• 한국가시화정보학회지
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• 제18권3호
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• pp.109-115
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• 2020

Five layers in mean flow are proposed by using the direct numerical simulation data of turbulent pipe flow up to Reτ = 3008. Viscous sublayer, buffer layer, mesolayer, log layer and core region are investigated. In the buffer layer, the viscous force is counterbalanced by the turbulent inertia from the streamwise mean momentum balance, and a log law occurs here. The overlap layer is composed of the mesolayer and the log layer. Above the buffer layer, the non-negligible viscous force causes the power law, and this region is the mesolayer, where it is the lower part of the overlap layer. At the upper part of the overlap layer, where the viscous force itself becomes naturally negligible, the log layer will appear due to that the acceleration force of the large-scale motions increases as the Reynolds number increases. In the core region, the velocity-defect form is satisfied with the power-law scaling.

• Wind and Structures
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• 제34권1호
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• pp.115-125
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• 2022

The high-Reynolds number flow around a rectangular cylinder, having streamwise to crossflow length ratio equal to 5 is analyzed in the present paper. The flow is characterized by shear-layer separation from the upstream edges. Vortical structures of different size form from the roll-up of these shear layers, move downstream and interact with the classical vortex shedding further downstream in the wake. The corresponding mean flow is characterized by a recirculation region along the lateral surface of the cylinder, ending by mean flow reattachment close to the trailing edge. The mean flow features on the cylinder side have been shown to be highly sensitive to set-up parameters both in numerical simulations and in experiments. The results of 21 Large Eddy Simulations (LES) are analyzed herein to highlight the impact of the lateral mean recirculation characteristics on the near-wake flow features and on some bulk quantities. The considered simulations have been carried out at Reynolds number Re=DU_∞/ν=40 000, being D the crossflow dimension, U_∞ the freestream velocity and ν the kinematic viscosity of air; the flow is set to have zero angle of attack. Some simulations are carried out with sharp edges (Mariotti et al. 2017), others with different values of the rounding of the upstream edges (Rocchio et al. 2020) and an additional LES is carried out to match the value of the roundness of the upstream edges in the experiments in Pasqualetto et al. (2022). The dimensions of the mean recirculation zone vary considerably in these simulations, allowing us to single out meaningful trends. The streamwise length of the lateral mean recirculation and the streamwise distance from the upstream edge of its center are the parameters controlling the considered quantities. The wake width increases linearly with these parameters, while the vortex-shedding non-dimensional frequency shows a linear decrease. The drag coefficient also linearly decreases with increasing the recirculation length and this is due to a reduction of the suctions on the base. However, the overall variation of C_D is small. Finally, a significant, and once again linear, increase of the fluctuations of the lift coefficient is found for increasing the mean recirculation streamwise length.

• 물과 미래
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• 제24권1호
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• pp.83-92
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• 1991

본 연구는 금강유역의 주요수위표 지점에서 하천유지유량을 산정하는데 그 목적이 있다. 하천유지 유량을 산정하는 방법에는 수요적인 측면과 공급적인 측면 두가지로 분류할 수 있다. 본 논문에서 하천유지유량의 개념은 평균갈수량과 환경보존유량중 큰 값으로 결정하였다. 하천유지유량의 산정에서 대청댐 상류지역은 평균갈수량으로, 대청댐 하류지역은 평균갈수량과 QUAL2E 모형으로 산정된 수질보전유량값 중 큰 값을 하천유지유량으로 결정하였다. 그 결과 대청댐 하류인 공주와 규암지점 모두 평균갈수량 값이 크게 나타났으며 그 값은 공주지점에서 33.82$m^3$/sec, 규암 지점은 51.51$m^3$/sec 이였다. 그러므로 금강유역의 하천유지유량의 산정은 평균갈수량으로 결정하는것이 타당하다고 판단되었다.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 춘계학술대회논문집E
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• pp.727-732
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• 2001

The purpose of this study is to compare the time mean velocity distribution, the time mean kinetic energy, and the time mean turbulence intensity between vertical and horizontal flow fields in a coaxial circular pipe by PIV measurement. Experiments are performed at a Reynolds number 2,000, measuring regions divided as the section regions A, B, C, D in flow fields. The angle of the high-frequency ultrasonic is selected in the direction of $45^{\circ}$ to the flow axes and it is reflected several times. In results, it is clarified that the effect of gravity is given in the vertical flow field compared with the horizontal flow field and the ultrasonic affects the turbulence enhancement. And kinetic energy and turbulence intensity with ultrasonic are shown slightly bigger than those in flow field without it.

• 대한기계학회논문집B
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• 제20권4호
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• pp.1472-1480
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• 1996

The phenomena of drag reduction using small quantities of a linear macromolecules has attracted the attention of experimental investigations. It is well known that drag reduction in single phase liquid flow is affected by polymer materials, molecular weight, polymer concentration, pipe diameter and flow velocity. But the research on drag reduction in two phase flow has not intensively investigated. Drag reduction can be applied to phase change system such as chemical reactor, pool and boiling flow, and to flow with cavitation which occurs pump impellers. The purpose of the present work is to evaluate the drag reduction by measuring pressure drop, mean liquid velocity, and turbulent intensity and determine the effects of polymer additives on drag reduction in horizontal two phase flow. Experimental results show higher drag reduction using co-polymer comparing with using polyacrylamide. Mean liquid velocities increase as adding more polymer, and turbulent intensities decrease as the distance for the wall in inversed.

• Journal of Mechanical Science and Technology
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• 제19권12호
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• pp.2270-2280
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• 2005

An experimental study is performed for turbulent swirling flow behind a circular cylinder using 2-D PIV technique. The Reynolds number investigated are 10,000, 15,000, 20,000 and 25,000. The mean velocity vector, time mean axial velocity, turbulence intensity, kinetic energy and Reynolds shear stress behind the cylinder are measured before and behind the round cylinder along the test tube. A comparison is included with non swirl flow behind a circular and square cylinder. The recirculation zones are showed asymmetric profiles.

• 대한토목학회논문집
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• 제36권3호
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• pp.407-416
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• 2016

본 연구에서는 미계측 유역의 평균갈수량 추정을 위한 지역회귀모형을 개발하고자 하였다. 12개 다목적댐과 4개의 용수댐에서 관측된 조절되지 않은 유입량 자료로부터 평균갈수량을 산정하였고, 이를 유역면적, 유역경사, 유역밀도, 연평균강수량, 유출곡선지수 등의 유역특성인자와의 상관분석을 통해 다양한 형태의 지역회귀모형을 개발하였다. 평균갈수량의 관측값과 추정값의 비교를 통해 각 회귀모형의 성능을 평가하였고, 유역면적, 연평균강수량, 유출곡선지수를 설명변량으로 하는 회귀모형이 가장 우수한 성능을 보였다. 또한 비유량법과 기존에 개발된 기존회귀모형과의 비교를 통해서 본 연구에서 개발한 모형의 적용성이 가장 우수한 것으로 분석되었다.

• 대한기계학회논문집B
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• 제20권7호
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• pp.2289-2297
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• 1996

Flows in the combustion chamber near the spark plug are measured using LDv.A single cylinder DOHC S.I. engine of compression ratio 9.5:1 with a transparent quartz window piston is used. Combustion chamber shape is semi-wedge type. Measured data are analyzed using the ensemble averaged analysis and the cycle resolved analysis which uses FFT Filtering. Turbulent intensity and mean velocity are studied in the main flow direction and the normal to main flow direction as a function of engine speeds. The results shows that the turbulent intensity obtained by the ensemble averaged analysis is greater than that calculated by the cycle resolved analysis. Especially, the ensemble averaged analysis shows increase in turbulence at the end of compression stroke although the cycle resolved analysis shows increase only in the cycle-by-cycle variation with no noticeable increase in turbulence. The mean velocity in the main flow direction increase as engine speed increase. But the mean velocity normal to the main flow does not show such increase. Turbulent intensity in both direction increase in proportion to engine speeds. The magnitude of turbulent intensity is about 0.3 ~ 0.4 times the mean piston speeds at the end of the compression stroke.

• Journal of Chest Surgery
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• 제26권2호
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• pp.148-153
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• 1993

To compare two methods of mammary pedicle graft preparations with free internal mammary artery flow, we studied 31 patients who had the left internal mammary artery harvested for coronary artery bypass grafting. The free flow was measured at the transected opening of 2 to 3 cm distal to the point of bifurcation on mean arterial pressure of 50 to 55 mmHg during cardiopulmonary bypass. Group I comprised 14 patients, whose grafts were sprayed and wrapped in sponges soaked in diluted papaverine solution (60 mg in 40 ml Hartmann's solution). An average 80 minutes after the preparations, free flow of the internal mammary artery ranged from 20 to 80 ml/min (mean 37.7 ml/min). Group II comprised 17 patients, who had internal mammary artery takedown under the exact conditions used in group I. The grafts were sprayed and wrapped in sponges soaked in the diluted papaverine solution as in group I. After an average of 28 minutes, free flow ranged from 8 to 28 ml/min (mean 17.6 ml/min). Intraluminal papaverine of the same dilution was then injected without any hydrostatic dilatation and flows increased upto 37 to 150 ml/min (mean 74.7 ml/min). This study shows that intraluminal papaverine preparation method markedly increases free mammary artery flow which is inadequate with external papaverine preparation.

• 대한기계학회논문집B
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• 제24권11호
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• pp.1417-1427
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• 2000

A flow stabilizer, which is made of a honeycomb and three different mesh screens, is located downstream of a butterfly-type valve, for the reduction of flow disturbances behind the valve. Mean flow and turbulence measurements as well as flow visualizations are conducted in the downstream region of the deepens the non-uniformity of the streamwise velocity component and turbulence. The mesh screens considerably reduce the turbulence and enhance the uniformity of mean velocities. The combination of the honeycomb and the three mesh screens results in an efficient reduction in the flow disturbances. In addition, the flow stabilizer proves to have a good performance in the suppression of turbulence at a short distance.