• Journal of the Korean Institute of Navigation
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• v.17 no.1
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• pp.99-105
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• 1993

Underwater acoustic positioning systems have been extensively used not only in surface position fixing but also in underwater position fixing. Recently, these systems have been applied in the field of installation and underwater inspection offshore platforms etc. But in these systems are included the fixing errors as results of a signal with noise and irregular motion of vessel by ocean waves. In this paper to improve the accuracy of the position fixing a Kalman filter is applied to the short baseline(SBL) acoustic positioning system. The optimal position obtained by the Kalman filter is compared with the raw position and it is confirmed that the former is more accurate than the latter.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.18 no.1
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• pp.92-99
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• 1993

On this study. Constructed the system detecting the reflective acoustic waves from the solid-samples using the 3MHZ focused-acoustic transducer and detected the V(Z) charateristics curves varied by the distance between the focusing position and the solid surface. The results of the experimental detection to various solid materials showed that the materials have their own unique shape of V(Z) curves. This paper suggested the possibility distinguishing the solid characteristics by analysing the V (Z) curves.

• Journal of the Korean Society of Visualization
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• v.18 no.2
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• pp.46-50
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• 2020

On-demand droplet generation with tunable droplet volume is fundamental in many droplet microfluidic applications. In this work, we propose an acoustofluidic method to produce water-in-oil droplets with prescribed volume in an on-demand manner. Surface acoustic waves produced from a slanted interdigital transducer are coupled with parallel laminar streams of dispersed and continuous phase fluids. Acoustic radiation force acting on the fluid interface enable generation of droplets in a microfluidic chip. We expect that the proposed acoustofluidic droplet generation method will serve as a promising tool for on-demand droplet generation with on-chip droplet volume control.

• Journal of the Korean Society of Fisheries and Ocean Technology
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• v.29 no.2
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• pp.109-116
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• 1993

Underwater acoustic positioning systems have been extensively used not only in surface position fixing but also in underwater position fixing. Recently, these systems have been applied in the field of installation and underwater inspection of offshore platforms etc. But in these systems are included the fixing errors as results of a signal with additive noise and irregular motion of vessel by ocean waves. To improve the accuracy of the position fixing a Kalman filter is applied to the supershort baseline (SSBL) acoustic positioning system with beacon mode in noise conditions. The position data obtained by the Kalman filter is compared with raw position data and it is confirmed in the simulation that the former is more accurate than the latter. And an indicator monitoring the filtering effect is described while ship's moving.

• Journal of the Korean Society for Nondestructive Testing
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• v.26 no.6
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• pp.390-402
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• 2006

The elastic properties and thickness of mullite environmental barrier coatings grown through chemical vapor deposition (CVD) on silicon carbide substrates were measured using frequency domain photoacoustic microscopy. In this technique, extremely narrow bandwidth surface acoustic waves are generated with an amplitude modulated laser source. A photorefractive crystal based interferometer is used to detect the resulting surface displacement. The complex displacement field is mapped as a function of source-to-receiver distance in order to extract the wavelength of the surface acoustic wave at a given excitation frequency, and the phase velocity is determined. The coatings tested exhibited spatial variations in thickness and mechanical properties. The measured surface wave dispersion curves were used to extract an effective value for the elastic modulus and the coating thickness. Nanoindentation was used to validate the measurements of the effective elastic modulus. The average elastic modulus measured through the coating thickness using nanoindentation is compared to the effective modulus found using the photoacoustic system. Optical microscopy is used to validate the thickness measurements. The results indicate that the photoacoustic microscopy technique can be used to estimate the effective elastic properties in coatings exhibiting spatial inhomogeneities, potentially providing valuable feedback for the optimization of the CVD growth process.

• The Journal of the Acoustical Society of Korea
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• v.36 no.3
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• pp.165-171
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• 2017

The effect of the concentric solid tube inserted inside the vibrator on the sound field distribution was analyzed for the sound waves focused on the center axis in the fluid - filled cylindrical piezoelectric transducer. The sound waves radiated from the inside of the cylindrical piezoelectric vibrator are transmitted through the fluid medium and are reflected or transmitted on the wall surface of the solid tube, and are focused on the central axis. At this time, the sound field distribution centered on the acoustic tube varies depending on the acoustic impedance and the thickness of the solid tube. In order to theoretically analyze this, the transfer matrix for each medium is derived, and the sound pressure level at the center axis is theoretically analyzed. For the acrylic tube with various thicknesses, the changing trend in the sound pressure level measured on the central axis agrees well with the result of the theoretical analysis, and it confirmed that the sound pressure formed at the center changes very sensitively with the thickness of the solid tube.

• Journal of the Korean Society for Nondestructive Testing
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• v.29 no.4
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• pp.344-350
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• 2009

The goal of this study is to introduce the theoretical background of acoustic nonlinearity in surface wave and to verify its characteristics by experiments. It has been known by theory that the nonlinear parameter of surface wave is proportional to the ratio of $2^{nd}$ harmonic amplitude and the power of primary component in the propagated surface wave, as like as in bulk waves. In this paper, in order to verify this characteristics we constructed a measurement system using contact angle beam transducers and measured the nonlinear parameter of surface wave in an Aluminum 6061 alloy block specimen while changing the distance of wave propagation and the input amplitude. We also considered the effect of frequency-dependent attenuation to the measurement of nonlinear parameter. Results showed good agreement with the theoretical expectation that the nonlinear parameter should be independent on the input amplitude and linearly dependent on the input amplitude and the $2^{nd}$ harmonic amplitude is linearly dependant on the propagation distance.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.26 no.7
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• pp.765-773
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• 2016

The vehicle interior noise caused by exterior fluid flow field is one of critical issues for product developers in a design stage. Especially, turbulence and vortex flow around A-pillar and side mirror affect vehicle interior noise through a side window. The reliable numerical prediction of the noise in a vehicle cabin due to exterior flow requires distinguishing between the aerodynamic (incompressible) and the acoustic (compressible) surface pressures as well as accurate computation of surface pressure due to this flow, since the transmission characteristics of incompressible and compressible pressure waves are quite different from each other. In this paper, effective signal processing technique is proposed to separate them. First, the exterior flow field is computed by applying computational aeroacoustics techniques based on the Lattice Boltzmann method. Then, the wavenumber-frequency analysis is performed for the time-space pressure signals in order to characterize pressure fluctuations on the surface of a vehicle side window. The wavenumber-frequency diagrams of the power spectral density shows clearly two distinct regions corresponding to the hydrodynamic and the acoustic components of the surface pressure fluctuations. Lastly, decomposition of surface pressure fluctuation into incompressible and compressible ones is successfully accomplished by taking the inverse Fourier transform on the wavenumber-frequency diagrams.

• Composites Research
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• v.13 no.1
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• pp.61-68
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• 2000

A new approach is presented for the analysis of transient waves propagating in anisotropic composite laminates. The wavelet transform (WT) using the Gabor wavelet is applied to the time-frequency analysis of dispersive flexural waves. It is shown that the peaks of the magnitude of WT in time-frequency domain is related to the arrival times of group velocity. Experiments are performed using a lead break as the simulated fracture source on the surface of quasi-isotropic and unidirectional laminates. For predictions of the dispersion of the flexural mode, Mindlin plate theory is shown to give good agreement with the experimental results. Based on the frequency-dependent arrival times and angular dependence of group velocities of flexural waves, the problem of source location in anisotropic laminates is considered and the results are given.

• Journal of the Korea Institute of Military Science and Technology
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• v.15 no.2
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• pp.138-146
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• 2012

Using Shear-free Ship wake theory it was predicted the detectable submarine's turbulent wake on the sea surface was about 12km long when there was no breaking waves on the sea surface. It means that there are sufficient detectable turbulent kinetic energies on the sea surface as well as in the water. In this paper, we have proposed some concepts of non acoustic anti-submarine surveillance systems; SAR for sea surface surveillance, LIDAR for sub-surface surveillance and propelled gliders for under -water surveillance.