• Journal of the Korean Society of Visualization
• /
• v.7 no.2
• /
• pp.35-41
• /
• 2010

Supersonic microjets acquire considerable research interest from a fundamental fluid dynamics perspective, in part because the combination of highly compressible flow at low-to-moderate Reynolds number is not very common, and in part due to the complex nature of the flow itself. In addition, microjets have a great variety engineering applications such as micro-propulsion, MEMS(Micro-Electro Mechanical Systems) components, microjet actuators and fine particle deposition and removal. Numerical simulations have been carried out at moderate nozzle pressure ratios and for different nozzle exit diameters to investigate and to understand in-depth of aerodynamic characteristics of supersonic microjets.

• Korean System Dynamics Review
• /
• v.11 no.1
• /
• pp.85-102
• /
• 2010

This paper explores the ways in which the functions of local educational authority in Korea can be analyzed by systems thinking, and puts forward some policy leverage strategies to enhance predictability of education policy effects and also to prevent unanticipated side effects arising from it. In dong so, we sketch causal diagrams to depict functional changes of local educational authority, based on before and after comparison, and attempt to derive policy intervention points to minimize unforseen reactions from the stake-holders concerned. These diagrams make it possible for educational policy-makers to capture the feedback, stocks and flows, time delays, and non-linearities they identify, although they have some limitations. This paper concludes that newly-designed functions and structures for local educational authority may be accepted by the stake-holders including teachers and unions, only if complex systems surrounding functional changes regarding local educational authority can be clearly understood and relevant policy measures can be effectively taken before the functional changes happen.

• Transactions of the Korean Society of Automotive Engineers
• /
• v.10 no.6
• /
• pp.1-10
• /
• 2002

As the preliminary stage for the countermeasure of thermal loading in miller cycle engine, coolant flows in the cylinder head of base engine including exhaust valve bridge were visualized and analyzed by using PIV technique. It was found that low coolant velocity regions were around exhaust valve bridge, around which stagnation of the coolant flow was observed due to the complex geometry configuration of water jacket. And velocity variation between each cylinder was remarkable. For the countermeasure of these, it is necessary to enhance coolant flow around exhaust valve bridge and to improve the deviation of coolant flow between each cylinder.

• Wind and Structures
• /
• v.9 no.5
• /
• pp.357-367
• /
• 2006

Unsteady 3D Reynolds Averaged Navier-Stokes (URANS) solver is used to simulate the turbulent flow past an isolated prismatic cylinder at Re=37,400. The aspect ratio of height to base width of the body is 5. The turbulence closure is achieved through a non-linear $k-{\varepsilon}$ model. The applicability of this model to predict unsteady forces associated with this flow is examined. The study shows that the present URANS solver with standard wall functions predicts all the major unsteady phenomena showing closer agreement with experiment. This investigation concludes that URANS simulations with the non-linear $k-{\varepsilon}$ model as a turbulence closure provides a promising alternative to LES with view to study flows having complex features.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
• /
• v.13 no.5
• /
• pp.398-410
• /
• 2001

The flow and heat transfer characteristics for three-dimensional mixed convection flows in a radiator flat tube with U--shaped grooves are analyzed numerically. The flow and temperature fields are calculated by using the modified SIMPLE algorithm for irregular geometry. One tube specification among the various flat tube exchangers is recommended by considering the heat transfer and pressure drop. The effects of variation of coolant flow conditions and external air conditions on the flow and the thermal characteristics for the selected tube are investigated. the results show that inlet velocity of coolant flow is the very important factor in heat transfer and pressure drop, and top side is better position than the others as fin cleave to tube.

• Asia pacific journal of information systems
• /
• v.10 no.2
• /
• pp.23-49
• /
• 2000

We proposed a conceptual design of the web-based agent model for global supply chain management(GSCM), where agents representing autonomous operational units, such as suppliers, factories, distribution center and customers, cooperate and are coordinated through the information exchange. The agent model assumed the hierarchical federated system. In the federated system, the agents of the same region are grouped and linked to the region-specific facilitator only through which communication between agents is allowed. The facilitator is responsible for monitoring and controlling the conversations consisting of the message flows across the agents. A web-based user presentation was also designed so that human users could involve in collaborative settings into the GSCM multi-agent system. In the conversation protocols which allow for complex coordinated behavior among agents, the KQML was extended to represent the messages. A GSCM scenario where the supply chain is formed upon customer order and supply decision is made was used to demonstrate the dynamics of the conversation protocols.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
• /
• v.12 no.11
• /
• pp.1020-1030
• /
• 2000

A crossflow finned tube evaporator has been investigated using tube-by-tube method. The refrigerant flows inside tubes while the air passes outside the finned tue. R134a for a refrigerant and wet air were used in this analysis. Three refrigerant circuitries, complex or simple, were compared. The effects of refrigerant circuitry and airside condensation on thermal performance such as heat transfer rate, enthalpy of refrigerant, temperature of refrigerant and air, pressure of refrigerant and average condensate layer thickness were investigated. It was found that this method could be applied to the analysis of finned tube evaporator for dehumidification and air conditioning.

• Journal of computational fluids engineering
• /
• v.14 no.4
• /
• pp.41-47
• /
• 2009

An immersed boundary method(IBM, Kim et al.(2001)) for simulating flows over complex geometries is applied to computation of three-dimensional Floquet stability of a periodic wake. Floquet stability analysis is employed to extract different modes of three-dimensional instability. To verify the present method, a fully-resolved Floquet stability calculation for flow past a circular cylinder is considered. There are two different instability modes with long(mode A) and short (mode B) spanwise wavelengths for the periodic wake of a circular cylinder. The critical Reynolds number and the most unstable spanwise wavelengths of modes A and B are computed using the present method, and compared with other authors' results currently available.

• Proceedings of the KSME Conference
• /
• 2008.11b
• /
• pp.2460-2464
• /
• 2008

The present paper suggests new method to know the effects of molecular diffusion and the helicity of microchannel flows on mixing in passive micromixers, which are essential components of a microfluidic chip. In this study, 'Helix Index' is newly defined as the magnitude of chaotic advection. Relationship between Helix Index and Mixing Index is analyzed numerically such as the wide range of Peclet and Reynolds numbers in three dimensional serpentine microchannel when using soluble solutions (water/glycerol). As a result, a simple algebraic equation is derived by this relationship based on a regression analysis. The algebraic equation is found to be able to accurately predict the mixing performance without solving the coupled, complex momentum and mass transfer equations.

• Proceedings of the SAREK Conference
• /
• 2007.11a
• /
• pp.229-234
• /
• 2007

Microscale heat transfer and microfluidics have become increasingly important to overcome some very complex engineering challenges. The use of very small passages to gain heat transfer enhancement is a well documented method for achieving high heat flux dissipation. As the passage size is decreased, the heat transfer performance increases but the pressure drop increases sharply when the passage size is reduced. In this study, the performance evaluation of micro plated heat exchangers under the counter flows with straight, V-shaped and Y-shaped channel are carried out.