• Journal of Mechanical Science and Technology
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• v.15 no.7
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• pp.941-950
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• 2001

A study of counter-current two-phase flow in narrow rectangular channels has been performed. Two-phase flow regimes were experimentally investigated in a 760mm long and 100mm wide test section with 2.0 and 5.0mm gap widths. The resulting flow regime maps were compared with the existing transition criteria. The experimental data and the transition criteria of the models showed relatively good agreement. However, the discrepancies between the experimental data and the model predictions of the flow regime transition become pronounced as the gap width increased. As the gap width increased the transition gas superficial velocities increased. The critical void fraction for the bubbly-to-slug transition was observed to be about 0.25. The two-phase distribution parameter for the slug flow was larger for the narrower channel. The uncertainties in the distribution parameter could lead to a disagreement in slug-to-churn transition between the experimental findings and the transition criteria. For the transition from churn to annular flow the effect of liquid superficial velocity was found to be insignificant.

• Proceedings of the KSME Conference
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• 2000.11b
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• pp.136-141
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• 2000

A study of counter-current two-phase flow in narrow rectangular channels has been performed. Two-phase flow regimes were experimentally studied in 760 mm long and 100 mm wide test sections with 2.0 and 3.0mm gaps. The resulting data have been compared to previous transition models. For the transition from bubbly to slug flow the superficial velocity of gas increased as the gap width increased. The comparison of experimental data to the transition model developed by Taitel and Barnea showed relatively good agreement for the bubbly-to-slug transition in the case of 2mm gap width. For the criteria of Mishima and Ishii to be applicable to the slug-to-churn transition the distribution parameter should be well defined for narrow channels. Even though the gap width of narrow channels increased the superficial gas velocity did not change for the transition form chum to annular flow regime. For the chum-to-annular transition the model of Taitel and Barnea showed discrepancies with experimental data, especially in the channel with larger gap.

• 한국전산유체공학회:학술대회논문집
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• 2004.03a
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• pp.23-26
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• 2004

Drag prediction is sought for the aifoil having laminar and turbulent flow characteristics with CFD code being unable to predict transition to turbulent flow. Laminar flow simulation presents some insight to the transition position. Separate simulations with laminar and turbulent flow and their combination estimate the drag of the airfoil containing laminar and turbulent flow characteristics.

• Wind and Structures
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• v.20 no.1
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• pp.15-36
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• 2015

Characterization of wind flows over a complex terrain, especially mountain-gorge terrain (referred to as the very complex terrain with rolling mountains and deep narrow gorges), is an important issue for design and operation of long-span bridges constructed in this area. In both wind tunnel testing and numerical simulation, a transition section is often used to connect the wind tunnel floor or computational domain bottom and the boundary top of the terrain model in order to generate a smooth flow transition over the edge of the terrain model. Although the transition section plays an important role in simulation of wind field over complex terrain, an appropriate shape needs investigation. In this study, two principles for selecting an appropriate shape of boundary transition section were proposed, and a theoretical curve serving for the mountain-gorge terrain model was derived based on potential flow theory around a circular cylinder. Then a two-dimensional (2-D) simulation was used to compare the flow transition performance between the proposed curved transition section and the traditional ramp transition section in a wind tunnel. Furthermore, the wind velocity field induced by the curved transition section with an equivalent slope of $30^{\circ}$ was investigated in detail, and a parameter called the 'velocity stability factor' was defined; an analytical model for predicting the velocity stability factor was also proposed. The results show that the proposed curved transition section has a better flow transition performance compared with the traditional ramp transition section. The proposed analytical model can also adequately predict the velocity stability factor of the wind field.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.13 no.5
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• pp.165-172
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• 1993

On the basis of Nikuradse laboratory experiments conducted in circular pipe with uniform roughness, five flow regimes are defined with respect to the characteristics of boundary layer such as laminar, transition laminar, smooth turbulent, transition turbulent and rough turbulent flows. Two cases are found for the transition laminar flow: one for the transition between laminar flow and smooth turbulent flow and the other for the one between laminar flow and rough turbulent flow. They all can be clearly determined by the relative roughness or the ratio of pipe diameter to the roughness. Explicit functions are developed for the estimation of pipe friction factor for the various flow conditions including turbulent flow regimes, which have excellent agreement with the Nikuradse laboratory data.

• Journal of computational fluids engineering
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• v.5 no.3
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• pp.40-46
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• 2000

Natural convection in a wide-gap horizontal annulus is considered, and the transition of flows from steady to oscillatory convection is investigated for the fluid with Pr=0.1. The unsteady streamfunction-vorticity equation is solved with finite difference method. As Rayleigh number is increased, the steady crescent-shaped flow bifurcates to a time-periodic flow with like-rotating eddies. And afterwards, a transition to an oscillatory multicellular flow with a counter-rotating eddy on the top of the annulus occurs. A transition from steady to an oscillatory flow occurs, but dual solutions and hysteresis phenomena are not observed.

• Water for future
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• v.27 no.2
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• pp.85-96
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• 1994

Numerical instability of Preissmann scheme is studied for unsteady flow analysis in a natural river. The solution strategies to overcome the instability problems are presented in this paper. The main causes of numerical instability of Preissmann scheme are transition flow, abrupt change in cross section, in-appropriate roughness coefficients, time step and distance step, rapidly rising hydrograph, dry bed and so on. Transition flow model is proposed for the analysis of the transition flow which changes from subcritical to supercritical or conversely. The subcritical and supercritical reaches are groped in the channel, then appropriate boundary conditions are introduced for each reach. The transition flow analysis produces stable solutions in calculating through the various transition conditions. Verification with an actual river system is necessary in the future.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.24 no.2
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• pp.169-176
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• 2000

Natural convection in a wide-gap horizontal annulus is considered, and the transition of flows and the bifurcation phenomenon are investigated for the fluids with Pr=0.2 and 0.3. At Pr=0.2, a bicellular flow pattern is observed at high Rayleigh number, and the solution is unique. At Pr=0.3, both the steady unicellular and bicellular flows exist above a certain critical Rayleigh number. For the fluids of Pr=0.2, the bicellular flow can be obtained by the impulsive heating of the inner cylinder, but it is not obtained from the zero initial condition for Pr=0.3. Hysteresis phenomena have not been observed. A transition from a bicellular flow to a unicellular flow occurs for Pr=0.3.

• Journal of Korean Society for Atmospheric Environment
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• v.21 no.5
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• pp.525-535
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• 2005

Using a three-dimensional computational fluid dynamics (CFD) model with the $k-{\varepsilon}$ turbulence closure scheme based on the renormalization group theory, flow regimes in urban street canyons are classified according to the building and street aspect ratios. The transition between skimming flow (SF) and wake interference flow (WIF) is determined with the size of double-eddy circulation generated behind the upwind building. The transition between WIF and isolated roughness flow (IRF) is determined with the flow reattachment distance from the upwind building. The critical aspect ratios at which the flow transition occurs are found and compared with those in previous studies. The results show that the flow-regime classification method used in this study is quite reasonable and that the values of the critical aspect ratios are generally consistent with those in fluid experiments or large-eddy simulation. The regression equation describing a relation between the building and street aspect ratios at the flow-regime transition is presented.

• Proceedings of the Korea Water Resources Association Conference
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• 2022.05a
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• pp.155-155
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• 2022

Low head, ogee spillways is popularly used to defense against floods as well as to provide water for irrigation. Spillway is also used to assess compliance with water quality regulations by controlling amount of discharge to the downstream of a channel. For the purpose of water resource management and/or environmental aspects as explained above, the flow discharge through spillways need to be correctly rated as a function of geometry and hydraulic variables. Typically, four flow conditions are encountered during the operation of spillway: (a) uncontrolled free flow (UF); (b) uncontrolled submerged flow (US); controlled free flow (CF); and controlled submerged flow (CS), and each condition has a unique rating equation. However, one of the tricky part of the spillway operation is finding correct flow type over the spillway because structures can operate under both submerged and free flow conditions, and the types are continuously changing over time depending on the amount of discharge, head water and tail water elevation. Quite obviously, if the wrong rating curve relationship is applied because of misjudgment of the flow type due to a transition, a serious error can occur. Thus, an hydraulic model study of one of spillway structure located in South Florida was conducted for the purpose of developing transition relationships. In this presentation, US to UF transition is highlighted.