. (2004; 2006). Based on these comparisons it was found that the v²-f model is most accurate for the URANS simulation; moreover, the k-ω model is also acceptable. However, the k-ε model was found to be unsuitable in this case. Therefore, v²-f model is proved to be an excellent choice for the analysis of flow with massive separation. Therefore, it is expected to be used in future by studies aiming to control the flow separation.

• Korean Journal of Fisheries and Aquatic Sciences
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• v.33 no.6
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• pp.514-519
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• 2000

In order to develop the method for the calculation of flow resistance acting on cage net, the relation between the velocity reduction factor and $S_n/S$, the ratio of total area of netting projected to the perpendicular to the water flow $S_n$ to wall area of netting S, was derived based on the numerical and experimental analysis of the wake flow through a netting twine simplified by a cylinder and a netting panel. The velocity was reduced in accordance with the velocity reduction factor when the flow passed the netting panel upstream of a cage net. The proposed method for the calculation of fluid force acting on a square cage net was based upon the assumption that it could be divided into four side panels and one bottom panel. It was proved that the force could be calculated by the sum of the drag forces acting on the individual netting panels.