• Journal of Ocean Engineering and Technology
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• v.24 no.1
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• pp.1-9
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• 2010

A series of experiments employing particle image velocimetry (PIV) technique was conducted to produce benchmark wave kinematics data for regular waves having four different wave slopes in 2-D wave tank. Water velocities and accelerations near the free surface of regular waves were computed from image pair obtained by PIV systems. With the measured wave velocity field, the wave accelerations were computed using a centered finite difference scheme. Both local and convective components of the total accelerations are obtained from experimental data. With increasing the wave slope, the horizontal velocity and the vertical accelerations near the wave crest obtained by PIV technique became larger than theoretical results, which are well-known phenomena of the wave nonlinearity. It is noted that the relative magnitude of convective acceleration to the local acceleration became larger with increasing wave slope.

• Journal of Environmental Science International
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• v.23 no.4
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• pp.543-552
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• 2014

Fixed Electromagnetic Wave Surface Velocimetry (Fixed EWSV) has been started to be used to measure flood discharge in the mountain stream, since it has various advantages such that it works well to continuously measure stream discharge even in the night time as well as very strong weather. On the contrary, the Fixed EWSV only measures single point surface velocity, thus it does not consider varying feature of the transverse velocity profile in the given stream cross-section. In addition, a conventional value of 0.85 was generally used as the ratio for converting the measured surface velocity into the depth-averaged velocity. These aspects could bring in error for accurately measuring the stream discharge. The capacity of the EWSV for capturing rapid flow velocity was also not properly validated. This study aims at conducting error analysis of using the EWSV by: 1) measuring transverse velocity at multiple points along the cross-section to assess an error driven by the single point measurement; 2) figuring out ratio between surface velocity and the depth-averaged velocity based on the concurrent ADCP measurements; 3) validating the capacity of the EWSV for capturing rapid flow velocity. As results, the velocity measured near the center by the fixed EWSV overestimated about 15% of the cross-sectional mean velocity. The converting ratio from the surface velocity to the depth-averaged velocity was 0.8 rather than 0.85 of a conventional ratio. Finally, the EWSV revealed unstable velocity output when the flow velocity was higher than 2 m/s.

• Journal of the Korean Society for Precision Engineering
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• v.14 no.10
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• pp.119-126
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• 1997

The relationship between the applied stresses and the change of elastic wave velocity has been established based on the acoustoelasticity theory. The non-uniform stress field in a loaded specimen has been evaluated from the surface acoustic wave velocity measured by the line-focus acoustic microscopy with the acoustoelastic constants obtained form a calibration test. The evaluated stresses are in good agreement with the results calculated by finite element method.

• Geophysics and Geophysical Exploration
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• v.7 no.1
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• pp.7-13
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• 2004

In this paper, we demonstrate that Common Mid-Point (CMP) cross-correlation gathers of multi-channel and multi-shot surface waves give accurate phase-velocity curves, and enable us to reconstruct two-dimensional (2D) velocity structures with high resolution. Data acquisition for CMP cross-correlation analysis is similar to acquisition for a 2D seismic reflection survey. Data processing seems similar to Common Depth-Point (CDP) analysis of 2D seismic reflection survey data, but differs in that the cross-correlation of the original waveform is calculated before making CMP gathers. Data processing in CMP cross-correlation analysis consists of the following four steps: First, cross-correlations are calculated for every pair of traces in each shot gather. Second, correlation traces having a common mid-point are gathered, and those traces that have equal spacing are stacked in the time domain. The resultant cross-correlation gathers resemble shot gathers and are referred to as CMP cross-correlation gathers. Third, a multi-channel analysis is applied to the CMP cross-correlation gathers for calculating phase velocities of surface waves. Finally, a 2D S-wave velocity profile is reconstructed through non-linear least squares inversion. Analyses of waveform data from numerical modelling and field observations indicate that the new method could greatly improve the accuracy and resolution of subsurface S-velocity structure, compared with conventional surface-wave methods.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.5 no.3
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• pp.204-211
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• 1993

Boundary element method is applied to simulate nonlinear water waves using Green's identity formula in a numerical wave flume. A system of linear equations is formulated from the governing equation and free surface boundary conditions in order to calculate velocity potential and water surface elevation at each nodal point. The velocity square terms are included in the dynamic free surface boundary condition. The free surface is treated as a moving boundary. the vertical variation of velocity potential being considered in calculating the time derivative of the velocity potential at the free surface. The present method is applied to simulate solitary wave and Stokes 2nd order wave, and shows excellent agreements with their theoretical values.

• Proceedings of the KSRS Conference
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• 2002.10a
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• pp.429-434
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• 2002

The Synthetic Aperture Radar (SAR) data are considered to contain the greatest amount of information among various microwave techniques developed for measuring ocean variables from aircraft or satellites. They have the potential of measuring wavelength, wave direction and wave height of the ocean waves. But, it is difficult to retrieve significant ocean wave heights and surface current from conventional SAR data, since the imaging mechanism of ocean waves by a SAR is determined by the three basic modulation processes arise through the tilt modulation, hydrodynamic modulation and velocity bunching which are poorly known functions. Along-Track Interferometric (ATI) SAR systems can directly detect the Doppler shift associated with each pixel of a SAR image and have been used to estimate wave fields and surface currents. However, the Doppler shift is not simply proportional to the component of the mean surface current. It includes also contributions associated with the phase velocity of the Brags waves and orbital motions of all ocean waves that are longer than Brags waves. In this paper, we have developed a new method for extracting the surface current vector using multiple-frequency (L- & C-band) ATI SAR data, and have generated surface wave height information.

• Korean Journal of Materials Research
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• v.11 no.6
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• pp.477-482
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• 2001

The velocity changes of magneto surface acoustic wave (MSAW) have been investigated in the MSAW devices composed of wedge type transducer and as-sputtered ($Fe_{1-x}$ $Co_{x}$ )$_{89}$ $Zr_{ 11}$ (x=0~1.0)amorphous films on glass substrates. The velocity changes of devices depended sensistively on exciting frequency of MSAW, applying the DC bias magnetic field. film thickness and film composition. Particularly. it was conformed that velocity changes increased with the increase of the exciting frequency of MSAW and the thickness of magnetic films. A device deposited x= 0.8 film along the MSAW propagation direction among the devices exhibited a large velocity change of 0.062% at 8.7 MHz for the applied field of 70 Oe.

• Journal of the Korean Geophysical Society
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• v.9 no.3
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• pp.231-240
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• 2006

The crustal structure of the Korean Peninsula was investigated by analyzing phase velocity dispersion data of Rayleigh waves. Earthquakes recorded by three component broad-band velocity seismographs during 1999-2004 in South Korea were used in this study. The fundamental mode Rayleigh waves were extracted from vertical components of seismograms by multiple filter technique and phase match filter method. Phase velocity dispersion curves of the fundamental mode signal pairs for 14 surface wave propagation paths on the great circle in the range 10 to 80 sec were computed by two-station method. Treating the shear velocity of each layer as an independent parameter, phase velocity data of Rayleigh wave were inverted. All the result models can be explained by a rather homogeneous crust of shear-wave velocity increasing from 2.8 to 3.25 km/sec from top to about 33 km depth without any distinctive crustal discontinuities and an uppermost mantle of shear-wave velocity between 4.55 and 4.67 km/sec. Our results turn out to agree well with recent study of Cho et al. (2006 b) based on the analysis of seismic background noises to recover short-period (0.5-20 sec) Rayleigh- and Love-wave group velocity dispersion characteristics.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.36 no.6
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• pp.1037-1048
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• 2016

In this study, nonlinear wave interaction in the presence of a uniform current is studied using numerical model, named CADMAS-SURF which is based on the Navier-Stokes equations coupled with Volume of Fluid for tracking free surface deformation. The original CADMAS-SURF developed for interaction of wave with structure is modified/extended to simulate nonlinear fluid dynamic motions within wave-current coexisting field. The capability of Numerical Wave-Current Tank (NWCT) in this study is validated by comparing with available existing laboratory experiments for both wave-following and wave-opposing current. The numerical results for interaction between wave and current are shown to be in good agreement with experimental data. Then, this study focused on the dynamic motions of the water velocity, surface elevation and vorticity within combined wave-current field in demonstrating complex nonlinear physical phenomena due to interaction between wave and current. In addition, NWCT is applied to simulate a more complex wave-current-structure field for wave propagating over a submerged breakwater associated with current. Detailed discussion including characteristics of velocity and vorticity fields and the relation between free surface and vorticity are given.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 1992.05a
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• pp.144-146
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• 1992

Surface acoustic wave(SAW) oscillator offers many attractive features for application to vapor sensors. The perturbation of SAW velocity by the hexafluoropropence plasma polymer thin film has been studied for relative humidity sensing. adsorption of moisture produces rapid aid changes in the properties of the film, resulting in a change in the velocity of surface acoustic waves and, hence, in the frequency of one SAW oscillator. The device used in our experiments have 55 MHZ SAW oscillator fabricated on a LiNbO substrate.