• The Journal of Korean Institute of Communications and Information Sciences
• /
• v.21 no.4
• /
• pp.1077-1088
• /
• 1996

In this paper, we propose a new method to analyze the performance degradation by timing jitters in the receivers of intensity modulation/direct detection digital optical communication systems where pulse-shaping filters are used to minimize intersymbol interference. The results obtained from the proposed analytical method show that conventional analytical methods underestimate the influence of timing jitters on the receiver performance. Using the proposed anlaytical method, we derive an analytic equation for approximated power penalty due to timing itters and obtain an exact power penalty by numerical analyses. Assuming Gaussian or uniform probability density function for timing jitters, we also show that assumption of Gaussian distribution for timing jitters yields more performance degration than that of uniform distribution.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
• /
• 1997.04a
• /
• pp.701-708
• /
• 1997

A direct boundary element method(DBEM) is developed for thin bodies whose surfaces are rigid or compliant. Th eHelmholtz integral equation and its normal derivative integral equation are adopted simultaneously to calculate the pressure on both sides of the thin body, instead of the jump values across it, to account for the different surface conditions of each side. Unlike the usual assumption, the normal velocity is assumed to be discontinuous across the thin body. In this approach, only the neutral surface of the thin body has to be discontinuous across the thin body. In this approach, only the neural surface of the thin body has to be discretized. The method is validated by comparison with analytic and/or numerical results for acoustic scattering and radiation from several surface conditions of the thin body; the surfaces are rigid when stationary or vibrating, and part of the interior surface is lined with a sound-absorbing material.

• Structural Engineering and Mechanics
• /
• v.24 no.6
• /
• pp.699-707
• /
• 2006

In the paper, a direct method of solution of the Navier equation is presented. An orthotropic thick hollow cylinder under a one-dimensional steady-state temperature distribution and a uniform magnetic field with general types of thermal and mechanical boundary conditions is considered. The Navier equation in terms of displacement is derived and solved analytically by the direct method, and magnetothermoelastic responses and perturbation of the magnetic field vector in the orthotropic thick hollow cylinder is described. The present method is suitable for orthotropic thick hollow cylinders placed in an axial magnetic field with arbitrary thermal and mechanical boundary conditions. Finally, numerical examples are carried out and discussed.

• 유체기계공업학회:학술대회논문집
• /
• 2003.12a
• /
• pp.671-677
• /
• 2003

Two-dimensional direct numerical simulation of the edge-tones by the finite difference lattice Boltzmann method (FDLBM) is presented. We use a new lattice BGK compressible fluid model that has an additional term and allow larger time increment comparing the conventional FDLBM, and also use a boundary fitted coordinates. We have succeeded in capturing very small pressure fluctuations result from periodically oscillation of jet around the edge. That pressure fluctuations propagate with the sound speed. It is clarified that the sound wave generated in rather wide region and individual vortices do not affect the sound wave propagation.

• Steel and Composite Structures
• /
• v.51 no.3
• /
• pp.225-241
• /
• 2024

For several reasons, cold-formed steel (CFS) beams are often manufactured with holes. Nevertheless, because of holes, the reduction in the web area causes a decrease in the bending strength. Edge stiffeners are presently added around the holes to improve the bending strength of flexural members. Therefore, this research studies CFSZ-beams with stiffened holes and investigates how edge stiffener affects bending strength and failure modes. Nonlinear analysis was carried out using ABAQUS software and the developed finite element (FE) model was verified against tests from previous studies. Using the verified FE model, a parametric study of 104 FE models was conducted to investigate the influence of key parameters on bending strength of Z- sections. The results indicated that the effect of holes is less noticeable in very thin Z-sections. Moreover, adding edge stiffeners around the holes improves the flexural capacity of Z-beams and sometimes restores the original bending capacity. Because the computational techniques used to solve the CFS buckling mode with stiffened holes are still unclear, a numerical method using constrained and unconstrained finite strip method (CUFSM) software was proposed to predict the elastic distortional buckling moment for a wide variety of CFSZ-sections with stiffened holes. A numerical method with two procedures was applied and validated. Upon comparison, the numerical method accurately predicted the distortional buckling moment of CFS Z-sections with stiffened holes.

• Journal of Korean Society of Coastal and Ocean Engineers
• /
• v.14 no.1
• /
• pp.19-33
• /
• 2002

A numerical method to efficiently secure necessary design informations of the hydraulic characteristics of rubble mound breakwater was attempted here. The method combines the exterior wave field with the interior wave field which is formulated incorporating porous media flow inside the breakwaters. An approximate method based on the long wave assumption was used for the exterior wave field while a boundary element method was used for the interior wave field. A hydraulic experiment was also performed to verify the validity of the numerical analysis. The numerical results were compared with experimental data and results from existing formulae. They generally agreed in both reflection and transmission coefficients. The calculated pore pressures also showed a similar pattern with experimental data, even if they gave some significant differences in their values fur some cases. The main cause of such differences can be attributed to the strongly nonlinear wave field occurring on the frontal slope of the breakwater. The direct input of dynamic pressures(measured from hydraulic experiment) into the numerical method was suggested as a promising method to enhance the predictability of pore pressures.

• Structural Engineering and Mechanics
• /
• v.11 no.2
• /
• pp.171-184
• /
• 2001

A direct discrete formulation suitable for the nonlinear analysis of masonry structures is presented. The numerical approach requires a pair of dual meshes, one for describing displacement fields, one for imposing equilibrium. Forces and displacements are directly used (instead of having to resort to a model derived from a set of differential equations). Associated and nonassociated flow laws are dealt with within a complementarity framework. The main features of the method and of the relevant computer code are discussed. Numerical examples are presented, showing that the numerical approach is able to describe plastic strains, damage effects and crack patterns in masonry structures.

• Journal of the Korea Institute of Information and Communication Engineering
• /
• v.7 no.2
• /
• pp.284-292
• /
• 2003

In this paper, we carried out the numerical examination of the initial value setting-up method to extend the range of optimal step parameter in a adaptive system which is controlled by LMS algorithm. For initial value setting-up methods, the general method which select the initial value randomly and the other method which applies the approximate value obtained from the direct method to initial value, were used. And then, we compared to the ranges of step parameter setting, the convergence speeds of mean-square-error, and the stabilities during the convergence process when the initial values were applied to the optimal directivity synthesis problem. According to the numerical simulation results, the initial value setting-up method by means of the direct method provides wider range for the step parameter, more efficient capability for convergence and stability, and more error correction ability than the general method.

• Journal of The Korean Astronomical Society
• /
• v.48 no.1
• /
• pp.67-73
• /
• 2015

We present a novel method to implement time-delayed propagation of radiation fields in cosmological radiative transfer simulations. Time-delayed propagation of radiation fields requires construction of retarded-time fields by tracking the location and lifetime of radiation sources along the corresponding light-cones. Cosmological radiative transfer simulations have, until now, ignored this "light-cone effect" or implemented ray-tracing methods that are computationally demanding. We show that radiative transfer calculation of the time-delayed fields can be easily achieved in numerical simulations when periodic boundary conditions are used, by calculating the time-discretized retarded-time Green's function using the Fast Fourier Transform (FFT) method and convolving it with the source distribution. We also present a direct application of this method to the long-range radiation field of Lyman-Werner band photons, which is important in the high-redshift astrophysics with first stars.

• Transactions of the Korean Society for Noise and Vibration Engineering
• /
• v.16 no.2 s.107
• /
• pp.132-140
• /
• 2006

This paper investigates the characteristics of dynamic responses of floating structures with connections under sea wave loads. Direct method using higher order boundary element method (HOBEM) and finite element method (FEM) is adopted for numerical analysis. A 500 m-long and 250-m width very large floating structure (VLFS) with four units are considered in numerical analysis. Hinge connection and spring connection with various strength are considered as connection types. Displacements and stresses of VLFS according to the connection types are compared considering wave period and heading angle reduction.