• Earthquakes and Structures
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• v.17 no.6
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• pp.533-543
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• 2019

This paper presents a study on the behavior of an RC bridge under near-field and far-field ground motions. For this purpose, a dynamic nonlinear finite element time history analysis has been conducted. The near-field and far-field records are chosen pairwise from the same events which are fits to the seismic design of the bridge. In order to perform an accurate seismic evaluation, the model has been analyzed under two vertical and horizontal components of ground motions. Parameters of relative displacement, residual displacement, and maximum plastic strain have been considered and compared in terms of near-field and far-field ground motions. In the following, in order to decrease the undesirable effects of near-field ground motions, a viscous damper is suggested and its effects have been studied. In this case, the results show that the near-field ground motions increase maximum relative and residual displacement respectively up to three and twice times. Significant seismic improvements were achieved by using viscous dampers on the bridge model. Somehow under the considered near-field ground motion, parameters of residual and relative displacement decrease dramatically even less than the model without damper under the far-field record of the same ground motion.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.13 no.10
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• pp.743-750
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• 2003

The equivalent source method(ESM) is used for the calculation of the internal pressure field for an enclosure which can have arbitrary boundary conditions and nay include internal objects which scatter the sound field. The advantage of using ESM is that it requires relatively low computing cost and is easy to model the internal diffracting objects. Typical ESM modeling uses two groups of equivalent source positions. One group includes the first order images of the source inside the enclosure. The Positions of the other group are usually on a spherical surface some distance outside the enclosure. The normal velocity on the surfaces of the enclosure walls is evaluated at a larger number of positions than there are equivalent sources. The sum of the squared difference between this velocity and the expected is minimized by adjusting the strength of the equivalent sources. This study is on the optimal far field sources positions when using the equivalent source method. In general, the far field sources are evenly distributed on a surface of a virtual sphere which is centered at the enclosure with a sufficiently large radius. In this study. optimal far field source locations are searched using simulated annealing method for various radii of spheres where far field sources are located. Simulation results showed that optimally located sources with adequate distance away from the enclosure center gave better result than sources with even distribution even with a smaller number of far field sources.

• The Journal of the Acoustical Society of Korea
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• v.35 no.1
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• pp.35-41
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• 2016

Based on correlation and least square method, a closed-form algorithm for estimating the location of mixed far-field and near-field source is presented using the Uniform Circular Array (UCA). Recently, for a homogeneous circular arrangement case, a correlation based closed-form algorithm is proposed to estimate 2-D angle (azimuth, elevation) and the extended algorithm is proposed to 3-D location (azimuth, elevation, range). These algorithms assume the far-field source or near-field source only. Therefore, for mixed source localization, the proposed algorithm estimates source location with the assumption of far-field source, and then estimates the range to distinguish the far-field from the near-field source. For both cases, numerical experiments have been performed, which confirmed the validity of the proposed algorithm.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.24 no.3
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• pp.316-323
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• 2013

This paper deals with the near-field to far-field calculation for far-field characteristics of antenna and electromagnetic compatibility(EMC) testing. Since the conventional EMC testing process is inefficient such as measurements of the wide band signals and mega structures, Stratton-Chu formula is used to predict the far-field emission by simple and direct process. The usefulness of Stratton-Chu formula is verified by comparing to the analytic solution of the uniform distribution aperture in free-space. In order to inspect the far-fields and to get the near-field values, full-wave simulation solver is utilized. Through the full-wave simulation about the patch antenna, calculated far-field results from Stratton-Chu formula are proven. The predicted magnitudes of the far-field are in error by less than 6 %.

• Proceedings of the KSME Conference
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• 2001.06b
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• pp.216-221
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• 2001

The equivalent source method(ESM) is used for the calculation of the internal pressure field for an enclosure which can have arbitrary boundary conditions and may include internal objects which scatter the sound field. The advantage of using ESM is that it requires relatively low computing cost and is easy to model the internal diffracting objects. In the ESM modelling, some of the equivalent positions are chosen to be the same as the first order images of the source inside the enclosure, some are positioned on a spherical surface some distance outside the enclosure. The normal velocity on the surfaces of the enclosure walls is evaluated at a larger number of positions than there are equivalent sources. The sum of the squared difference between this velocity and the expected is minimized by adjusting the strength of the equivalent sources. This study is on the optimal equivalent source positions, the far field sources. Typically, the far field sources are evenly distributed on a surface of a virtual sphere which is centered at the enclosure with a sufficiently large radius. In this study, optimal far field source locations are searched using simulated annealing method and simulation results showed that optimally located sources gave better accuracy even with a smaller number of far field sources.

• ETRI Journal
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• v.29 no.2
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• pp.135-142
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• 2007

The small anechoic chambers built by many small-/medium-sized companies and universities present difficulties in testing electrically large antennas because the chamber size cannot satisfy the far-field criterion of large antennas. In this paper, a method for Fresnel-region measurement on a cylindrical surface with variation of the measurement height is proposed and verified by both calculations and experiments. We implement the proposed method using a direct far-field measurement system by adding a few supporting structures. The results show good accuracy.

• 한국전산유체공학회:학술대회논문집
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• 2011.05a
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• pp.199-201
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• 2011

NACA0012 Airfoil was simulated with Computational Fluid Dynamics(CFD) and the aerodynamic characteristics was analyzed for various far-field boundary distances ranging from 10 airfoil chord to 50 chord Drag coefficient distribution was dependent on the far-field distance and circulation, integrated along the loop inside the flow region, was also dependent. It was turned out that some corrections based on the circulation should be added to the far-field boundary condition for accurate airfoil simulation.

• International Journal of Naval Architecture and Ocean Engineering
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• v.12 no.1
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• pp.680-690
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• 2020

In ocean environment, the sound speed gradient of seawater has an important influence on far field sound propagation. The FEM/BEM is used to decouple the vibroacoustic radiation of the spherical shell, and the Green function of the virtual source chain is adopted for decoupling. For far field radiated Sound Pressure Level (SPL), the Beam Displacement Ray normal Mode (BDRM) is employed. The vibration and near-/far-field radiated SPL of spherical shell is analyzed in shallow sea uniform layer, negative/positive gradient, negative thermocline environment, and deep-sea sound channel. Results show that the vibroacoustic radiation of spherical shell acted at 300Hz can be analogous to dipole. When the radiated field of the spherical shell is dominated by large-grazing-angle waves, it can be analogous to vertically distributed dipole, and the far field radiated SPL is lower; while similar to horizontally distributed dipole if dominated by small-grazing-angle waves, and the far field SPL is high.

• Korean Journal of Optics and Photonics
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• v.9 no.3
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• pp.168-174
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• 1998

We have measured nonlinear refractive index and nonlinear absorption coefficient of optical materials by using a far-field phase modulation technique. The phase variation of the probe beam in the nonlinear material is transformed into the spatial phase modulation in the far-field so that the spatial distribution of the optical intensity in conjunction with the computer simulation analysis can give the nonlinear optical constants. We have obtained the nonlinear refractive indices and nonlinear absorption coefficient of $CS_2$ and $BaF_2$ by fitting the experimental values and numerical simulation analysis of far-field measurements. The nonlinear refractive indices of $CS_2$ and $BaF_2$were obtained as $1.2{\times}10^{-11}$ esu and $1.0{\times}10^{-13}$ esu, respectively at 616 nm, and the nonlinear absorption coefficient of BaF$_2$as $5.0{\times}10^{-11}$cm/W at 308nm. These measured values were in good agreement with previous reports.

• Journal of Ocean Engineering and Technology
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• v.24 no.6
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• pp.1-6
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• 2010

A far field approximate solution of the slowly varying force on a 3 dimensional offshore structure in gravity ocean waves is presented. The first order potential, or at least the far field form of the Kochin function, of each frequency wave is assumed to be known. The momentum flux of the fluid domain is formulated to find the time variant force acting on the floating body in bichromatic waves. The second order difference frequency force is identified and extracted from the time variant force. The final solution is expressed as the circular integration of the product of Kochin functions. The limiting form of the slowly varying force is identical to the mean drift force. It shows that the slowly varying force components caused by the body disturbance potential can be evaluated at the far field.