• Transactions of the Korean Society of Mechanical Engineers B
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• v.22 no.7
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• pp.915-926
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• 1998

Velocity fields of near turbulent was behind a porous wind fence were measured using the 2-frame PTV method in a circulating water channel. The fences used in this study had different geometric porosity(.epsilon.) of 0, 20, 40 and 65%. The fence was embedded in a thin laminar boundary layer, i.e., .delta./H ~ = 0.1. Reynolds number based on the fence height H and free stream velocity(U$\_$o/) was about 8,400. As a result, a recirculating flow region was formed behind the fence for the .epsilon.=0% and 20% wind fence. For the wind fences having porosity larger than .epsilon.=40%, it was difficult to see separation bubbles behind the fence. The .epsilon.=20% porous fence reveals the maximum velocity reduction, however, the turbulent intensity and Reynolds shear stress are much greater than those of .epsilon.=40% fence. Among the wind fence tested in this study, the porous wind fence of .epsilon.=40% porosity is the most effective for abating wind erosion.

• Journal of Mechanical Science and Technology
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• v.17 no.4
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• pp.562-570
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• 2003

This experimental study concerns the characteristics of vortex flow in a concentric annulus with a diameter ratio of 0.52, whose outer cylinder is stationary and inner one is rotating. Pressure losses and skin friction coefficients have been measured for fully developed flows of water and of 0.4% aqueous solution of sodium carboxymethyl cellulose (CMC), respectively, when the inner cylinder rotates at the speed of 0～600 rpm. Also, the visualization of vortex flows has been performed to observe the unstable waves. The results of present study reveal the relation of the bulk flow Reynolds number Re and Rossby number Ro with respect to the skin friction coefficients. In somehow, they show the existence of flow instability mechanism. The effect of rotation on the skin friction coefficient is significantly dependent on the flow regime. The change of skin friction coefficient corresponding to the variation of rotating speed is large for the laminar flow regime, whereas it becomes smaller as Re increases for the transitional flow regime and. then, it gradually approach to zero for the turbulent flow regime. Consequently, the critical (bulk flow) Reynolds number Re$\_$c/ decreases as the rotational speed increases. Thus, the rotation of the inner cylinder promotes the onset of transition due to the excitation of Taylor vortices.

• Proceedings of the Korean Geotechical Society Conference
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• 2000.02a
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• pp.93-112
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• 2000

The permeability of converter slag, replacing material of sand mat on improving soft clay foundation, was evaluated in the laboratory. The effects of grain size, flow water time and aging were investigated using sea and fresh water. Converter slag being submerged with fresh water, the coefficients of permeability in A and B samples less than 10mm grain sizes were measured as 6.52${\times}$10$\^$-2/cm/sec and 5.99${\times}$10$\^$-1/cm/sec, while changed as 1.88${\times}$10$\^$-2/cm/sec, 3.86${\times}$10$\^$-1/cm/sec under sea water condition. Also, the condition of turbulent flow may exit and was experimentally identified from the relationship between hydraulic gradient and seepage velocity. After 180 days on using sea water, the coefficients of permeability of sample A and B samples decreased ten times smaller than those initial values. And after that time continually decreased as for till 360 days. The reduction of permeability coefficient was considered to influence filled with voids in high-calcium quicklime(CaO). However, in-situ coefficient of permeability was practically satisfactory.

• Proceedings of the Korean Geotechical Society Conference
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• 2000.02a
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• pp.12-31
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• 2000

The permeability of converter slag, replacing material of sand mat on improving soft clay foundation, was evaluated in the laboratory. The effects of grain size, flow water time and aging were investigated using sea and fresh water. Converter slag being submerged with fresh water, the coefficients of permeability in A and B samples less than 10mm grain sizes were measured as 6.52${\times}$10$\^$-2/cm/sec and 5,99${\times}$10$\^$-1/cm/sec, while changed as 1,88${\times}$10$\^$-2/cm/sec, 3.86${\times}$10$\^$-1/cm/sec under sea water condition. Also, the condition of turbulent flow may exit and was experimentally identified from the relationship between hydraulic gradient and seepage velocity. After 180 days on using sea water, the coefficients of permeability of sample A and B samples decreased ten times smaller than those initial values. And after that time continually decreased as for till 360 days. The reduction of permeability coefficient was considered to influence filled with voids in high-calcium quicklime(CaO). However, in-situ coefficient of permeability was practically satisfactory.

• Journal of Advanced Marine Engineering and Technology
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• v.28 no.6
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• pp.1026-1036
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• 2004

In this study, single-phase heat transfer experiments were conducted with Oblong Shell and Plate heat exchanger using water. An experimental water loop has been developed to measure the single-phase heat transfer coefficient and pressure drop in a vertical Oblong Shell and Plate heat exchanger. Downflow of hot water in one channel receives heat from the cold water upflow of water in the other channel. Similar to the case of a plate heat exchanger, even at a very low Reynolds number, the flow in the Oblong Shell and Plate heat exchanger remains turbulent. The present data show that the heat transfer coefficient and pressure drop increase with the Reynolds number. Based on the present data, empirical correlations of the heat transfer coefficient and pressure drop in terms of Nusselt number and friction factor were proposed. Also, performance prediction analyses for Oblong Shell and Plate heat exchanger were executed and compared with experiments. $\varepsilon$-NTU method was used in this prediction program. Independent variables are flow rates and inlet temperatures. Compared with experimental data, the accuracy of the program is within the error bounds of $\pm$5% in the heat transfer rate.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.29 no.3B
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• pp.247-257
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• 2009

In the present paper, turbulent open-channel flows with alternate vegetated zones are numerically simulated using threedimensional model. The Reynolds-averaged Navier-Stokes Equations are solved with the ${\kappa}-{\varepsilon}$ model. The CFD code developed by Olsen(2004) is used for the present study. For model validation, the partly vegetated channel flows are simulated, and the computed depth-averaged mean velocity and Reynolds stress are compared with measured data in the literature. Comparisons reveal that the present model successfully predicts the mean flow and turbulent structures in vegetated open-channel. However, it is found that the ${\kappa}-{\varepsilon}$ model cannot accurately predict the momentum transfer at the interface between the vegetated zone and the non-vegetated zone. It is because the ${\kappa}-{\varepsilon}$ model is the isotropic turbulence model. Next, the open channel flows with alternate vegetated zones are simulated. The computed mean velocities are compared well with the previously reported measured data. Good agreement between the simulated results and the experimental data was found. Also, the turbulent flows are computed for different densities of vegetation. It is found that the vegetation curves the flow and the meandering flow pattern becomes more obvious with increasing vegetation density. When the vegetation density is 9.97%, the recirculation flows occur at the locations opposite to the vegetation zones. The impacts of vegetation on the flow velocity and the water surface elevation are also investigated.

• Journal of Power System Engineering
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• v.17 no.6
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• pp.95-101
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• 2013

A stereo PIV measurements in a circulating water channel has been performed to investigate the skin friction reduction mechanism of the outer-layer vertical blades first devised by Hutchins. In a recent PIV measurement study, considerable skin friction reduction was achieved as much as 2.73%~7.95% by outer-layer vertical blades array. In the present study, the influence of vertical blades array upon the characteristics of the turbulent coherent structures was analyzed by proper orthogonal decomposition method. It is observed that the vortical structures are cut and deformed by blades array and also the turbulent intensity and the Reynolds stress were weakened by the blades. These phenomena strongly associate the skin-friction drag reduction mechanism in the turbulent boundary layer flow.

• Journal of radiological science and technology
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• v.25 no.1
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• pp.55-62
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• 2002

Aneurysm-mimicking findings were frequently visualized due to hemodynamical causes of dephasing effects around area of A-com artery during magnetic resonance angiography(MRA) and these kind of phenomena have not been clearly known yet. We investigated the hemodynamical patterns of dephasing effect around area of the A-com artery that might be a cause of false intracranial aneurysms on MRA. For experimental study, We used hand-made silicon phantoms of the asymmetric A-com artery as like a bifurcation configuration. In a closed circulatory system with UHDC computer driven cardiac pump system. MRA and fast digital subfraction angiography(DSA) involved the use of these phantoms. Flow patterns were evaluated with axial and coronal imaging of MRA(2D-TOF, 3D-TOF) and DSA of Phantoms constructed from an automated closed-type circulatory system filled with glycerol solution [circulation fluid(glycerol:water = 1:1.4)]. These findings were then compared with those obtained from computational fluid dynamic(CFD) for inter-experimental correlation study. Imaging findings of MRA, DSA and CFD on inflow zone according to the following: a) MRA demonstrated high signal intensity zone as inflow zone on silicon phantom; b) Patterns of DSA were well matched with MRA on trajectory of inflow zone; and c) CFD were well matched with MRA on the pattern of main flow. Imaging findings of MRA. DSA and CFD on turbulent flow zone according to the following: a) MRA demonstrated hyposignal intensity zone at shoulder and axillar zone of main inflow; b) DSA delineated prominent vortex flow at the same area. The hemodynamical causes of signal defect, which could Induce the false aneurysm on MRA, turned out to be dephasing effects at axilla area of bifurcation from turbulent flow as the results of MRA, DSA and CFD.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.29 no.3B
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• pp.239-245
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• 2009

Levees, the hydro-engineering structure, are similar to earth dams in aspects of shape and structure. However, they are different from earth dams in the external force conditions. As a levee is the structure that is complexly affected by the flow and the water stage in the river, it may be unreasonable to analyze the seepage safety as previous studies derived from the neglect of river flow. In this study, an experiment was conducted to investigate flow structures in a trapezoidal open-channel and the influence of the channel flow on the seepage through a levee. Flow structures in a trapezoidal open-channel were distinguished from a rectangular open-channel such as velocity and bottom shear stress distributions. In case with the flow velocity of 0.5 m/s, seepage water heads were higher 10 percent as compared with the stagnant case. This result is caused by dynamic heads, secondary currents, turbulent fluctuation forces, and various physical factors. It is suggested that external force boundary considered in terms of the flow as well as the water stage is proper to seepage analyses.

• Proceedings of the Korea Water Resources Association Conference
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• 2007.05a
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• pp.1254-1257
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• 2007

본 연구에서는 유류-유사 분리기내에서 유체의 흐름거동을 상용 3차원 수치모형인 FLOW-3D를 이용하여 해석하였다. 유류-유사 분리기는 도심지의 우수로 인한 유출발생시 동반되는 이물질을 여과시키는 기능을 가진 지하구조물이다. 우수로 인한 유출이 발생하여 유류-유사 분리기내로 흘러들어오는 유입수는 3차원적 거동을 하고 다양한 흐름특성을 갖는다. 따라서, 분리기내에서의 흐름거동을 정확히 해석하는데 많은 어려움이 있다. 본 연구에서는 정류판(baffle)과 유류흡착기(oil skimmer)로 구성된 유류-유사 분리기에서의 유류와 유사를 포함하고 있는 유출수의 복잡한 거동을 해석하기 위해 수치모의를 실시하였다. 유류와 유사에 대한 포착률(oil and grit trap)은 유류-유사 분리기내에서 유체의 흐름거동과 관계된다. 따라서, 본 연구에서는 유류와 유사의 포착률을 향상시키기 위한 수치모의의 기초적인 단계로서 유류-유사 분리기내에서 유류흡착기(oil skimmer)를 설치한 경우와 설치하지 않은 경우에 대한 유체의 흐름거동을 FLOW-3D를 이용하여 3차원 수치모의를 실시하였다. 수치모의를 통해 유류흡착기의 유무에 따른 유류-유사 분리기내에서의 흐름거동을 알 수 있었으며, 유류흡착기를 설치한 경우엔 유류 흡착기에 의해 여과될 수 있는 유류와 유사에 대한 각각의 포획률이 높아짐을 알 수 있었다.