• Journal of Advanced Marine Engineering and Technology
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• v.31 no.2
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• pp.166-172
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• 2007

The flow around a rotating circular cylinder near a plane wall is investigated by the measurement of the lift acting on the cylinder and by the flow visualization using the hydrogen bubble technique in the circulating water tank. The experimental parameters are the rotating direction of the cylinder. the space ratios $H/D(H/D=0.05{\sim}0.5)$ between cylinder and plane wall and the velocity ratios ${\alpha}({\alpha}=0{\sim}{\pm}2.0)$. In the case of clockwise, the lift on the rotating circular cylinder was increased with the reduction of the space ratios and with the velocity ratios, the upper separation point was more shifted in the rotating direction with them. In the case of anticlockwise, the absolute value of the lift on the rotating circular cylinder was increased with increasing the space ratios and the velocity ratios. the lower separation point was more shifted in the rotating direction with them.

• 한국가시화정보학회:학술대회논문집
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• 2006.12a
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• pp.42-47
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• 2006

The flow around a rotating circular cylinder near a plane wall is investigated by the measurement of the lift acting on the cylinder and by the flow visualization using the hydrogen bubble technique in the circulating water tank. The experimental parameters are the rotating direction of the cylinder, the space ratios H/D$(H/D=0.05\sim0.5)$ between cylinder and plane wall and the velocity ratios $\alpha(\alpha=0\sim{\pm}2.0)$. In the case of clockwise, the lift on the rotating circular cylinder was increased with the reduction of the space ratios and with the velocity ratios, the upper separation point was more shifted in the rotating direction with them. In the case of anticlockwise, the absolute value of the lift on the rotating circular cylinder was increased with the space ratios and with the velocity ratios, the lower separation point was more shifted in the rotating direction with them.

• Wind and Structures
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• v.19 no.2
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• pp.219-232
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• 2014

The characteristics of turbulent boundary layers over hilly terrain depend strongly on the hill slope and upstream condition, especially inflow turbulence. Numerical simulations are carried out to investigate the neutrally stratified turbulent boundary layer over two-dimensional hills. Two kinds of hill shape, a steep one with stable separation and a low one without stable separation, two kinds of inflow condition, laminar turbulent, are considered. An auxiliary simulation, based on the local differential quadrature method and recycling technique, is performed to simulate the inflow turbulence be imposed at inlet boundary of the turbulent inflow, which preserves very well in the computational domain. A large separation bubble is established on the leeside of the steep hill with laminar inflow, while reattachment point moves upstream under turbulent inflow condition. There is stable separation on the side of low hill with laminar inflow, whilw not turbulent inflow. Besides increase of turbulence intensity, inflow can efficiently enhance the speedup around hills. So in practice, it is unreasonable to study wind flow over hilly terrain without considering inflow turbulence.

• Advances in aircraft and spacecraft science
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• v.10 no.4
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• pp.303-315
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• 2023

The primary goal of this research is to investigate flow separation phenomena using various turbulence models. Also investigated are the effects of free-stream turbulence intensity on the flow over a NACA 0018 airfoil. The flow field around a NACA 0018 airfoil has been numerically simulated using RANS at Reynolds numbers ranging from 100,000 to 200,000 and angles of attack (AoA) ranging from 0° to 18° with various inflow conditions. A parametric study is conducted over a range of chord Reynolds numbers for free-stream turbulence intensities from 0.1 % to 0.5 % to understand the effects of each parameter on the suction side laminar separation bubble. The results showed that increasing the free-stream turbulence intensity reduces the length of the separation bubble formed over the suction side of the airfoil, as well as the flow prediction accuracy of each model. These models were used to compare the modeling accuracy and processing time improvements. The K- SST performs well in this simulation for estimating lift coefficients, with only small deviations at larger angles of attack. However, a stall was not predicted by the transition k-kl-omega. When predicting the location of flow reattachment over the airfoil, the transition k-kl-omega model also made some over-predictions. The Cp plots showed that the model generated results more in line with the experimental findings.

• Journal of the Korean Society of Visualization
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• v.19 no.1
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• pp.88-93
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• 2021

For a flow normal to a circular disk, the flow separation occurs from the edge of the disk and the flow recirculation zone exists behind the disk. Many existing studies conducted simulations of flow normal to a circular disk under low Reynolds numbers. Some studies performed LES or DES simulations under high Reynolds numbers. However, comparative study for different RANS models for high Reynolds numbers is very limited. This study presents numerical simulations of a flow normal to a circular disk using Realizable k-ε model and SST k-ω model. The recirculation bubble length and drag coefficient were compared with the experimental data. The SST k-ω model showed the excellent predictions for the recirculation bubble length and drag coefficient.

• Journal of Korea Water Resources Association
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• v.48 no.4
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• pp.281-290
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• 2015

Numerical simulations of free surface flow over a broad-crested rectangular weir are conducted by using the volume of fraction (VOF) method and three different turbulence models, the k-${\varepsilon}$, RNG k-${\omega}$ and k-${\omega}$ SST models. The governing equations are solved by a second-order accurate finite volume method and the grid sensitivity study of solutions is carried out. The numerical results are evaluated by comparing the solutions with experimental and numerical results of Kirkgoz et al. (2008) and some non-dimensionalized experimental results obtained by Moss (1972) and Zachoval et al. (2012). The results show that the present numerical model can reasonably reproduce the experimental results, while three turbulent models yield different numerical predictions of two distinct zones of flow separation, the first zone is in front of the upstream edge of the weir and the second is created immediately behind the upstream edge of the weir where the flow is separated to form the separation bubble. The standard k-${\varepsilon}$ model appears to significantly underestimate the size of both separation zones and the k-${\omega}$ SST model slightly over-estimates the first separation zone in front of the weir. The RNG k-${\varepsilon}$ model predicts both separation zones in overall good agreement with the experimental measurement, while the k-${\omega}$ SST model yields the best numerical prediction of separation bubble at the upstream edge of the weir.

• Korean Chemical Engineering Research
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• v.47 no.2
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• pp.195-202
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• 2009

As a basic research of developing two-interconnected fluidized beds system for selective solid circulation, a solid separator was developed to separate fine and coarse particles by means of particle size difference with particle size separation system equipped with metal screen. The effects of gas velocity, height of solid separator, and separation area on the solid separation rate were investigated as well. The solid separation rate increased as the gas velocity, height of solid separator, and separation area increased. As the gas velocity and height of the solid separator increased, the variation of the solid separation rate was consistent with that of bubble size. Consequently, coarse($212{\sim}300{\mu}m$) and fine($63{\sim}106{\mu}m$) particles were separated using the solid separator and the solid separation rate was ranged from 4.4 to 127 g/min. We also proposed two interconnenced fluidized beds system for sorption enhanced water-gas shift process equipped with the developed solid separator.

• Wind and Structures
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• v.7 no.5
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• pp.317-332
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• 2004

Wind tunnel experiments were conducted under highly turbulent and disturbed flow conditions over a solid/perforated plate with a long splitter plate in its plane of symmetry. The effect of varied level of perforation of the normal plate on fluctuating velocities and fluctuating pressures measured across and along the separation bubble was studied. The different perforation levels of the normal plate; that is 0%, 10%, 20%, 30%, 40% and 50% are studied. The Reynolds number based on step height was varied from $4{\times}10^3$ to $1.2{\times}10^4$. The shape and size of the bubble vary with different perforation level of the normal plate that is to say the bubble is reduced both in height and length up to 30% perforation level. For higher perforation of the normal plate, bubble is completely swept out. The peak turbulence value occurs around 0.7 to 0.8 times the reattachment length. The turbulence intensity values are highest for the case of solid normal plate (bleed air is absent) and are lowest for the case of 50% perforation of the normal plate (bleed air is maximum in the present study). From the analysis of data it is observed that $\sqrt{\overline{u^{{\prime}2}}}/(\sqrt{\overline{u^{{\prime}2}}})_{max}$, (the ratio of RMS velocity fluctuation to maximum RMS velocity fluctuation), is uniquely related with dimensionless distance y/Y', (the ratio of distance normal to splitter plate to the distance where RMS velocity fluctuation is half its maximum value) for all the perforated normal plates. It is interesting to note that for 50% perforation of the normal plate, the RMS pressure fluctuation in the flow field gets reduced to around 60% as compared to that for solid normal plate. Analysis of the results show that the ratio [$C^{\prime}_p$ max/$-C_{pb}(1-{\eta})$], where $C^{\prime}_p$ max is the maximum coefficient of fluctuating pressure, $C_{pb}$ is the coefficient of base pressure and ${\eta}$ is the perforation level (ratio of open to total area), for surface RMS pressure fluctuation levels seems to be constant and has value of about 0.22. Similar analysis show that the ratio $[C^{\prime}_p$ max/$-C_{pb}(1-{\eta})]$ for flow field RMS pressure fluctuation levels seems to be constant and has a value of about 0.32.

• Proceeding of EDISON Challenge
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• 2016.11a
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• pp.4-8
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• 2016

본 연구에서는 플라잉 디스크를 이용하여 회전 운동의 유무에 따른 공력특성의 변화를 비교 분석하였다. 회전 운동이 발생하면 표면에 비대칭적으로 압력이 분포하기 때문에 롤링 모멘트가 발생하고 마그너스 효과로 측력이 발생하고 피칭모멘트, 회전감쇠모멘트가 발생하고 롤 안정성이 증가한다.

• 유체기계공업학회:학술대회논문집
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• 2006.08a
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• pp.187-190
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• 2006

In the present study, effects of tree-stream turbulence and surface trip wire on the flow past a sphere at $Re\;=\;0.4\;{\times}\;10^5\;{\sim}\;2.8\;{\times}\;10^5$ are investigated through wind tunnel experiments. Various types of grids are installed upstream of the sphere in order to change the tree-stream turbulence intensity. In the case of surface trip wire, 0.5mm and 2mm trip wires are attached from $20^{\circ}\;{\sim}\;90^{\circ}$ at $10^{\circ}$ interval along the streamwise direction. To investigate the flow around a sphere, drag measurement using a load cell, surface-pressure measurement, surface visualization using oil-flow pattern and near-wall velocity measurement using an I-type hot-wire probe are conducted. In the variation of free-stream turbulence, the critical Reynolds number decreases and drag crisis occurs earlier with increasing turbulence intensity. With increasing Reynolds number, the laminar separation point moves downstream, but the reattachment point after laminar separation and the main separation point are fixed, resulting in constant drag coefficient at each free-stream turbulence intensity. At the supercritical regime, as Reynolds number is further increased, the separation bubble is regressed but the reattachment and the main separation points are fixed. In the case of surface trip wire directly disturbing the boundary layer flow, the critical Reynolds number decreases further with trip wire located more downstream. However, the drag coefficient after drag crisis remains constant irrespective of the trip location.