• Transactions of the Korean Society of Mechanical Engineers B
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• v.22 no.1
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• pp.34-49
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• 1998

In this study, turbulent flows in a grooved channel are numerically investigated by Large Eddy Simulation (LES). Especially, a parametric study is carried out to study effects of length and depth of a groove on large-scale flow structures. For one test case, comparison of LES results with those of DNS reveals a good agreement even though the number of grid points of LES is only 6.5% of that of DNS. This confirms that LES is a suitable tool for a parametric study of turbulent flows. The subsequent parametric study using LES shows that the large-scale turbulent structures are significantly affected by the geometry of the groove. Especially, when the length of the groove is short such that the recirculation region occupies the entire groove, the turbulent flow in the groove becomes very weak in both mean and fluctuation quantities.

• Transactions of the Korean Society of Mechanical Engineers
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• v.18 no.7
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• pp.1866-1872
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• 1994

Turbulent flow in a channel with a square rib periodically mounted on one wall is studied by large-eddy simulation(LES). An efficient 3D Navier-Stokes solver has been written for this geometry using a fractional step method and a multi-grid technique. The Reynolds number considered is 82, 000 based on the mean velocity above the obstacle height. Near-wall turbulence is approximated by a wall-layer model based on the turbulence intensity at the grid point nearest a solid wall. The results show a good qualitative agreement with experiments currently available for a single rib, indicating that LES can be a useful tool in simulating complex turbulent flows.

• Journal of the KSME
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• v.33 no.12
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• pp.1063-1068
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• 1993

왕복식엔진에서 연소과정은 실린더내 유체유동에 지배되므로 최적조건의 엔진설계를 위해서는 실린더내 유체유동을 효과적으로 이용하는 것이 필요하다. 연소과정에 중요한 영향을 미치는 압축말기 연소실내 난류강도는 흡입과정시 생성된 유동의 에너지가 압축과정을 거치면서 작은 스케일의 에디(eddy)로 깨지면서 발생된다. 연소과정시 이러한 에디들은 초기화염생성을 촉진 시키고 화염전파속도를 증진시키는 역할을 함으로써 실린더내 유체유동에 대한 이해증진을위해 실린더내 평균속도 및 난류유동을 측정하는 연구들이 많이 진행되고 있다. 엔진유동은 매사이 클의 유동이 엄밀히 주기적인 운동을 하지 않고, 각 사이클의 유동이 비정상유동을 하며, 유동의 생성 및 소멸이 매우 짧은 특성을 가진다. 따라서 산란입자가 측정체적을 통과할 때 속도데이 터가 발생하는 LDV(laser Doppler velocimetry) 측정에 있어서 레이저빔의 산란광노이즈 감소와 산란입자의 효율적인 공급으로 데이터 발생률을 높이는 것이 어려운 점이다. 이 글에서는 엔진 유동의 LDV측정시 고려해야 할 문제점들, 실험장치구성, 그리고 데이터처리 방법과 주요측정 결과에 대해 본 연구팀에서 지금까지 수행한 연구결과를 토대로 하여 기술하고자 한다.

• Proceedings of the KSME Conference
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• 2004.04a
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• pp.1442-1446
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• 2004

The heat (mass) transfer characteristics on the blade surface of a first-stage turbine rotor cascade has been investigated by employing the naphthalene sublimation technique. A four-axis profile measurement system is employed for the measurements of the local heat (mass) transfer coefficient on the curved blade surface. The experiments are carried out for two free-stream turbulence intensities of 1.2% and 14.7%. The high free-stream turbulence results in more uniform distributions of heat load on the both pressure and suction surfaces and in an early boundary-layer separation on the suction surface. The heat (mass) transfer enhancement on the suction surface due to the endwall vortices is found to be relatively small under the high free-stream turbulence.

• Transactions of the Korean Society of Mechanical Engineers
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• v.18 no.9
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• pp.2423-2430
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• 1994

Several factors of the efficient combustion process are shape of combustion chamber, position of spark plug, turbulence flow and so on. the shape of combustion chamber and position of spark plug are constrained to geometrically, and then it could not make a change the shape easily. But the turlence flow in combustion chamber have a great influence on combustion phenomena, and which is much easier to control relatively. And since characteristics of turbulence flow would be very important to the stability of combustion and performances, This study is also essential to future engine-low emission and lean burn engine. This paper shows that the visualization of the turbulence flow of single cylinder engine by using 2way, $45^{\circ}$ inclined and 2 channel hot wire probe through the park plug hole. We also study the characteristics of turbulence flow by means of ensemble averaged mean velocity, turvulence intensity and integral length scale.

• Journal of Korea Water Resources Association
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• v.56 no.spc1
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• pp.1059-1069
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• 2023

Dam-break flow occurs when an elevated dam suddenly collapses, resulting in the catastrophic release of rapid and uncontrolled impounded water. This study compares laminar and turbulent closure models for simulating three-dimensional dam-break flows using OpenFOAM. The Reynolds-Averaged Navier-Stokes (RANS) model, specifically the k-ε model, is employed to capture turbulent dissipation. Two scenarios are evaluated based on a laboratory experiment and a modified multi-layered block obstacle scenario. Both models effectively represent dam-break flows, with the turbulent closure model reducing oscillations. However, excessive dissipation in turbulent models can underestimate water surface profiles. Improving numerical schemes and grid resolution enhances flow recreation, particularly near structures and during turbulence. Model stability is more significantly influenced by numerical schemes and grid refinement than the use of turbulence closure. The k-ε model's reliance on time-averaging processes poses challenges in representing dam-break profiles with pronounced discontinuities and unsteadiness. While simulating turbulence models requires extensive computational efforts, the performance improvement compared to laminar models is marginal. To achieve better representation, more advanced turbulence models like Large Eddy Simulation (LES) and Direct Numerical Simulation (DNS) are recommended, necessitating small spatial and time scales. This research provides insights into the applicability of different modeling approaches for simulating dam-break flows, emphasizing the importance of accurate representation near structures and during turbulence.

• Journal of Korea Water Resources Association
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• v.55 no.11
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• pp.955-967
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• 2022

Existing particle tracking models predict vertical displacement of particles based on the terminal settling velocity in the stagnant water. However, experimental results of the present study confirmed that the settling velocity of particles is influenced by the turbulence effects in turbulent flow, consistent with the previous studies. The settling velocity of particles and turbulent characteristics were measured by using PTV and PIV methods, respectively, in order to establish relationship between the particle settling velocity and the ambient turbulence. It was observed that the settling velocity increase rate starts to grow when the particle diameter is of the same order as Kolmogorov length scale. Compared with the previous studies, the present study shows that the graphs of the settling velocity increase rate according to the Stokes number have concave shapes for each particle density. In conclusion, since the settling velocity in the natural flow is faster than in the stagnant water, the existing particle tracking model may estimate a relatively long time for particles to reach the river bed. Therefore, the results of the present study can help improve the performance of particle tracking models.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.20 no.3
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• pp.1040-1049
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• 1996

The high loading combustion is accomplished by making the turbulent intensity strong and the scale small in the premixed combustor. The Da-mkoler number, which is decreased by short turbulent characteristic time or by long chemical reaction time, can make the distributed reaction flame. So we developed a doubled jet burner for high loading combustion. The doubled jet burner was designed to make the scale of the flame small by the effect of impingement and increasing shear stress with doubled jet. We investigated the turbulence characteristics of unburned mixture and visualized several flames with the typical schlieren photography. Then we studied the influence of several factors that related the scale of flame. Consequently, the doubled jet burner can make the eddy very small. And we can obtain the detail information of the flame scale through ADSF(the Average Distance between Successive Fringes) in the micro- schlieren photography. The ADSF is not a exact flame scale, but it has qualitative trend with increasing turbulent intensity. The ADSF is diminished remarkably with increasing turbulent intensity. The reason is that strong turbulent intensity makes the flame zone thick and flamelets numerous. We can confirm this fact by the signal analysis of ion currents.

• Proceedings of the KSME Conference
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• 2001.06d
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• pp.759-764
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• 2001

Heat (mass) transfer characteristics have been investigated on the endwall of a large-scale linear turbine cascade passage under a combustor-level high free-stream turbulence with a large length scale. Local heat (mass) transfer coefficients are measured by using the naphthalene sublimation technique. The result shows that local heat (mass) transfer on the endwall is greatly enhanced in the central region of the turbine passage, but there is no noticeable change in the local heat (mass) transfer in the region suffering severe heat load. Under the high free-stream turbulence, the local heat (mass) transfer coefficient shows more uniform distribution and its average value across the whole endwall region is increased by 26% of that at low turbulence condition. The heat (mass) transfer data on the endwall strongly supports that well-organized vortices near the endwall tends to suffer an suppression by the high free-stream turbulence.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.24 no.10
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• pp.1409-1416
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• 2000

In this study, turbulent flows in a planar combustor which has a square rib-type flame holder are numerically investigated by Large Eddy Simulation(LES). Firstly, the flow fields with or without jet injection downstream of the flame-holder are examined using uniform inlet velocity. Comparison of the present LES results with experimental one shows a good agreement. Secondly, to investigate mixing of oxidizer(air) and fuel injected behind the flame holder, the scalar-transport equation is introduced and solved. From the instantaneous flow and scalar fields, complex and intense mixing phenomena between fuel and jet are observed. It is shown that the ratio of jet to blocked air velocity is an important factor to determine the flow structure. Especially, when the ratio is large enough, the fuel jet penetrates the main vortices shed from the flame holder, resulting in significant changes in the flow and scalar fields.