• Transactions of the Korean Society of Mechanical Engineers
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• v.16 no.10
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• pp.1971-1978
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• 1992

A turbulent flow in a conical diffuser with total divergence angle of 8.deg. was numerically studied. The low Reynolds number k-.epsilon. model(Launder-Sharma model) was adopted to simulate the turbulence. The continuity and time averaged Navier-Stokes equations in a nonorthogonal coordinate system were solved by a finite volume method based on the fully elliptic formulation. The low Reynolds number k-.epsilon. model reasonably simulates the pressure recovery and the mean velocity components. However, there are also considerable discrepancies between predicted and measured shear stress distribution on the wall and turbulent kinetic energy distributions. It is necessary to investigate the flow structure at the entry of the diffuser, numerically as well as experimentally.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.68 no.2
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• pp.262-268
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• 2019

Previous dynamic models of hysteresis motor use an extended induction machine equivalent circuit or somewhat different equivalent circuit with conventional one, which makes unsatisfiable results. In this paper, the hysteresis dynamic characteristics of the motor rotor are analyzed using the inverse Preisach model and the hysteresis motor equivalent circuit considering eddy current effect. The hysteresis loop for the rotor ring is analyzed under full-load voltage source static state. The calculated hysteresis loop is then approximated to an ellipse for simplicity of dynamic computation. The permeability and delay angle of the elliptic loop apply to the dynamic analysis model. As a result, it is possible to dynamically analyze the hysteresis motor according to the applied voltage and the rotor material. With this method, the motor speed, generated torque, load angle, rotor current as well as synchronous entry time, hunting effect can be calculated.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.33 no.9
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• pp.1-8
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• 2005

This paper explored the effects of separation control through the use of pulsating jet blowing on a two dimensional elliptical airfoil. To develop an active control technique of flow separation, a flow control actuator utilizing continuous/pulsed jet of pressurized air was designed and installed in a wind tunnel testing model of elliptic wing. PIV measurement and flow visualization of the wing near field were conducted to access the feasibility and effectiveness of the pulsed jet blowing on controlling the stall of the elliptical wing in subsonic flow. PIV experimental results show that separation control can provide significant reduction in turbulent flow wake and separation bubbles by jet blowing. The pulsating jet blowing is more effective on the separation control than continuous one. Increased jet frequency suppressed the turbulent separated flow wake effectively at even higher AOAs.

• Journal of the Society of Naval Architects of Korea
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• v.30 no.1
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• pp.94-103
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• 1993

This study deals with an investigation on the tip vortex generated by an elliptic wing with section shapes of NACA 0020. The flow structure on the wing surface is investigated by using tufts test as well as observing the cavitation pattern. The surface pressure on a foil surface is measured to complement the visualized flow field. Results show that a strong spanwise pressure gradient is a definite contributor on the formation of tip vorex, and the fluids from both sides contribute to the evolutionary process of tip vortex. On the other hand, a series of experiments are conducted to investigate the detailed structure of tip-vortex at various angles of attack. The tip-vortex formation and development are observed by producing a cavitation, and then by a laser sheet technique in conduction with a dye injection method. The shape of tip-vortex and the distance between a vortex core and the trailing vortex sheet are found to vary with the angle of attack. Overall features of tip flow are evaluated to complement the vortex model based on inviscid theory.

• Bulletin of the Society of Naval Architects of Korea
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• v.11 no.1
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• pp.9-16
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• 1974

To contribute towards more accurate estimation of the virtual inertia coefficient for the horizontal vibration of ships, three dimensional correction factor $J_H$ for the added mass of finitely long elliptic prismatic bars in horizontal vibration in a free surface of an ideal fluid are calculated. In the problem formulation Dr. T. Kumai's quasi-finite length concept[1,11,12] is employed. Now that, in Dr. Kumai's work[1] for the horizontal vibration the mathematical model was a circular cylinder, the principal aim of the authors' work is to investigate the influence of the beam-draft ratio B/T on $J_H$. The numerical results of this work are shown in Fig.3 graphically, from which we may recognize that the influence of B/T on $J_H$ is remarkable as much as that of the length-draft ratio L/T(refer to Fig.1 also). In Fig.3 the curves for B/T=2.00 are of those based on Dr. Kumai's result[1]. On the other hand, the experimental data obtained by Burril et al.[9] for the horizontal vibration of finitely long prismatic bars of various cross-section shapes are compared with the theoretical added mass coefficients defined by combination of the authors' $J_H$ from Fig.3 and two dimensional coefficients $C_H$ obtained by Lewis form approximation for the corresponding sections. They are in reasonable correspondence with each other as shown in Fig.2. Finally, considering that the longitudinal profile of full-form ship's hull is well resembled to that of an elliptic cylinder and that the influences of other factors such as the sectional area coefficient and the shape of section contour itself can be well merged in the two dimensional added mass coefficient, the authors recommend that the data given in Fig.3 may be successfully adopted for the three dimensional correction factor the added mass in the horizontal vibration of hull-form ships.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.36 no.11B
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• pp.1305-1314
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• 2011

Initial entry section has vulnerability which exposes plain text parameter in Mobile WIMAX environment which is the 4th generation technology. Each node message need to be encrypted to prevent the third party attack or message leakage. In this paper, we propose Mobile WIMAX initial entry section encryption using Elliptic Curve Cryptosystem. We have compared proposed model with existing model using OPNET simulator tool. He delay rate has increased little bit in initial entry section than before after the comparison, but it has shown more effective in average delay and throughput than encryption applied other existing model.

• ETRI Journal
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• v.22 no.4
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• pp.25-31
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• 2000

Design of secure and efficient public-key encryption schemes under weaker computational assumptions has been regarded as an important and challenging task. As far as ElGamal-type encryption schemes are concerned, some variants of the original ElGamal encryption scheme based on weaker computational assumption have been proposed: Although security of the ElGamal variant of Fujisaki-Okamoto public -key encryption scheme and Cramer and Shoup's encryption scheme is based on the Decisional Diffie-Hellman Assumption (DDH-A), security of the recent Pointcheval's ElGamal encryption variant is based on the Computational Diffie-Hellman Assumption (CDH-A), which is known to be weaker than DDH-A. In this paper, we propose new ElGamal encryption variants whose security is based on CDH-A and the Elliptic Curve Computational Diffie-Hellman Assumption (EC-CDH-A). Also, we show that the proposed variants are secure against the adaptive chosen-ciphertext attack in the random oracle model. An important feature of the proposed variants is length-efficiency which provides shorter ciphertexts than those of other schemes.

• 한국지구물리탐사학회:학술대회논문집
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• 2005.05a
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• pp.233-236
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• 2005

Polarization filtering based on the multicomponent complex trace analysis method is performed to reject Rayleigh waves and to enhance S waves. To test the polarization filter, synthetic seismic data were constructed for a simple two-layer model based on the finite difference method. Rayleigh waves with elliptic motion are eliminated effectively and P and S waves with linear motions are well separated each other.

• Journal of Ocean Engineering and Technology
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• v.20 no.6 s.73
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• pp.61-66
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• 2006

Two iterative solvers are applied to solve the modified mild slope equation. The elliptic formulation of the governing equation is selected for numerical treatment because it is partly suited for complex wave fields, like those encountered inside harbors. The requirement that the computational model should be capable of dealing with a large problem domain is addressed by implementing and testing two iterative solvers, which are based on the Stabilized Bi-Conjugate Gradient Method (BiCGSTAB) and Generalized Conjugate Gradient Method (GCGM). The characteristics of the solvers are compared, using the results for Berkhoff's shoal test, used widely as a benchmark in coastal modeling. It is shown that the GCGM algorithm has a better convergence rate than BiCGSTAB, and preconditioning of these algorithms gives more than half a reduction of computational cost.

• Journal of the Korean Mathematical Society
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• v.40 no.3
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• pp.461-486
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• 2003

It is known that the CR geometries of Levi non-degen-erate hypersurfaces in $\C^n$ and of the elliptic or hyperbolic CR submanifolds of codimension two in $\C^4$ share many common features. In this paper, a special class of normalized coordinates is introduced for any CR manifold M which is one of the above three kinds and it is shown that the explicit expression in these coordinates of an isotropy automorphism $f{\in}Aut(M)_o {\subset}Aut(M),\;o{\in}M$, is equal to the expression of a corresponding element of the automorphism group of the homogeneous model. As an application of this property, an extension theorem for CR maps is obtained.