• International Journal of Fluid Machinery and Systems
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• v.3 no.1
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• pp.11-19
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• 2010

The performance of the crossover system of a centrifugal compressor stage consisting of static components of $180^{\circ}$ U-bend, return channel vanes and exit ducting with a $90^{\circ}$ bend is investigated. This study is confined to the assessment of performance of the crossover system by varying the shape of the return channel vanes. For this purpose two different types of Return Channel Vanes (RCV1 and RCV2) were experimentally investigated. The performance of the crossover system is discussed in terms of total pressure loss coefficient, static pressure recovery coefficient and vane surface pressure distribution. The experimentation was carried out on a test setup in which static swirl vanes were used to simulate the flow at the exit of an actual centrifugal compressor impeller with a design flow coefficient of 0.053. The swirl vanes are connected to a mechanism with which the flow angle at the inlet of U-bend could be altered. The measurements were taken at five different operating conditions varying from 70% to 120% of design flow rate. On an overall assessment RCV1 is found to give better performance in comparison to RCV2 for different U-bend inlet flow angles. The performance of RCV2 was verified using numerical studies with the help of a CFD Code. Three dimensional sector models were used for simulating the flow through the crossover system. The turbulence was predicted with standard k-$\varepsilon$, 2-equation model. The iso-Mach contour plots on different planes and development of secondary flows were visualized through this study.

• Proceedings of the Korean Vacuum Society Conference
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• 2010.02a
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• pp.381-381
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• 2010

$(ZrO_2)_{0.66}(HfO_2)_{0.34}$ thin films as gate dielectrics have been proposed to overcome the problems of tunneling current and degradation mobility inachieving a thin equivalent oxide thickness. An extremely thin $SiO_2$ layer is used in order to separate the carrier in MOSFET channel from the dielectric field fluctuation caused by phonons in the dielectric which decreases the carrier mobility. The electronic and optical properties influenced the device performance to a great extent. $(ZrO_2)_{0.66}(HfO_2)_{0.34}$ dielectric films on p-Si (100) were grown by atomic layer deposition method, for which the conduction band offsets, valence band offsets and band gapswere obtained by using X-ray photoelectron spectroscopy and reflection electron energy loss spectroscopy. The band gap, valence and conduction band offset values for $(ZrO_2)_{0.66}(HfO_2)_{0.34}$ dielectric thin film, grown on Si substrate were about 5.34, 2.35 and 1.87 eV respectively. This band alignment was similar to that of $ZrO_2$. In addition, The dielectric function (k, $\omega$), index of refraction n and the extinction coefficient k for the $(ZrO_2)_{0.66}(HfO_2)_{0.34}$ thin films were obtained from a quantitative analysis of REELS data by comparison to detailed dielectric response model calculations using the QUEELS-$\varepsilon$(k, $\omega$)-REELS software package. These optical properties are similar with $ZrO_2$ dielectric thin films.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.26 no.2
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• pp.278-287
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• 2002

The objective of the paper was to calculate the pressure drop and to investigate the recirculation region of the conical orifices used in Kwang-yang Iron & Steel Company. The flow field with water used as a working fluid was the turbulent flow for Reynolds number of 2$\times$10$^4$. The effective parameters fur the pressure drop and the recirculation region were the conical orifice\`s inclined angle ($\theta$) against the wall, the interval(S) between orifices, the relative angle of rotation($\alpha$) of the orifices, the shape of the orifice's hole(circle, rectangle, triangle) having the same area, the number(N) of the orifice's holes having the same mass flow rate, and the thickness(t) of the orifices. It was fecund that the shape of the orifice's hole, the number of the orifice's holes and the thickness of the orifice affected the total pressure drop a lot and that the conical orifice's inclined angle against the wall, the relative angle of rotation of the orifices, the number of the orifice's holes and the thickness of the orifices affected the center location of the recirculation region. The PISO algorithm with FLUENT code was employed to analyze the flow field.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.37 no.12
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• pp.1503-1512
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• 2013

In this study, we proposed an indentation evaluation method for fracture toughness using cohesive finite element simulations. First, we examined the effect of material properties (yield strain, Poisson's ratio, and elastic modulus) on crack size during Vickers indentation and then generated a regression formula that explains the relations among fracture toughness, indentation load, and crack size. We also proposed another indentation formula for fracture toughness evaluation using the contact size a and E/H (H: hardness). Finally, we examined the relation between the crack size and the indenter shapes. Based on this, we can generate from the formula obtained using the Vickers indenter a formula for an indenter of different shapes. Using the proposed method, fracture toughness is directly estimated from indentation data.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2008.11a
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• pp.299-304
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• 2008

Starting characteristics of the axi-symmetric second throat exhaust diffuser (STED) with zero-secondary flows are numerically investigated. Renolds-Average Navier-Stokes equations with a standard ${\kappa}-{\varepsilon}$ turbulence model incorporated with enhanced wall treatment are solved to simulate the diffusing evolutions of the nozzle plume. Minimum (optimum) starting pressure difference of 20$\sim$25% between 1-D theory and the measured data validated from previous results[5] is also applied to predict the range of an effective diffuser expansion ratio (Ad/At) in this system.

• Steel and Composite Structures
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• v.31 no.5
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• pp.503-516
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• 2019

This work presents a dynamic investigation of functionally graded (FG) plates resting on elastic foundation using a simple quasi-3D higher shear deformation theory (quasi-3D HSDT) in which the stretching effect is considered. The culmination of this theory is that in addition to taking into account the effect of thickness extension (${\varepsilon}_z{\neq}0$), the kinematic is defined with only 4 unknowns, which is even lower than the first order shear deformation theory (FSDT). The elastic foundation is included in the formulation using the Pasternak mathematical model. The governing equations are deduced through the Hamilton's principle. These equations are then solved via closed-type solutions of the Navier type. The fundamental frequencies are predicted by solving the eigenvalue problem. The degree of accuracy of present solutions can be shown by comparing it to the 3D solution and other closed-form solutions available in the literature.

• Journal of the Korea Institute of Information Security & Cryptology
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• v.21 no.3
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• pp.67-74
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• 2011

In this paper, we presented a dynamic cyber attack tree which can describe an attack scenario flexibly for an active cyber attack model could be detected complex and transformed attack method. An attack tree provides a formal and methodical route of describing the security safeguard on varying attacks against network system. The existent attack tree can describe attack scenario as using vertex, edge and composition. But an attack tree has the limitations to express complex and new attack due to the restriction of attack tree's attributes. We solved the limitations of the existent attack tree as adding an threat occurrence probability and 2 components of composition in the attributes. Firstly, we improved the flexibility to describe complex and transformed attack method, and reduced the ambiguity of attack sequence, as reinforcing composition. And we can identify the risk level of attack at each attack phase from child node to parent node as adding an threat occurrence probability.

• Journal of the Korean Society of Industry Convergence
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• v.25 no.1
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• pp.61-70
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• 2022

Fire damage time in high-rise buildings and wildland fire increasing every year. The use of high-pressure fire pumps is required to effectively extinguish fires. Reflecting the curvature effect of the fire hose occurring at the actual fire fighting site, this study provides a database of pressure drop, discharge velocity and maximum discharge height through C FD numerical analysis and it can provide using standards for fire extinguishing. Two Reynolds numbers of 200000 and 400000 were numerically analyzed at 0° -180° bending with water of 25℃ as a working fluid in hoses with a diameter of 65mm, a length of 15m, and a radius of curvature of 130mm. Realizable k-ε turbulence model was used and standard wall function was used. The pressure drop increases as the bending angle increases, and the maximum value at 90° and then decreases. The increasing rate is greater than the decrease. The velocity of the secondary flow also decreases after having the maximum value at 90°. The decreasing rate is greater than the increase. The turbulent kinetic energy increases to 120° and decreases with the maximum value. Pressure drop, velocity of the secondary flow, and turbulence kinetic energy are measured larger in the second bending region than in the first bending region.

• Journal of Biomedical Engineering Research
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• v.25 no.5
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• pp.403-414
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• 2004

In the accompanying paper, we proposed a real. time volumetric imaging method using a cross array based on receive dynamic focusing and synthetic aperture focusing along lateral and elevational directions, respetively. But synthetic aperture methods using spherical waves are subject to beam spreading with increasing depth due to the wave diffraction phenomenon. Moreover, since the proposed method uses only one element for each transmission, it has a limited transmit power. To overcome these limitations, we propose a new real. time volumetric imaging method using cross arrays based on synthetic aperture technique with linear wave fronts. In the proposed method, linear wave fronts having different angles on the horizontal plane is transmitted successively from all transmit array elements. On receive, by employing the conventional dynamic focusing and synthetic aperture methods along lateral and elevational directions, respectively, ultrasound waves can be focused effectively at all imaging points. Mathematical analysis and computer simulation results show that the proposed method can provide uniform elevational resolution over a large depth of field. Especially, since the new method can construct a volume image with a limited number of transmit receive events using a full transmit aperture, it is suitable for real-time 3D imaging with high transmit power and volume rate.

• Journal of Energy Engineering
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• v.8 no.2
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• pp.224-232
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• 1999

Latent heat storage system using micro-encapsuled phase change material is effective method for floor heating of house and building. The temperature profile in capsule block and flow rate of hot water are important parameters for the development of heat storage system. In the present study, a mathematical model based on 3-D, non-steady state, Navier-Stokes equations, scalar conservation equations and turbulence model ($\kappa$-$\varepsilon$), is used to predict the temperature profiles in capsule and the velocity vectors in hot water pipe. The multi-block grids and fine grids embedding are used to join the circle in hot water pipe and square in capsule block. The phase change process of the capsule is quite complex not only because the size of phase change material is very small, but also because phase change material is mixed with the cement to form thermal storage block. In calculation, it's assumed that the phenomena of phase change is limited only the thermal properties of phase change material and the change of boundary is not happened in capsule. The purpose of this study is to calculate the temperature profiles in capsule block and velocity vectors in hot water pipe using the numerical calculation. Two kinds of thermal boundary condition were considered, the first (case 1) is the adiabatic condition for the both outside surfaces of the wall, the second (case 2) is the case in which one surface is natural convection with atmosphere and another surface is adaibatic. Calculation results are shown that the temperature profile in capsule block for case 1 is higher than that for case 2 due to less heat loss in adaibatic surface. Specially, in the domain of near Y=0, the difference of temperature is greater in case 1 than in case 2. The detailed experimental data of capsule block on the temperature profile and the thermal properties such as specific heat and coefficient of heat transfer with the various temperature are required to predict more exact phenomena of heat transfer.