• 한국멀티미디어학회논문지
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• 제18권2호
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• pp.189-198
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• 2015

Despite the fact that traffic engineering techniques have been comprehensively utilized in the past to enhance the performance of communication networks, the distinctive characteristics of Software Defined Networking (SDN) demand new traffic engineering techniques for better traffic control and management. Considering the behavior of traffic, large flows normally carry out transfers of large blocks of data and are naturally packet latency insensitive. However, small flows are often latency-sensitive. Without intelligent traffic engineering, these small flows may be blocked in the same queue behind megabytes of file transfer traffic. So it is very important to identify large flows for different applications. In the scope of this paper, we present an approach to detect large flows in real-time without even a short delay. After the detection of large flows, the next problem is how to control these large flows effectively and prevent network jam. In order to address this issue, we propose an approach in which when the controller is enabled, the large flow is mitigated the moment it hits the predefined threshold value in the control application. This real-time detection, marking, and controlling of large flows will assure an optimize usage of an overall network.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2010년 춘계학술대회논문집
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• pp.2-4
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• 2010

Due to rapid progress in the performance of high-end computers, numerical prediction of fluid flow and flow-induced sound is expected to become a vital tool for aero- and hydro- dynamic design of various flow-related products. This presentation focuses on the applications of large-scale numerical simulations to complex engineering problems with a particular emphasis placed on the low-speed flows. Flow field computations are based on a large eddy simulation that directly computes all active eddies in the flow and models only those eddies responsible for energy dissipations. The sound generated from low-speed turbulent flows are computed either by direct numerical simulation or by decoupled methods, according to whether or not the feedback effects of the generated sound onto the source flow field can be neglected. Several numerical examples are presented in order to elucidate the present status of such computational methods and discussion on the future prospects will also be given.

• 대한기계학회논문집B
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• 제30권4호
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• pp.364-372
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• 2006

The objective of this study is to evaluate the efficiency and the prediction accuracy of developed large eddy simulation (LES) program for complex turbulent flows, such as recirculating and swirling flows. To save the computational cost, a Beowulf cluster system consisting 16 processors was constructed. The flows in backward-facing step and dump combustor were examined as representative recirculating and swirling flows. Firstly, a direct numerical simulation (DNS) for laminar backward-facing step flows was previously conducted to validate the overall performance of program. Then LES was carried out for turbulent backward-facing step flows. The results of laminar flow showed a qualitative and quantitative agreement between simulations and experiments. The simulations of the turbulent flow also showed reasonable results. Secondly, LES results for non-swirling and swirling flows in a dump combustor were compared with the results of Reynolds-averaged Navier-Stokes (RANS) using standard $k-{\varepsilon}$ model. The results show that LES has a better performance in predicting the mean axial and azimuthal velocities, comer recirculation zone (CRZ) and center toroidal recirculation zone (CTRZ) than those of RANS. Finally, it was examined the capability of LES for the description of unsteady phenomena.

• 한국철도학회:학술대회논문집
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• 한국철도학회 2008년도 추계학술대회 논문집
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• pp.815-829
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• 2008

Debris flows are a natural hazard which looks like a combination of flood, land and rock slide. Large rainfall in July 2006 produced several large scale debris flows and many small debris flows that resulted in loss of life and considerable property and railway damage, as was widely reported in the national media. The hazard "debris flow" is still insufficiently researched. Furthermore debris flows are very hard to predict. Flexible Ring net barriers are multi-functional mitigation devices commonly applied to rock fall or floating wood protection in floods, snow avalanches and also mud flows or granular debris flows, if properly dimensioned for the process or processes for which they are intended. Overtopping of the barriers by debris flows and sediment transport is possible, supporting the design concept that a series of barriers may be used to stop volumes of debris larger than are possible using only one barrier. The future for these barrier concepts looks promising because these barriers represent the state of art for such applications and are superior to many other available options.

• International Journal of Automotive Technology
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• 제7권4호
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• pp.415-422
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• 2006

Swirl and tumble flows in an engine in-cylinder have been simulated by using a three-dimensional computational fluid dynamics code, and the results are validated in comparison with experimental data. The large eddy simulation based on the Smagorinsky model and the fractional step method is adopted to describe the turbulence of in-cylinder flows and to save computing time, respectively. The main purpose of this study is connected with the effect of various conditions of intake flows on formation and development of in-cylinder tumble and swirl motions. The engine speeds considered are 1000 rpm and 3000 rpm for intake flows with inclination angles between $-10^{\circ}$ and $20^{\circ}$ at deflection angles of $0^{\circ}$, $22.5^{\circ}$, and $30^{\circ}$. The results are discussed by visualizing flow fields and by evaluating parameters in relation to vortex intensity such as swirl and tumble ratios.

• 인터넷정보학회논문지
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• 제10권3호
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• pp.131-140
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• 2009

최근 정보의 시각화를 연구하는 쪽에서는 그래프의 시각화에 많은 관심을 갖고 있는데, 그 중 사회망 (social network)의 시각화에 특히 집중하고 있다. 하지만 아직까지 사회망 내에서의 정보의 흐름을 시각화하는 방법에 대해서는 깊이 있는 연구가 진행되지 않고 있었다. 정보의 흐름은 사회망의 구조와 밀접하게 연관되어 있고, 또한 실제적인 상호관계의 동적인 구성을 보여주기 때문에 사회망의 구조적 특징보다 더 유용한 정보를 담고 있다. 따라서 정보의 흐름을 시각화하는 것은 매우 중요하다. 본 논문에서는 초대형 사회망을 마치 온라인 지도서비스를 이용하듯이 탐색하고, 사회망 내에서의 정보의 흐름을 관찰할 수 있는 방법에 관하여 제안한다. 이를 위하여 (i) 초대형 사회망을 2차원 그래프에 맵핑하는 방법과, (ii) 줌-인, 줌-아웃 기능 등을 활용하여 그래프를 탐색하는 방법, 그리고 (iii) 효율적인 질의 처리 프레임웍을 구축하는 방법을 고안하였다. 이 방법들을 이용하여 초대형 사회망을 제한적인 공간과 한정된 자원을 이용하여 효과적으로 표현할 수 있고, 이에 기반을 두어 사회망에서의 정보의 흐름을 시각화할 수 있게 된다.

• 대한기계학회논문집B
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• 제22권1호
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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.

• International Journal of Fluid Machinery and Systems
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• 제2권4호
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• pp.346-352
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• 2009

The objective of the present work is to improve numerical predictions of unsteady turbulent swirling flows in the draft tubes of hydraulic power plants. We present Large Eddy Simulation (LES) results on a simplified draft tube consisting of a straight conical diffuser. The basis of LES is to solve the large scales of motion, which contain most of the energy, while the small scales are modeled. LES strategy is here preferred to the average equations strategies (RANS models) because it resolves directly the most energetic part of the turbulent flow. LES is now recognized as a powerful tool to simulate real applications in several engineering fields which are more and more frequently found. However, the cost of large-eddy simulations of wall bounded flows is still expensive. Bypass methods are investigated to perform high-Reynolds-number LES at a reasonable cost. In this study, computations at a Reynolds number about 2 $10^5$ are presented. This study presents the result of a new near-wall model for turbulent boundary layer taking into account the streamwise pressure gradient (adverse or favorable). Validations are made based on simple channel flow, without any pressure gradient and on the data base ERCOFTAC. The experiments carried out by Clausen et al. [1] reproduce the essential features of the complex flow and are used to develop and test closure models for such flows.

• 기술발표회
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• 통권2006호
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• pp.220-231
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• 2006

Debris flows are a natural hazard which looks like a combination of flood, land and rock slide. Large rainfall in July 2006 produced several large scale debris flows and many small debris flows that resulted in loss of life and considerable property and highway damage, as was widely reported in the national media. The hazard "debris flow" is still insufficiently researched Furthermore debris flows are very hard to predict. In this paper, a general over view of the debris flow problems along the highway, a generic way for the design and dimensioning of flexible barrier systems will be presented. A brief description of the various unique barrier types will be provided, too. The future for these barrier concepts looks promising because these barriers represent the state of art for such applications and are superior to many other available options.

• 한국가시화정보학회지
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• 제18권2호
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• pp.28-34
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• 2020

In this study, we numerically investigated the thermal stratification in solar seasonal thermal storage tanks. The vertical in/out flows were unsuitable for the thermal stratification in a large scale. The effect of an aspect ratio (AR) on the thermal stratification was investigated. When AR was less than 2, water adheres and flows along the upper wall due to buoyance and the surface effect. Thereafter, hot water flows down and a large scale vortex occurs in entire tank. For high AR, jet flows ejected from the inlet pipe impinges to the opposite wall and splits. The divided flows create two vortex flows in the upper and lower regions. These different flows strongly influence temperature and thermal stratification. The thermal stratification was evaluated in terms of the thermocline thickness and degree of stratification. Compared to ARs, the maximum degree of stratification was obtained with AR of 5 having the minimum thermocline thickness.