• Journal of Magnetics
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• v.21 no.3
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• pp.450-459
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• 2016

Experimental results and data of numerical simulations are presented, concerning generation of wideband radio frequency (RF) oscillations in a nonlinear transmission line (NLTL) which contains a pre-magnetized core of ferrite material. Emphasis is made on the means for extracting the RF signal from the line, in order to radiate it into free space. Antennas of two types that can be used for the purpose are considered, both featuring a coaxial design. This permits availing of the principal advantages of coaxial systems, such as operation in the mode of a traveling TEM wave; wide range of the transmitted frequencies, and a reasonably simple design. The antennas studied, specifically a disc-cone dipole (DCD) and an impulse reflector antenna ('Half-IRA', or HIRA type) differ significantly in effective width of the radiated spectrum and in spatial characteristics of the radiated field in far region.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.13 no.3
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• pp.254-261
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• 2001

It is well known that when long waves propagate from deep ocean onto a continental shelf with a very steep continental slope, the waves reflected from the shore can not propagate offshore and are re-reflected from the continental slope so that large water level fluctuations are induced near the shore. Liu(1986) has analyzed this phenomenon by assuming a topography which has a depth discontinuity along a semicircular offshore boundary, but his solution is erroneous. In the present paper, we correct his analytical solutions for a straight shoreline and a rectangular harbor. The corrected solution is then compared with the numerical results of the Galerkin finite element model of Jeong et al.(1998), which is based on the extended mild-slope equation.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.20 no.7
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• pp.2397-2408
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• 1996

A numerical study was made of the control of transient oscillatory flow modes in Czochralski convection. The reduction of temperature oscillation was achieved by changing the rotation rate of crystal rod, .OMEGA.$_{S}$=.OMEG $A_{S0}$(1+ $A_{S}$sin(2.pi. $f_{S}$/ $t_{p}$t)). The temporal behavior of oscillation flow was scrutinized over broad ranges of two parameters, i.e., the rotation amplitude( $A_{S}$.leq.0.5) and the nondimensional frequency (0.9.leq. $f_{S}$.leq.1.5). The mixed convection parameter was ranged 0.225.leq.Ra/PrR $e^{2}$.leq.0.929, which encompassed the buoyancy-and forced-dominant convection regimes. Computational results revealed that the temperature oscillations could be reduced effectively by a proper adjustment of the control parameters. The uniformity of temperature distribution near the crystal rod was examined. The control of oscillatory flow modes was also made for a realistic, low value of Pr.

• Journal of Advanced Marine Engineering and Technology
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• v.29 no.8
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• pp.929-937
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• 2005

A lattice BGK model based on a finite difference scheme with an internal degree of freedom is employed and it is shown that a diatomic 9as such as air is successfully simulated In a weak compressive wane problem and Coutte flow, the validity and characteristics of the applied model are examined. With the model. furthermore. we present a 2-dimensional edge tones to predict the frequency characteristics of discrete oscillations of a jet-edge feedback cycle by the FDLB model (I.D.F FDLBM) in which any specific heat ratio $\gamma$ can be chosen freely. The jet is chosen long enough in order to guaranteed the Parabolic velocity profile of a jet at the outlet. and the edges have of an angle of $\alpha$=$23^{0}$ and $20^{0}$. A sinuous instability wane with real frequency resulting from Periodic oscillation of the jet around the edge is propagated on the upper and lower of wedge.

• Structural Engineering and Mechanics
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• v.53 no.3
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• pp.443-453
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• 2015

Cable-beam structures characterized by variable stiffness nonlinearities are widely found in various structural engineering applications, for example in space deployable structures. Space deployable structures in orbit experience both high temperature caused by sun's radiation and low temperature by Earth's umbral shadow. The space temperature difference is above 300K at the moment of exiting or entering Earth's umbral shadow, which results in structural thermally induced vibration. To understand the thermally induced oscillations, the analytical expression of Boley parameter of cable-beam structures is firstly deduced. Then, the thermally induced vibration of cable-beam structures is analyzed using finite element method to verify the effectiveness of Boley parameter. Finally, by analyzing the obtained numerical results and the corresponding Boley parameters, it can be concluded that the derived expression of Boley parameter is valid to evaluate the occurrence conditions of thermally induced vibration of cable-beam structures and the key parameters influencing structural thermal flutter are the cable stiffness and thickness of beams.

• Journal of Korea Water Resources Association
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• v.42 no.10
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• pp.785-793
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• 2009

Numerical model which can simulate the surge tank for unsteady flow was developed in the present study. Furthermore, reliability model which can calculate the probability of pipe breakage regarding unsteady effect was developed. For the risk estimation of pipe breakage and functional estimation of surge tank, probability of pipe breakage for pipe network with surge tank was calculated regarding unsteady effect. From the results, it was found that unsteady flow significantly increase the probability of pipe breakage and surge tank considerably decrease probability of pipe breakage as damping out the pressure oscillations.

• Korea-Australia Rheology Journal
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• v.21 no.1
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• pp.47-58
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• 2009

We have investigated the transient behavior of 1D fully developed Poiseuille viscoelastic flow under finite pressure gradient described by the Oldroyd-B and Leonov constitutive equations. For analysis we employ a simple $2^{nd}$ order discretization scheme such as central difference for space and the Crank-Nicolson for time approximation. For the analysis of the Oldroyd-B model, we also apply the analytical solution, which is obtained again in this work in terms of elementary solution procedure simpler than the previous one (Waters and King, 1970). Both models demonstrate qualitatively similar solutions, but their eventual steady flowrate exhibits noticeable difference due to the absence or presence of shear thinning behavior. In the inertialess flow, the flowrate instantaneously attains a large value corresponding to the Newtonian creeping flow and then decreases to its steady value when the applied pressure gradient is low. However with finite liquid density the flow field shows severe fluctuation even accompanying reversals of flow directions. As the assigned pressure gradient increases, the flowrate achieves its steady value significantly higher than its value during oscillations after quite long period of time. We have also illustrated comparison between 1D and 2D results and possible mechanism of complex 2D flow rearrangement employing a previous solution of [mite element computation. In addition, we discuss some mathematical points regarding missing boundary conditions in 2D modeling due to the change of the type of differential equations when varying from inertialess to inertial flow.

• Journal of the Institute of Electronics Engineers of Korea TC
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• v.40 no.8
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• pp.340-348
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• 2003

In this paper, we propose a novel formulation to solve a time-domain combined field integral equation (CFIE) for analyzing the transient electromagnetic scattering response from closed conducting bodies. Instead of the conventional marching-on in time (MOT) technique, tile solution method in this paper is based on the moment method that involves separate spatial and temporal testing procedures. Triangular patch vector functions are used for spatial expansion and testing functions for three-dimensional arbitrarily shaped closed structures. The time-domain unknown coefficient is approximated as a basis function set that is derived from tile Laguerre functions with exponentially decaying functions. These basis functions are also used as the temporal testing. Numerical results computed by the proposed method arc stable without late-time oscillations and agree well with the frequency-domain CFIE solutions.

• Journal of applied mathematics & informatics
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• v.15 no.1_2
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• pp.29-51
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• 2004

In this paper, we propose a new mixed finite element method, called the characteristics-mixed method, for approximating the solution to Burgers' equation. This method is based upon a space-time variational form of Burgers' equation. The hyperbolic part of the equation is approximated along the characteristics in time and the diffusion part is approximated by a mixed finite element method of lowest order. The scheme is locally conservative since fluid is transported along the approximate characteristics on the discrete level and the test function can be piecewise constant. Our analysis show the new method approximate the scalar unknown and the vector flux optimally and simultaneously. We also show this scheme has much smaller time-truncation errors than those of standard methods. Numerical example is presented to show that the new scheme is easily implemented, shocks and boundary layers are handled with almost no oscillations. One of the contributions of the paper is to show how the optimal error estimates in $L^2(\Omega)$ are obtained which are much more difficult than in the standard finite element methods. These results seem to be new in the literature of finite element methods.

• Journal of Mechanical Science and Technology
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• v.19 no.9
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• pp.1790-1800
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• 2005

The examination on the operating mechanism of a pulsating heat pipe (PHP) using visualization revealed that the working fluid in the PHP oscillated to the axial direction by the contraction and expansion of vapor plugs. This contraction and expansion is due to the formation and extinction of bubbles in the evaporating and condensing section, respectively. In this paper, a theoretical model of PHP was presented. The theoretical model was based on the separated flow model with two liquid slugs and three vapor plugs. The results show that the diameter, surface tension and charge ratio of working fluid have significant effects on the performance of the PHP. The following conclusions were obtained. The periodic oscillations of liquid slugs and vapor plugs were obtained under specified parameters. When the hydraulic diameter of the PHP was increased to d=3mm, the frequency of oscillation decreased. By increasing the charging ratio from 40 to 60 by volume ratio, the pressure difference between the evaporating section and condensing section increased, the amplitude of oscillation reduced, and the oscillation frequency decreased. The working fluid with higher surface tension resulted in an increase in the amplitude and frequency of oscillation. Also the average temperature of vapor plugs decreased.