• Transactions of the Korean Society of Mechanical Engineers B
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• 제26권7호
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• pp.927-936
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• 2002

An experimental study has been conducted to investigate the heat/mass transfer characteristics within a film cooling hole of square cross-section for various blowing ratios and Reynolds numbers. The experiments have been performed using a naphthalene sublimation method and the flow field has been analyzed by numerical calculation using a commercial code. A duct flow enters into a film cooling hole in a cross-direction. For the film cooling hole with square cross-section, it is observed that the reattachment of separated flow and the vortices within the hole enhance considerably the heat/mass transfer around the hole entrance region. The heat/mass transfer on the leading edge side of hole exit region increases as the blowing ratios decrease because the main flow induces a secondary vortex. Heat/mass transfer patterns within the square film cooling hole are changed little with the various Reynolds numbers.

• Journal of computational fluids engineering
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• 제18권1호
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• pp.1-6
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• 2013

Missile and fighter aircraft have been challenged by low restoring nose-down pitching moment at high angle of attach. The consequence of weak nose-down pitching moment can be resulting in a deep stall condition. Especially, the pressure oscillation has a huge effect on noise generation, structure damage, aerodynamic performance and safety, because the flow has strong unsteadiness at high angle of attack. In this paper, the unsteady aerodynamics coefficients were analyzed at high angle of attack up to 50 degrees around two dimensional NACA0012 airfoil. The two dimensional unsteady compressible Navier-Stokes equation with a LES turbulent model was calculated by OHOC (Optimized High-Order Compact) scheme. The flow conditions are Mach number of 0.3 and Reynolds number of $10^5$. The lift, drag, pressure, entropy distribution, etc. are analyzed according to the angle of attack. The results of average lift coefficients are compared with other results according to the angle of attack. From a certain high angle of attack, the strong vortex formed by the leading edge are flowing downstream as like Karman vortex around a circular cylinder. The primary and secondary oscillating frequencies are analyzed by the effects of these unsteady aerodynamic characteristics.

• Journal of computational fluids engineering
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• 제18권1호
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• pp.91-97
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• 2013

In the present study, the effects of contaminant accumulation and surface roughness on the aerodynamic performance of wind turbine blade sections were numerically investigated by using a flow solver based on unstructured meshes. The turbulent flow over the rough surface was modeled by a modified ${\kappa}-{\omega}$ SST turbulence model. The calculations were made for the NREL S809 airfoil with varying contaminant sizes and positions at several angles of attack. It was found that as the contaminant size increases, the degradation of the airfoil performance becomes more significant, and this trend is further amplified near the stall condition. When the contaminant is located at the upper surface near the leading edge, the loss in the aerodynamic performance of the blade section becomes more critical. It was also found that the surface roughness leads to a significant reduction of lift, in addition to increased drag.

• 유체기계공업학회:학술대회논문집
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• 유체기계공업학회 2005년도 연구개발 발표회 논문집
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• pp.473-480
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• 2005

Clocking effects of a stator on the performance and internal flow in an UTRC 1.5 stage axial turbine are investigated using a three-dimensional unsteady flow simulation. Six relative positions of two rows of stator are investigated by positioning the second stator being clocked in a step of 1/6 pitch. The relative efficiency benefit of about 1% is obtained depending on the clocking positions. However, internal flows have some different characteristics from that in the previous study at the best and worst efficiency positions, since be first stator wake is mixed out with the rotor wake before arriving at the leading edge of the second stator. Instead of the first stator wake, it is found that the wake interaction of the first stator and rotor has a important role on a relative efficiency variation at each clocking position. The time-averaged local efficiency along the span at the maximum efficiency is more uniform than that at the minimum efficiency. That is, the spanwise efficiency distribution at the minimum efficiency has larger values in mid-span but smaller values near the hub and casing in comparison to those at the maximum efficiency. Moreover, the difference between maximum and minimum instantaneous efficiencies during one period is found to be smaller at the maximum efficiency than at the minimum efficiency.

• The KSFM Journal of Fluid Machinery
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• 제19권2호
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• pp.21-26
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• 2016

The objective of present study is to assess the performance of the first stage compressor in a total 3-stage 5000 HP-level turbo compressor. CFD commercial code, CFX has been used to predict three-dimensional flow characteristics inside of the impeller. Shear Stress Transport (SST) model has been used to simulate turbulent flows through Reynolds-averaged Navier-Stokes (RANS) equations. Grid dependency has been also checked to get optimal grid distribution. Numerical results have been compared with the experimental test results to elucidate performance characteristics of the present compressor. In addition, flow characteristics of the impeller only have been studied for various blade configurations. Angular offset in leading edge of the blade has been selected for the optimal blade design. Performance characteristics in region of low mass flow rate and high pressure ratio between the impeller entrance and exit have been investigated for the selection of optimal blade design. Also, flow instability such as stall phenomena has been studied and anti-stall characteristics have been checked for various blade configurations in the operational window.

• Journal of the Korean Society of Surveying, Geodesy, Photogrammetry and Cartography
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• 제22권4호
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• pp.349-357
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• 2004

In order to manage the waterfront efficiently, we are to find the efficiency of producing method for digital orthophoto that presents natural form on lands and digital orthophoto cadastral map that presents the range of physical situation and property on lands in this study. Through this study, We can check that digital orthophoto cadastral maps for the boundary management of the waterfront are utilized widely at various field except the main purpose. Thus, we resolve the leading subjects that include the accuracy evaluation on produced maps and the bond edge of map sheet for the cadastral maps in order to drive activistic utilization and application.

• Wind and Structures
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• 제30권1호
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• pp.55-67
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• 2020

Wind tunnel test is one of the most important means to study the flutter performance of bridges, but there are blockage effects in flutter test due to the size limitation of the wind tunnel. On the other hand, the size of computational domain can be defined by users in the numerical simulation. This paper presents a study on blockage effects of a simplified box girder by computation fluid dynamics (CFD) simulation, the blockage effects on the aerodynamic characteristics and flutter performance of a long-span suspension bridge are studied. The results show that the aerodynamic coefficients and the absolute value of mean pressure coefficient increase with the increase of the blockage ratio. And the aerodynamic coefficients can be corrected by the mean wind speed in the plane of leading edge of model. At each angle of attack, the critical flutter wind speed decreases as the blockage ratio increases, but the difference is that bending-torsion coupled flutter and torsional flutter occur at lower and larger angles of attack respectively. Finally, the correction formula of critical wind speed at 0° angle of attack is given, which can provide reference for wind resistance design of streamlined box girders in practical engineering.

• Journal of the Korean Society of Fisheries and Ocean Technology
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• 제21권2호
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• pp.137-141
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• 1985

In this paper, the author investigate viscous effects on the characteristics of TP620 hydrofoil. The pressure distribution on the foil section in non-cavitating flow should be considered its characteristics of displacement thickness due to viscous effects. Theoretical potential theory, which neglects viscous effects do not agree with this analysis, especially at leading edge region of the foil. And, the efficiency of TP620 hydrofoil considering viscous effects is a little lower than that of the foil, which neglected viscous effects.

• KSII Transactions on Internet and Information Systems (TIIS)
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• 제10권11호
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• pp.5419-5436
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• 2016

Enhancing the performance of computing systems has been an important topic since the invention of computers. The leading-edge technologies of multicore and virtualization dramatically influence the development of current IT systems. We study performance attributes of response time (RT), throughput, efficiency, and scalability of a virtualized Web system running on a multicore server. We build virtual machines (VMs) for a Web application, and use distributed stress tests to measure RTs and throughputs under varied combinations of virtual cores (VCs) and VM instances. Their gains, efficiencies and scalabilities are also computed and compared. Our experimental and analytic results indicate: 1) A system can perform and scale much better by adopting multiple single-VC VMs than by single multiple-VC VM. 2) The system capacity gain is proportional to the number of VM instances run, but not proportional to the number of VCs allocated in a VM. 3) A system with more VMs or VCs has higher physical CPU utilization, but lower vCPU utilization. 4) The maximum throughput gain is less than VM or VC gain. 5) Per-core computing efficiency does not correlate to the quality of VCs or VMs employed. The outcomes can provide valuable guidelines for selecting instance types provided by public Cloud providers and load balancing planning for Web systems.

• International Journal of Aeronautical and Space Sciences
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• 제16권1호
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• pp.8-18
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• 2015

A numerical simulation for a nonslender BWB UCAV configuration with a rounded leading edge and span of 1.0 m was performed to analyze its aerodynamic characteristics. Numerical results were compared with experimental data obtained at a free stream velocity of 50 m/s and at angles of attack from -4 to $26^{\circ}$. The Reynolds number, based on the mean chord length, is $1.25{\times}106$. 3D multi-block hexahedral grids are used to guarantee good grid quality and to efficiently resolve the boundary layer. Menter's shear stress transport model and two transition models (${\gamma}-Re_{\theta}$ model and ${\gamma}$ model) were used to assess the effect of the laminar/turbulent transition on the flow characteristics. Aerodynamic coefficients, such as drag, lift, and the pitching moment, were compared with experimental data. Drag and lift coefficients of the UCAV were predicted well while the pitching moment coefficient was underpredicted at high angles of attack and influenced strongly by the selected turbulent models. After assessing the pressure distribution, skin friction lines and velocity field around UCAV configuration, it was found that the transition effect should be considered in the prediction of aerodynamic characteristics of vortical flow fields.