• Journal of Auto-vehicle Safety Association
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• v.13 no.3
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• pp.13-19
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• 2021

Fog is a phenomenon caused by condensation of water vapor in the atmosphere, which is when very fine drops of water float in the atmosphere and the distance of visible is less than 1km. Fog dispersion technology is a technology that removing or weakening fog by using artificial methods to reduce damage caused by fog. It is applied differently depending on the temperature of fog generation rather than the cause of fog. This study conducted an experimental study on the fog dispersion mechanism in order to minimize damage caused by fog on the road, and studied two methods of over-cooling dispersion using solid-carbon-dioxide as a dissipated particle and dissipating fog particles through thermal acoustic waves. As a result the two methods proved experimentally that were capable of dissipating fog.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2004.03a
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• pp.155-164
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• 2004

In order for studying pressure-coupled dynamic responses of droplet vaporization, open-loop experiment of an isolated droplet vaporization exposed to pressure perturbations in stagnant gaseous environment is numerically conducted, Governing equations are solved for flow parameters at gas and liquid phases separately and thermodynamic parameters at the interfacial boundary are matched for problem closure. For high-pressure effects, vapor-liquid interfacial thermodynamics is rigorously treated. A series of parametric calculations in terms of mean pressure level and wave frequencies are carried out employing a n-pentane droplet in stagnant gaseous nitrogen. Results show that wave instability in view of pressure-coupled vaporization response seems more susceptible at higher pressures and higher wave frequencies. Mass evaporation rate responding to pressure waves is amplified with increase in pressure due to substantial reduction in latent heat of vaporization. Augmentation of perturbation frequency also enhances amplification due to the reduction of phase differences between pressure perturbation and surface temperature fluctuation.

• Journal of the Korean Society for Nondestructive Testing
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• v.24 no.1
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• pp.15-21
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• 2004

It is possible to detect flaws in pipelines without interruption using all EMAT transducer because it is a non-contact transducer which can transmit ultrasonic waves into specimens without couplant. And it ran easily generate guided waves desired in each specific problem by altering the design of coil and magnet. In the present work, EMAT systems have been fabricated to generate surface waves, and selectively the plate wave of $A_1\;or\;S_1$ mode. The surface wave of 1.5MHz showed a good signal-to-noise ratio without distortion in its propagation along a pipeline, while the $S_1$ mode of 800kHz and the $A_1$ mode of 940kHz were distorted according to their dispersive properties. The wider the excitation pulse becomes, the better the mode selectivity of the plate waves becomes. A pipe of 256mm inner diameter and 5.5m thickness with 5 flaws was used for comparing the flaw detectability among the modes under consideration.

• The Journal of the Acoustical Society of Korea
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• v.28 no.4
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• pp.356-362
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• 2009

In modern society, most people of the world are densely populated in cities so that the traffic sound has a very significant meaning. people tend to classify traffic sound as a noise pollution while they are likely to categorize most nature sound as positive. In this paper, we applied various forms of FFT filters into white noise belonged in nature sound to find frequency characteristics of white noise which preferred by people and confirm its correlation with nature sound. In addition, we conducted an analysis through the comparison of various traffic and nature sound waveforms and spectra. As a result of analysis, the traffic sound have characteristics which sound energy had concentrated on specific frequency bandwidth and point of time compared to nature sound. And we confirmed the fact that these characteristics had negative elements to which could affect to people. Lastly, by letting the subjects listen directly to both traffic and nature sound through brainwave experiment using electrode, the study measured the energy distribution of alpha waves and beta waves. As a result of experiments, it has been noted that urban sound created a noticeably larger amount of beta waves than nature sound; on the contrary, nature sound generated positive alpha waves. These results could directly confirm the negative effects of traffic sound and the positive effects of nature sound.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.25 no.6
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• pp.549-558
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• 2012

A topology optimization method for phononic crystals is developed for the design of sound barriers, using the level set approach. Given a frequency and an incident wave to the phononic crystals, an optimal shape of periodic inclusions is found by minimizing the norm of transmittance. In a sound field including scattering bodies, an acoustic wave can be refracted on the obstacle boundaries, which enables to control acoustic performance by taking the shape of inclusions as the design variables. In this research, we consider a layered structure which is composed of inclusions arranged periodically in horizontal direction while finite inclusions are distributed in vertical direction. Due to the periodicity of inclusions, a unit cell can be considered to analyze the wave propagation together with proper boundary conditions which are imposed on the left and right edges of the unit cell using the Bloch theorem. The boundary conditions for the lower and the upper boundaries of unit cell are described by impedance matrices, which represent the transmission of waves between the layered structure and the semi-infinite external media. A level set method is employed to describe the topology and the shape of inclusions. In the level set method, the initial domain is kept fixed and its boundary is represented by an implicit moving boundary embedded in the level set function, which facilitates to handle complicated topological shape changes. Through several numerical examples, the applicability of the proposed method is demonstrated.

• The Journal of the Acoustical Society of Korea
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• v.34 no.5
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• pp.335-342
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• 2015

Phononic crystals are composite materials consisting of a periodic arrangement of scattering inclusions in a host material. One of the most important properties of phononic crystals is the existence of band gaps, i.e., ranges of frequencies at which acoustic waves cannot propagate through the structure. The present study aims to investigate theoretically and experimentally the acoustic band structures in two-dimensional (2D) phononic crystals consisting of periodic square arrays of stainless steel solid cylinders with a diameter of 1 mm and a lattice constant of 1.5 mm in water. The theoretical dispersion relation that depicts the relationship between the frequency and the wave vector was calculated along the ${\Gamma}X$ direction of the first Brillouin zone using the finite element method to predict the band structures in the 2D phononic crystals. The transmission and the reflection coefficients were measured in the 2D phononic crystals with 1, 3, 5, 7, and 9 layers of stainless steel cylinders stacked in the perpendicular direction to propagation at normal incidence. The theoretical dispersion relation exhibited five band gaps at frequencies below 2 MHz, the first gap appearing around a frequency of 0.5 MHz. The location and the width of the band gaps experimentally observed in the transmission and the reflection coefficients appeared to coincide well with those determined from the theoretical dispersion relation.

• Geophysics and Geophysical Exploration
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• v.11 no.1
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• pp.15-25
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• 2008

Ocean-bottom seismometer (OBS) positions are one of the key parameters in an OBS-airgun seismic survey for crustal structure study. To improve the quality of these parameters, we have developed a new method of determining OBS positions, using airgun shot data and bathymetric data in addition to available distance measurements by acoustic transponders. The traveltimes of direct water waves emitted by airgun shots and recorded by OBSs are used as important information for determining OBS locations, in cases where there are few acoustic transponder data (<3 sites). The new method consists of two steps. A global search is performed as the first step, to find nodes of the bathymetric grid that are the closest to explaining the observed direct water-wave traveltimes from airgun shots, and acoustic ranging using a transponder system. The use of precise 2D bathymetric data is most important if the bottom topography near the OBS is extremely rough. The locations of the nodes obtained by the first step are used as initial values for the second step, to avoid falling into local convergence minima. In the second step, a non-linear inverse method is executed. If the OBS internal clock shows large drift, a secondary correction for the OBS internal clock is obtained, as well as the OBS location, as final results by this method. We discuss the error and the influence of each measurement used in the determination of OBS location.

• Geophysics and Geophysical Exploration
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• v.23 no.1
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• pp.38-49
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• 2020

Full-waveform inversion (FWI) is an optimization process of fitting observed and modeled data to reconstruct high-resolution subsurface physical models. In acoustic FWI (AFWI), pressure data acquired using a marine streamer has mainly been used to reconstruct the subsurface P-wave velocity models. With recent advances in marine seismic-acquisition techniques, acquiring multi-component data in marine environments have become increasingly common. Thus, AFWI strategies must be developed to effectively use marine multi-component data. Herein, we proposed an AFWI strategy using horizontal and vertical particle-acceleration data. By analyzing the modeled acoustic data and conducting sensitivity kernel analysis, we first investigated the characteristics of each data component using AFWI. Common-shot gathers show that direct, diving, and reflection waves appearing in the pressure data are separated in each component of the particle-acceleration data. Sensitivity kernel analyses show that the horizontal particle-acceleration wavefields typically contribute to the recovery of the long-wavelength structures in the shallow part of the model, and the vertical particle-acceleration wavefields are generally required to reconstruct long- and short-wavelength structures in the deep parts and over the whole area of a given model. Finally, we present a sequential-inversion strategy for using the particle-acceleration wavefields. We believe that this approach can be used to reconstruct a reasonable P-wave velocity model, even when the pressure data is not available.

• Geophysics and Geophysical Exploration
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• v.25 no.4
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• pp.177-188
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• 2022

Recently, the application of distributed acoustic sensors (DAS), which can replace geophones and seismometers, has significantly increased along with interest in micro-seismic monitoring technique, which is one of the CO₂ storage monitoring techniques. A significant amount of temporally and spatially continuous data is recorded in a DAS monitoring system, thereby necessitating fast and accurate data processing techniques. Because event detection and seismic phase picking are the most basic data processing techniques, they should be performed on all data. In this study, a machine learning-based P, S wave phase picking algorithm was developed to compensate for the limitations of conventional phase picking algorithms, and it was modified using a transfer learning technique for the application of DAS data consisting of a single component with a low signal-to-noise ratio. Our model was constructed by modifying the convolution-based EQTransformer, which performs well in phase picking, to the ResUNet structure. Not only the global earthquake dataset, STEAD but also the augmented dataset was used as training datasets to enhance the prediction performance on the unseen characteristics of the target dataset. The performance of the developed algorithm was verified using K-net and KiK-net data with characteristics different from the training data. Additionally, after modifying the trained model to suit DAS data using the transfer learning technique, the performance was verified by applying it to the DAS field data measured in the Pohang Janggi basin.

• Journal of Energy Engineering
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• v.21 no.2
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• pp.133-137
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• 2012

Among various clean energy technologies, the solar energy technology has been widely used in various fields such as photovoltaic power generation and solar water/space heating. These days, special attention is drawn on its conversion into acoustic energy along with waste heat as a means to promote clean energy utilization. This work was carried out to investigate the possibility of converting solar energy into acoustic waves, especially, its performance characteristics for a single resonance tube (20.2 mm in ID). Variations are made for the stack length and its position as well as power supply. For a resonance tube of 200mm, an average sound pressure of 114.5 dB was measured with a stack length of 25.6mm at 5cm from the closed end. When the power supply was increased to 35W, an average sound pressure of 117.29 dB was detected with a frequency of 500Hz. There was an increase in frequency, 630 Hz (115.7dB), with a shorter resonance tube of 150mm.