• Journal of Korea Water Resources Association
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• v.30 no.6
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• pp.587-595
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• 1997

Experimental study on unsaturated flow in the soil is important to understand the characteristics of the water flow. Measurement of unsteady-state water movement using the traditional equipment (e.g. tensiometer) has a problem that requires relatively a long response time. In this study a quick measurement method of soil water flow using TDR is introduced. TDR consists of an electronic function generator which generates a squared wave, and an oscilloscope which catches the reflected wave. The wave is reflected where both the impedance of the transmission line and the propagation velocity are changed. The water content can be obtained from the travel time measured by means of TDR because the dielectric constant is affected by the change of soil water content. From the result of TDR calibration. TDR measurement error for the oven dried soil was found to be less than 3.5%. This supports that TDR is a viable technique to measure the unsteady-state water movement.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.20 no.2
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• pp.191-206
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• 2007

The general theoretical solutions for the wavespeed and damping derived in Part 1 of this work, are incorporated into the computer code. In this paper the code is used in a parametric study of the influence of excitation frequency and variations in material properties on propagation velocity and damping. Compressional wave velocity for waves of the first kind is shown to vary as a function of the frequency-permeability product, with a zone where wavespeed transitions from a lower bound value to a higher bound value with increasing values of the product. Damping is seen to be a maximum where the rate of change in wavespeed is greatest. Waves of the second kind also show a transition in wavespeed from near zero at low values of the frequency-permeability product to an upper bound value at higher values of the product.

• Journal of the Korean Society of Propulsion Engineers
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• v.18 no.4
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• pp.43-49
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• 2014

This experimental study has been conducted to scrutinize the effects of an ultrasonic standing wave (USW) on the propagating velocity and structure of methane/air premixed flame. Propagating flame was caught by high-speed Schlieren photography, and the variation of flame-behavior was analyzed in detail. It is revealed that horizontal splitting in burnt zone is resulted by the USW, and the flame propagation velocity is augmented due to the strengthened chemical reaction. Evolutionary feature of the flame perturbed by USW, maintaining a pseudo-symmetry of top and bottom flame-front about the propagation axis tends to be free from buoyancy effect.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.30 no.2
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• pp.39-50
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• 2018

In this study, the nonlinear interaction of irregular waves with wave breaking and currents around permeable submerged breakwater was investigated with the aid of olaFlow model which is open source CFD software published under the GPL license. The irregular wave performance of olaFlow applied in this study was verified by comparing and evaluating the target frequency spectrum and the generated frequency spectrum for applicability to irregular waves. Based on the applicability of this numerical model to irregular wave fields, in the coexistence fields of irregular waves and currents, the characteristics of wave height, frequency spectrum, breaking waves, averaged velocity and turbulent kinetic energy around porous submerged breakwater with the respect to the beach type and current direction versus wave propagation were carefully investigated. The numerical results revealed that the shape of wave breaking on the crown of the submerged breakwater and the formation of the mean flow velocity around the structure depend greatly on the current directions and the type of the beach. In addition, it was found that the wave height fluctuation due to the current direction with respect to the wave propagation is closely related to the turbulent kinetic energy.

• Composites Research
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• v.18 no.1
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• pp.38-44
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• 2005

In this paper, the group velocity dispersion curves of the $S_o$ symmetric mode in unidirectional CFRP plate was calculated as varying the propagating direction. The group velocity curve was obtained with the group velocities of the $S_o$ symmetric mode corresponding to 0.2 MHz-mm under the first cut-off frequency in the dispersion curves, and corrected by introducing the slowness curve. The velocities of the $S_o$ symmetric mode in the unidirectional CFRP plate were measured as varying the propagating direction and compared with the col?rotted group velocity curve. The measured velocities were good agreement with the corrected group velocity curve except near the fiber direction which was called the cusp region. It implies that the direction of the group velocities incline toward the fiber direction of the unidirectional CFRP plates when the propagation direction is not accorded with the principal axis. It is supposed that this phenomenon rerults from the preferential propagating the energy toward the direction with the faster propagation velocity.

• 한국지구물리탐사학회:학술대회논문집
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• 2006.06a
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• pp.17-22
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• 2006

Shear modulus is directly linked to material's stiffness and is one of the most critical engineering parameters. Seismically, shear-wave velocity (Vs) is its best indicator. Although methods like refraction, down-hole, and cross-hole shear-wave surveys can be used, they are generally known to be tougher than any other seismic methods in field operation, data analysis, and overall cost. On the other hand, surface waves, commonly known as ground roll, are always generated in all seismic surveys with the strongest energy, and their propagation velocities are mainly determined by Vs of the medium. Furthermore, sampling depth of a particular frequency component of surface waves is in direct proportion to its wavelength and this property makes the surface wave velocity frequency dependent, i.e., dispersive. The multichannel analysis of surface waves (MASW) method tries to utilize this dispersion property of surface waves for the purpose of Vs profiling in 1-D (depth) or 2-D (depth and surface location) format. The active MASW method generates surface waves actively by using an impact source like sledgehammer, whereas the passive method utilizes those generated passively by cultural (e.g., traffic) or natural (e.g., thunder and tidal motion) activities. Investigation depth is usually shallower than 30 m with the active method, whereas it can reach a few hundred meters with the passive method. Overall procedures with both methods are briefly described.

• Journal of the Korean Geotechnical Society
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• v.29 no.1
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• pp.5-12
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• 2013

The critical velocity effect on railway trackbed means the amplification of vibration energy when the train running-speed and group velocity of ground surface wave are superimposed. It is called a pseudo-resonance phenomenon of time domain. In the past, it was not issued because the train speed was low and the ground group velocity was higher. But since the high-speed train is introduced, critical velocity reported causing a track irregularity. So far, theoretical analysis has been performed because of the complexity of formation process. However it requires reasonable consideration which is similar to actual track and trackbed conditions. In the present paper, finite element analysis to verify the critical velocity effect is performed considering each track structure and trackbed supporting stiffness. As a result, the deformation amplification caused by the critical velocity effect is verified to analyze each supporting stiffness and track system.

• Steel and Composite Structures
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• v.51 no.2
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• pp.185-202
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• 2024

In this research paper, and for the first time, wave propagations in sigmoidal imperfect functionally graded material plates are investigated using a simplified quasi-three-dimensionally higher shear deformation theory (Quasi-3D HSDTs). By employing an indeterminate integral for the transverse displacement in the shear components, the number of unknowns and governing equations in the current theory is reduced, thereby simplifying its application. Consequently, the present theories exhibit five fewer unknown variables compared to other Quasi-3D theories documented in the literature, eliminating the need for any correction coefficients as seen in the first shear deformation theory. The material properties of the functionally graded plates smoothly vary across the cross-section according to a sigmoid power law. The plates are considered imperfect, indicating a pore distribution throughout their thickness. The distribution of porosities is categorized into two types: even or uneven, with linear (L)-Type, exponential (E)-Type, logarithmic (Log)-Type, and Sinus (S)-Type distributions. The current quasi-3D shear deformation theories are applied to formulate governing equations for determining wave frequencies, and phase velocities are derived using Hamilton's principle. Dispersion relations are assumed as an analytical solution, and they are applied to obtain wave frequencies and phase velocities. A comprehensive parametric study is conducted to elucidate the influences of wavenumber, volume fraction, thickness ratio, and types of porosity distributions on wave propagation and phase velocities of the S-FGM plate. The findings of this investigation hold potential utility for studying and designing techniques for ultrasonic inspection and structural health monitoring.

• Journal of the Korean Geotechnical Society
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• v.22 no.8
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• pp.43-54
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• 2006

The scope of this paper covers the applications of bender element tests in consolidation, tomography, and liquefaction. Loading and unloading time during consolidation are evaluated based on shear wave velocity. As S-wave velocity is dependent on effective stress, the loading step may be determined. However, cautions are required due to the different mechanism between the settlement and effective stress criteria. The stress history may be evaluated because the S-wave shows the cement controlled regime and stress controlled regimes. A fixed frame complemented with bender elements permits S-wave tomography The tomography system is tested at low confinement within a true triaxial cell. Results show that shear wave velocity tomography permits monitoring changes in the velocity field which is related to the average effective stress. To monitor the liquefaction phenomenon, S-wave trans-illumination is implemented with a high repetition rate to provide detailed information on the evolution of shear stiffness during liquefaction. The evolution of shear wave propagation velocity and attenuation parallel the time-history of excess pore pressure during liquefaction. Applications discussed in this paper show that bender elements can be a very effective tool for the detection of shear waves in the laboratory.

• Journal of the Korean GEO-environmental Society
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• v.25 no.2
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• pp.5-14
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• 2024

Reinforced concrete bridge decks are the first to be damaged by vehicle loads and rain infiltration. Concrete deterioration primarily occurs owing to the corrosion of rebars and other metal components by chlorides used for snow and ice melting. The structural condition and concrete deterioration of the bridge decks within the pavement were evaluated using ground-penetrating radar (GPR) survey data. To evaluate concrete deterioration in bridges, it is necessary to develop GPR data analysis techniques to accurately identify deteriorated locations and rebar positions. GPR exploration involves the acquisition of reflection and diffraction wave signals due to differences in radar wave propagation velocity in geotechnical media. Therefore, a full-waveform inversion (FWI) method was developed to evaluate the deterioration of reinforced concrete bridge decks by estimating the radar wave propagation velocity in geotechnical media using GPR data. Numerical experiments using a GPR velocity model confirmed the deterioration phenomena of bridge decks, such as concrete delamination and rebar corrosion, verifying the applicability of the developed technology. Moreover, using the synthetic GPR data, FWI facilitates the determination of rebar positions and concrete deterioration locations using inverted velocity images.