• Transactions of the Korean hydrogen and new energy society
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• v.23 no.4
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• pp.293-299
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• 2012

The serpentine flow channel is widely used in polymer electrolyte membrane fuel cells (PEMFCs) to prevent flooding phenomena because it effectively removes liquid water in the flow channel. The pressure drop between inlet and outlet increases as compared with straight channels due to minor losses associated with the corners of the turning configurations. This results in a strong pressure gradient between adjacent channels in specific regions, where some amount of reactant gas can be delivered to catalyst layers by convection through a gas diffusion layer (GDL). The enhancement of the convective flow in the GDL, so-called bypass flow, affects fuel cell performance since the bypass flow influences the reactant transport and thus its concentration over the active area. In the present paper, for the bipolar plate design, a simple analytic model has been proposed to predict the bypass flow in the serpentine type flow channels and validated with three-dimensional numerical simulation results.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 1998.04a
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• pp.13-13
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• 1998

A simple model of the tip leakage flow models of the rotor downstream flow is developed, based on Lakshminarayana's theoretical concept on the tip clearance flow and the experimental data published in open literature. And new spanwise distribution models of deviation angle and pressure loss coefficient due to the tip leakage flow are formulated for use in association with the streamline curvature method as a through flow analysis. Combining these new models and previous deviation and loss models due to secondary flow, a robust streamline curvature method is established for flow analysis of single-stage, subsonic axial turbines with wide ranges of turning angle, aspect ratio and blading type. At the exit from rotor rows, the flow variables are mixed radially according to a spanwise transport equation. The proposed streamline curvature method is tested against a forced vortex type turbine as well as a free vortex type one. The results show that the spanwise variations of flow angle, axial velocity and loss coefficients at rotor exit are predicted with good accuracy, being comparable to a steady three-dimensional Navier-Stokes analysis. This simple and fast flow analysis is found to be very useful for the turbine design at the initial design phase.

• Journal of the Korean Society for Precision Engineering
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• v.22 no.11 s.176
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• pp.151-158
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• 2005

Micro-machine tool is essential in the micro/meso cutting for the sake of saving of space, resources, and energy. In this research, a micro-turning lathe was fabricated with piezoelectric feed drive mechanism, and motion of each axis was generated by stepwise mechanism with two piezo actuators. The resolution to drive the axis was $0.05{\mu}m$ and position accuracy less than $2{\mu}m$ was assured. From the positioning experiment, piezo feed mechanism is good enough for the micro machine tools. Many fuming experiments were carried out with diamond-cutting tools to evaluate cutting capability of a machine tool. Continuous flow type chip could be obtained even if the cutting speed was very low due to small diameter of workpiece. However, thorough investigation about machineability in micro/meso cutting is inevitable to assure high quality surface roughness in micro machine tool.

• Proceedings of the Korean Society of Machine Tool Engineers Conference
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• 1999.10a
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• pp.359-364
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• 1999

Due to rapid growth of die and mould industries, it is urgently required to maximize the productivity and the efficiency of machining. In recent years, owing to the development of new kinds of material, die and mould materials are much harder and it is more difficult to cut. In this study, the workpiece SKD11(HRC45) is cut with TiAlN coated tungsten-carbide cutting tools. To find the general characteristics of difficult-to-cut materials, orthogonal turning test is performed. Orthogonal cutting theory can be expanded to oblique cutting model. The oblique cutting process in the small cutting edge element has been analyzed as orthogonal cutting process in the plane containing the cutting velocity vector and chip-flow vector. Hence, with the orthogonal cutting data obtained from orthogonal turning test, the cutting forces can be analyzed through oblique cutting model. The simulation results have shown a fairy good agreement with the test results.

• 유체기계공업학회:학술대회논문집
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• 2005.12a
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• pp.173-177
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• 2005

The heat/mass transfer characteristics on the plane tip surface of a high-turning first-stage turbine rotor blade has been investigated by employing the naphthalene sublimation technique. The heat/mass transfer coefficient is measured for four tip clearance height-to-chord ratios of h/c = 1.0%, 2.0%, 3.0%, and 4% at the Reynolds number of $2.09{\times}105$. The result shows that at lower h/c, there exists a strong flow separation/re-attachment process, which results in severe thermal load along the pressure-side comer. As h/c increases, the re-attachment is occurred further downstream of the pressure-side comer with lower thermal load. At higher h/c, a pair of vortices on the tip surface near the leading edge are found along the pressure-side and suction-side comers, and the pressure-side tip vortex have significant influence even on the mid-chord local heat transfer.

• Proceedings of the Korean Society of Machine Tool Engineers Conference
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• 1995.03a
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• pp.152-157
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• 1995

In Turning It is good selection of cutting condition and cutting tools that influence upon the accuracy of dimension manufacturing efficiency and extension of tool life. Among them especially the identification of cutting force due to the change of cutting conditions which exerts a great influence on the turning is very important. In this study the cutting resistance due to the change of cutting conditions was caculated by using the energy method and good agreement in shown between theoritical and experimental results which were tested for the cutting resistance at the cemented carbide cutting tools with workpieces of SM20C and SM 45C.

• Ocean Systems Engineering
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• v.7 no.4
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• pp.435-460
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• 2017

The main objective of this study is to investigate the turning and zig-zag maneuvering performance of the well-known naval surface combatant DTMB (David Taylor Model Basin) 5415 hull with URANS (Unsteady Reynolds-averaged Navier-Stokes) method. Numerical simulations of static drift tests have been performed by a commercial RANS solver based on a finite volume method (FVM) in an unsteady manner. The fluid flow is considered as 3-D, incompressible and fully turbulent. Hydrodynamic analyses have been carried out for a fixed Froude number 0.28. During the analyses, the free surface effects have been taken into account using VOF (Volume of Fluid) method and the hull is considered as fixed. First, the code has been validated with the available experimental data in literature. After validation, static drift, static rudder and drift and rudder tests have been simulated. The forces and moments acting on the hull have been computed with URANS approach. Numerical results have been applied to determine the hydrodynamic maneuvering coefficients, such as, velocity terms and rudder terms. The acceleration, angular velocity and cross-coupled terms have been taken from the available experimental data. A computer program has been developed to apply a fast maneuvering simulation technique. Abkowitz's non-linear mathematical model has been used to calculate the forces and moment acting on the hull during the maneuvering motion. Euler method on the other hand has been applied to solve the simultaneous differential equations. Turning and zig-zag maneuvering simulations have been carried out and the maneuvering characteristics have been determined and the numerical simulation results have been compared with the available data in literature. In addition, viscous effects have been investigated using Eulerian approach for several static drift cases.

• 한국전산유체공학회:학술대회논문집
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• 2011.05a
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• pp.331-336
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• 2011

The flow fields over multi-element airfoils with lift-enhancing flat-plate tabs were numerically investigated. Common choice of the height of the lift-enhancing tabs usually ranges from 0.25% to 1.25% of the reference airfoil chord, and in this study the effect of the position of the tab with l%-chord height was studied by varying the distance of the tab from the trailing edge ranging from 0.5% to 2% of the reference chord. In this paper, the effects of lift-enhancing tabs with various position were studied at a constant Reynolds number on a two-element airfoil with a slotted flap. Computed streamlines show that the additional turning caused by the tab reduces the amount of separated flow on the flap.

• Transactions of Materials Processing
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• v.20 no.8
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• pp.611-618
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• 2011

Flow forming is an incremental forming process in which rollers are used to form cylindrical parts with repeated turning of both roller and starting material. Both sheet and tube can be used as the starting material. The process is highly useful for producing hollow shaped parts from a tube, with the benefit of the average strain in the final shape being significantly lower than that from a sheet material. In the present study, the flow forming process was studied and optimized for producing a hollow shaped part from seamless steel tube by both experiment and numerical analysis. Upon considering the difficulty of forming seamless steel sheet, the thickness reduction was distributed over several tool paths. In the end, an optimum process condition was attained, and the experiment verified the simulation results.

• Advances in aircraft and spacecraft science
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• v.9 no.5
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• pp.415-431
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• 2022

Secondary flows have a huge impact on losses generation in modern low pressure gas turbines (LPTs). At design point, the interaction of the blade profile with the end-wall boundary layer is responsible for up to 40% of total losses. Therefore, predicting accurately the end-wall flow field in a LPT is extremely important in the industrial design phase. Since the inlet boundary layer profile is one of the factors which most affects the evolution of secondary flows, the first main objective of the present work is to investigate the impact of two different inlet conditions on the end-wall flow field of the T106A, a well known LPT cascade. The first condition, labeled in the paper as C1, is represented by uniform conditions at the inlet plane and the second, C2, by a flow characterized by a defined inlet boundary layer profile. The code used for the simulations is based on the Discontinuous Galerkin (DG) formulation and solves the Reynolds-averaged Navier-Stokes (RANS) equations coupled with the Spalart Allmaras turbulence model. Secondly, this work aims at estimating the influence of viscosity and turbulence on the T106A end-wall flow field. In order to do so, RANS results are compared with those obtained from an inviscid simulation with a prescribed inlet total pressure profile, which mimics a boundary layer. A comparison between C1 and C2 results highlights an influence of secondary flows on the flow field up to a significant distance from the end-wall. In particular, the C2 end-wall flow field appears to be characterized by greater over turning and under turning angles and higher total pressure losses. Furthermore, the C2 simulated flow field shows good agreement with experimental and numerical data available in literature. The C2 and inviscid Euler computed flow fields, although globally comparable, present evident differences. The cascade passage simulated with inviscid flow is mainly dominated by a single large and homogeneous vortex structure, less stretched in the spanwise direction and closer to the end-wall than vortical structures computed by compressible flow simulation. It is reasonable, then, asserting that for the chosen test case a great part of the secondary flows details is strongly dependent on viscous phenomena and turbulence.