• Journal of the Optical Society of Korea
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• v.3 no.2
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• pp.41-46
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• 1999

Spatial and temporal dynamics arising in a photorefractive crystal(BaTiO3) during the process of double phase conjugation was studied experimentally. We studied the dynamical effects caused by the buildup of the diffraction grating and turn on of phase conjugated beams, as well as the spatial effects caused by the finite transverse coupling of beams and the propagation direction of beams. We observed conical emission in DPCM. We believe that various temporal and spatial instabilities are due to movement of the nonlinear grating. For a real beam coupling and constructive interaction of interference fringes in the crystal, we observed steady, periodic, irregular temporal behavior. And, by the calculation of the correlation index, we found that the spatial correlation decreased as the transverse interaction region was increased.

• Geomechanics and Engineering
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• v.28 no.5
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• pp.455-461
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• 2022

Numerical assessment of the dynamic stability behavior of nonlocal beams rested on elastic foundation has been provided in the present research. The beam is made of fucntional graded (FG) porous material and is exposed to thermal and humid environments. It is also consiered that the beam is subjected to axial periodic mechanical load which especific exitation frequency leading to its instability behavior. Beam modeling has been performed via a two-variable theory developed for thick beams. Then, nonlocal elasticity has been used to establish the governing equation which are solved via Chebyshev-Ritz-Bolotin method. Temperature and moisture variation showed notable effects on stability boundaries of the beam. Also, the stability boundaries are affected by the amount of porosities inside the material.

• Nuclear Engineering and Technology
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• v.50 no.7
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• pp.1051-1059
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• 2018

Fast periodic pulsed reactor is a type of reactor in which the fission bursts are formed entirely with external reactivity modulation with a specified time periodicity. This type of reactors could generate much larger intensity of neutron beams for experimental use, compared with the steady state reactors. In the design of fast periodic pulsed reactors, the time dependent simulation of the power pulse is majorly based on a point kinetic model, which is known to have limitations. A more accurate calculation method is desired for the design analyses of fast periodic pulsed reactors. Monte Carlo computer code MCNP6 is used for this task due to its three dimensional transport capability with a continuous energy library. Some new routines were added to simulate the rotation of the movable reflector parts in the time dependent calculation. Fast periodic pulsed reactor IBR-2M was utilized to validate the new routines. This reactor is periodically in prompt supercritical state, which lasts for ${\sim}400{\mu}s$, during the equilibrium state. This generates long neutron fission chains, which requires tremendously large amount of computation time during Monte Carlo simulations. Russian Roulette was applied for these very long neutron chains in MCNP6 calculation, combined with other approaches to improve the efficiency of the simulations. In the power pulse of the IBR-2M at equilibrium state, there is some discrepancy between the experimental measurements and the calculated results using the point kinetics model. MCNP6 results matches better the experimental measurements, which shows the merit of using MCNP6 calculation relative to the point kinetics model.

• Transactions of the Korean Society of Machine Tool Engineers
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• v.11 no.1
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• pp.52-60
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• 2002

The purpose of this research is to study the vibration and acoustic pressure radiation from a thin isotropic flat plate stiffened by a rectangular array of beams, and excited by a time harmonic point force. These constructions on aircraft and ship structures are often subjected to fiequency dependent pressure fluctuations and forces. Forces from the these excitations induce structural vibrations in a wide range of fiequencies, which may cause such things as acoustic fatigue and internal cabin noise in the aircraft. It is thus important that the response characteristics and vibration modes of such periodic structures be horn. From this theoretical model, the sound pressure levels(SPL) in a semi-infinite fluid(water) bounded by the plate with the variation in the locations of an external time harmonic point farce on the plate can be calculated efficiently using three numerical tools such as the Gauss-jordan method the LU decomposition method md the IMSL numerical package.

• Proceedings of the KIEE Conference
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• 1989.07a
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• pp.629-632
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• 1989

The scattered light intensity from a spherical particle passing through the cross-over region of two coherent laser beams, varies periodically. Photodetection of this light beams produces a periodic signal of varying amplitude. The phase of the signal varies with the particle size and refractive index, the beam crossing angle and wavelength, and the position and size of the scattered ligth collecting aperture. In this paper the phase variation with respect to the particle absorptive index of retraction, collecting lens size and beam crossing angle is calculated using both Mie scattering theory and reflection theory. The two theories show good agreement in phase predictions, especially for large absorptive indices and for small collection lenses. Both theories predict phase to be inversely proportional to the beam crossing angle.

• Proceedings of the Korean Society of Machine Tool Engineers Conference
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• 1998.03a
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• pp.214-220
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• 1998

The determination of sound pressure radiated from periodic plate structures is fundamental in the estimation of noise levels in aircraft fuselages and ship hull structures. As a robust approach to this problem, here a very general and comprehensive analytical model for prediction the sound radiated by a vibration plate stiffened by periodically spaced orthogonal symmetric beams subjected to a sinusoidally time varying point load is developed. The plate is assumed to be infinite in extent, and the beams are considered to exert both line force and moment reactions on it. Structural damping is included in both plate and beam materials. From this theoretical model, the sound pressure levels on axis in a semi-infinite fluid(water) bounded by the plate with the variation in the loactions of an external using three numerical tools such as the Gauss-Jordan method, the LU decomposition method and the IMSL numerical package.

• Advances in Computational Design
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• v.5 no.2
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• pp.177-193
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• 2020

In the present research, dynamic analysis of functionally graded (FG) graphene-reinforced beams under thermal loading has been carried out based on finite element approach. The presented formulation is based on a higher order refined beam element accounting for shear deformations. The graphene-reinforced beam is exposed to transverse periodic mechanical loading. Graphene platelets have three types of dispersion within the structure including uniform-type, linear-type and nonlinear-type. Convergences and validation studies of derived results from finite element approach are also presented. This research shows that the resonance behavior of a nanocomposite beam can be controlled by the GPL content and dispersions. Therefore, it is showed that the dynamical deflections are notably influenced by GPL weight fractions, types of GPL distributions, temperature changes, elastic foundation and harmonic load excitation frequency.

• Structural Engineering and Mechanics
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• v.37 no.2
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• pp.163-175
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• 2011

The results of experimental and numerical investigations on reinforced concrete beams, with different longitudinal rebars affected by corrosive processes are presented in this paper. Different diameters and/or different distributions of longitudinal rebars were employed keeping constant the total section in each analyzed case, (maintaining a constant stirrup diameter and distribution). The rebars were subjected to accelerated corrosion in the experimental study. Electrochemical monitoring of the process, periodic measuring of the cover cracking and gravimetry of the rebars were performed through the test. Some building recommendations are obtained in order to be considered by designers of concrete structures. The numerical simulation was carried out through the application of the Finite Element Method (FEM), employing plane models, and using linear-elastic material model. The cracking process was associated with the evolution of the tensile stresses that were originated. This numerical methodology allows the monitoring of the mechanical behavior until the beginning of the cracking.

• Structural Engineering and Mechanics
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• v.72 no.4
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• pp.525-540
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• 2019

The purpose of the present work was to study the dynamic instability of a three-layered, symmetric sandwich beam subjected to a periodic axial load resting on nonlinear elastic foundation. A higher-order theory was used for analysis of sandwich beams with soft core on elastic foundations. In the higher-order theory, the Reddy's third-order theory was used for the face sheets and quadratic and cubic functions were assumed for transverse and in-plane displacements of the core, respectively. The elastic foundation was modeled as nonlinear's type. The dynamic instability regions and free vibration were investigated for simply supported conditions by Bolotin's method. The results showed that the responses of the dynamic instability of the system were influenced by the excitation frequency, the coefficients of foundation, the core thickness, the dynamic and static load factor. Comparison of the present results with the published results in the literature for the special case confirmed the accuracy of the proposed theory.

• Advances in aircraft and spacecraft science
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• v.3 no.4
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• pp.471-502
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• 2016

This paper presents the dynamic instability analysis of un-damped elastically supported piezoelectric functionally graded (FG) beams subjected to in-plane static and dynamic periodic thermomechanical loadings with uncertain system properties. The elastic foundation model is assumed as one parameter Pasternak foundation with Winkler cubic nonlinearity. The piezoelectric FG beam is subjected to non-uniform temperature distribution with temperature dependent material properties. The Young's modulus and Poison's ratio of ceramic, metal and piezoelectric, density of respective ceramic and metal, volume fraction exponent and foundation parameters are taken as uncertain system properties. The basic nonlinear formulation of the beam is based on higher order shear deformation theory (HSDT) with von-Karman strain kinematics. The governing deterministic static and dynamic random instability equation and regions is solved by Bolotin's approach with Newmark's time integration method combined with first order perturbation technique (FOPT). Typical numerical results in terms of the mean and standard deviation of dynamic instability analysis are presented to examine the effect of slenderness ratios, volume fraction exponents, foundation parameters, amplitude ratios, temperature increments and position of piezoelectric layers by changing the random system properties. The correctness of the present stochastic model is examined by comparing the results with direct Monte Caro simulation (MCS).