• Transactions of the Korean Society of Mechanical Engineers
• /
• v.14 no.6
• /
• pp.1408-1416
• /
• 1990

A powerful and efficient method for finding approximate solutions to initial-boundary-value problems in the transient coupled thermoelasticity is formulated in time domain using the finite element technique with time-marching strategy. The final system equations can be derived by the Guritin's variational principle using the definition of convolution integral. But, the finite element formulation for the equations of motion is modified by differentiating in time. Numerical results to some test problems are compared with analytical and other sophisticated approximate solutions. Stable responces are observed in all the given examples irrespective of incremental time steps and mesh shapes. In addition, it is shown that good numerical results are obtained even in coarser mesh or larger time step comparing to other numerical methods.

• Journal of the Korean Society for Aeronautical & Space Sciences
• /
• v.33 no.2
• /
• pp.54-59
• /
• 2005

Nonlinear aeroelastic analyses of a missile control fin are performed considering backlash and dynamic stiffness of actuator. Doublet-Hybrid method is used for the calculation of subsonic unsteady aerodynamic forces, and aerodynamic forces are approximated by the minimum-state approximation. For nonlinear flutter analysis backlash is represented by a free-play and is linearized by using the describing function method. Also, dynamic stiffness is function of frequency and is calculated by solving equation of motion for actuator. The linear and nonlinear flutter analyses show that the aeroelastic characteristics are significantly dependent on the backlash and dynamic stiffness. From the nonlinear flutter analysis, various types of limit cycle oscillations are observed in a range of air speeds below the linear divergent flutter boundary. The nonlinear flutter characteristics and the nonlinear aeroelastic responses are also investigated in the time domain.

• Journal of the Korean Geotechnical Society
• /
• v.21 no.8
• /
• pp.45-53
• /
• 2005

Shallow slope failures in residual soil during periods of prolonged infiltration are common over the world. Therefore, this study examines an approximate method to estimate the influence of infiltration on surficial stability of slopes by one-dimensional infiltration model. Modified GAML model based on the Green-Ampt model was extended to predict the infiltration behavior of two-layered slope. Then, the model has been considered to evaluate the likelihood of shallow slope failure which is induced by a particular rainfall event that accounts for the rainfall intensity and duration for various return periods in two-layered soil profile. The results obtained from the approximate method were compared with those obtained from numerical analyses. According to the results, with the use of properly estimated input parameters, the proposed method was found to give good results that agree reasonably well to those of the more rigorous finite element analyses.

• Journal of Broadcast Engineering
• /
• v.2 no.1
• /
• pp.1-7
• /
• 1997

In this paper, a microstrip patch antenna was analyzed by using FDTD method. Firstly, the electric field in the microstrip patch antenna was obtained by approximating a Maxwell's equation to a finite difference equation by means of Yee's algorithm. In this case, Mur's 1st approximation and dispersive boundary condition(BBC) were applied to an absorbing boundary condition. We also analyzed a single microstrip patch antenna by using the FDTD method, then calculating the propagative process in the wave of a return loss. Also, as the result that FDTD was applied to 2-array antenna designed to increase the gain of antenna, the measured results was in relatively good accordance with the values calculated by the FDTD method. The calculated impedance, return loss and VSWR were comparatively good. And these results were In relatively good accordance with the measured values.

• Journal of the Computational Structural Engineering Institute of Korea
• /
• v.15 no.4
• /
• pp.703-713
• /
• 2002

Recently, an improved EFG(Element-Free Galerkin) crack analysis technique, which includes a discontinuous approximation and a singular basis function on the auxiliary supports, was developed. The technique is able to accurately analyze the crack propagation problem without any modification of the analysis model; however, it shows some dependency on the analysis parameters used. In this study, the effect of analysis parameters such as the size of compact support, dilation parameter, the smoothness of shape function around the crack tip, and the number of node using auxiliary supports on the accuracy of solution has been investigated. Through a patch test with a crack, relative L₂ error norm of stresses and the stress intensity factor were computed and compared for various analysis parameters and the results were presented as guidelines for adequate choice of analysis parameters.

• KSCE Journal of Civil and Environmental Engineering Research
• /
• v.7 no.3
• /
• pp.241-248
• /
• 1987

The theoretical backgrounds of the several methods were surveyed and reviewed to fin out the adequate one to determine equivalent modal damping values in solving the dynamic problem of soil-structure interaction by mode superposition method. Furthermore the rigorous damping matrix of equation of motion was obtained through component mode synthesis technique and used in direct integration of the equation. The analytical results by direct integration method were compared with those of mode superposition approach using the various sets of equivalent modal damping values calculated by the methods to be reviewed. Two types of superstructures and four kinds of subsurface conditions were considered and combined to make soil-structure coupled models. It was realized that dissipating energy method gives the equivalent modal damping values which lead the most similar results to direct integration ones. In case of fixed base, the responses of all methods except stiffness weighted approach are almost equal to those of direct integration method.

• Journal of the Computational Structural Engineering Institute of Korea
• /
• v.30 no.4
• /
• pp.329-334
• /
• 2017

The structures, which are made up with the huge number of degrees-of-freedom and the assembly of substructures, have a great complexity. In order to increase the computational efficiency, the analysis models have to be simplified. Many substructuring techniques have been developed to simplify large-scale engineering problems. The techniques are very powerful for solving nonlinear problems which require many iterative calculations. In this paper, a modal derivatives-based model order reduction method, which is able to capture the stretching-bending coupling behavior in geometrically nonlinear systems, is adopted and investigated for its performance evaluation. The quadratic terms in nonlinear beam theory, such as Green-Lagrange strains, can be explained by the modal derivatives. They can be obtained by taking the modal directional derivatives of eigenmodes and form the second order terms of modal reduction basis. The method proposed is then applied to a co-rotational finite element formulation that is well-suited for geometrically nonlinear problems. Numerical results reveal that the end-shortening effect is very important, in which a conventional modal reduction method does not work unless the full model is used. It is demonstrated that the modal derivative approach yields the best compromised result and is very promising for substructuring large-scale geometrically nonlinear problems.

• Korean Journal of Optics and Photonics
• /
• v.8 no.4
• /
• pp.277-282
• /
• 1997

The complex refractive index of Diamond-like Carbon (DLC) thin films, which can be applied to optical devices or electrical devices, have been determined using optical methods. DLC thin films are grown on Si(100) substrates and vitreous silica substrates respectively, using the technique of plasma enhanced chemical vapor deposition (PECVD). The spectroscopic ellipsometry data($\psi$, $\Delta$) and the transmission spectra of these DLC films are obtained. These optical spectra are analyzed with the help of the Sellmeier dipersion relation and a quantum mechanically derived dispersion relation. Using spectroscopic ellipsometry data at their transparent region, the refractive index and the effective thickness of DLC films on vitreous silica are model calculated, Then the transmission spectra are inverted to yield the extinction coefficient spectra k(λ) at absorbing region. These spectra are fit to the quantum mechanical dispersion relation and the best fit dispersion constants are determined. The complex refractive indices are easily calculated with these constants. The spectroscopic ellipsometry data at the absorbing region in model calculated to give the packing densities and the degrees of surface microroughness of DLC films. Discussions are made in correlation with the growth condition of DLC films.

• Magazine of the Korea Concrete Institute
• /
• v.7 no.6
• /
• pp.186-196
• /
• 1995

In this study, parameters such as unit cement weight and placing temperature which influence on temperature rise and temperature rise velocity are investigated through adiabatic tests for the domestic ordinary portland cement(0PC). Adiabatic temperature rise suggested by Korean Concrete Spec. are compared with that from this experimental results. As a result of this study, adiabatic temperature rise of OPC suggested spec. is overestimated. Also it is shown that 2-parameter equation suggested in the spec. overestimate heat evolution at early age and reasonable prediction of heat evolution can be obtained by using 3-parameter equation. Results of numerical analysis by using the input data from this test and the suggested values from spec. shows similar temperatures. However thermal stresses pridicted using input value from spec. may result 20% more than that from this test in case of externally restricted state.

• Nuclear Engineering and Technology
• /
• v.22 no.4
• /
• pp.359-370
• /
• 1990

This study concerns with comparing low-dimensional reactor kinetics methods with a three-dimensional kinetics method to be used for safety analysis of light water reactors in order to suggest means of preparing input parameters required for low-dimensional methods. For this purpose a one-dimensional finite difference two-group diffusion theory code ODTRAN and a third-order Hermit polynomial-based point kinetics code POTRAN are developed and used to obtain low-dimensional solutions to the LRA-BWR kinetics benchmark problem. The results are compared with a three-dimensional modified Borresen's coarse-mesh solution of the kinetics problem by CMSNACK code. Through this comparison some simple but practical means of preparing input parameters of low-dimensional kinetics analysis methods are suggested.