• 대한토목학회논문집
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• 제13권3호
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• pp.173-183
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• 1993

장주기파(長週期波)의 침입(侵入)에 의한 항내(港內) 수면교란(水面攪亂) 현상(現象)을 분석(分析)하기 위하여 동해항(東海港)의 항외(港外), 항입구(港入口) 및 항내(港內)의 4개(個) 정점(定點)에서 약 1개월간(1992. 2~3) 수압식(水壓式) 파고계(波高計)를 이용하여 현장관측(現場觀測)을 실시(實施)하였다. 관측자료(觀測資料)의 스펙트럼 분석(分析) 결과(結果) 동해항(東海港)의 Helmholtz natural period는 약 17.1분, 제(第)2 첨두주기(尖頭週期)는 약 5.5분이 되는 것으로 나타났다. Helmholtz natural period에서의 증폭비(增幅比)는 약 10으로 영일제(迎日濟)의 약 2에 비해 상당히 크게 나타났으나 관측기간중(觀測期間中)의 항내(港內)에서의 해수의(海水位) 변위(變位) 진폭(振幅)은 약 10cm로 비교적 작은 것으로 나타났다.

• 동력기계공학회지
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• 제9권1호
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• pp.5-13
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• 2005

Acoustic emission(AE) is defined as the transient elastic waves thar are generated by the rapid release of energy. The advantage of AE is that very early crack growth can be detected well before a highly stressed component may fail. At present, an exact diagnosis is the most reliable means for determining the soundness of structures during power plant operations. AE monitoring has been applied successfully in power plants to determine mechanical problems, pressure vessel integrity and external valves leaks, vacuum leaks, the onset of cavitation in pumps and valves, the presence of flow(or no flow) in piping and heat exchange equipment, etc. Acoustic emission(AE) technology has recently strengthened its application base, and practitioners' understanding of the technique's fundamentals. This paper introduces the methods of a survey and assessment on AE monitoring applications in nuclear, fossil and hydraulic power plant. The main objective of this paper was to obtain information on various applications of AE technology in power plant.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2006년도 춘계학술대회논문집
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• pp.858-861
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• 2006

In this paper, a study on the noise reduction in a mobile fuel cell system is presented. Among various fuel cell systems around 20W capacities designed for mobile electronic devices, the active direct methanol fuel cell (DMFC) systems have been recently developed. In such systems, the primary noise source is the air pump which provides sufficient air flow ($5{\sim}6$ liter/min) for electrochemical reaction with methanol fuel while the noise contributions from other auxiliary parts are relatively small. Especially, the discrete noise tones generated by the air pump are dominant and those frequency peaks related to the rotor harmonics are needed to be suppressed by a silencer. Therefore. the Herschel/Quinke (HQ) tubes, which use the out-of-phase cancellation of acoustic waves propagating through direct and indirect pathways, are applied to the inlet of the air pump. Performance of noise reduction with HQ silencer is analytically estimated by calculating the transmission. The length and number of thin HQ tubes are optimized to decrease the radiated noise. As a result, the sound pressure level could be successfully reduced by about 10 dB after applying three serially connected HQ tubes.

• 비파괴검사학회지
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• 제19권6호
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• pp.420-425
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• 1999

Time of flight diffraction(TOFD)법은 초음파의 회절현상을 이용한 검사방법으로 일반적인 pulse echo 법에 비해 검사수행 속도가 빠르며, 보다 알기 쉽게 결함을 유무를 표시하고 검사표면의 상태와 결함의 방향에 의한 검사 결과가 기존에 방법에 비해 안정적인 특징을 갖는다. 그러나 TOFD 법은 결함의 위치가 검사체의 표면 근처에 위치하는 경우 표면을 따라 전파되는 lateral wave의 영향으로 결함의 검출에 어려움이 따른다. 본 연구에서는 TOFD 법을 이용한 초음파 탐상 시스템을 개발하였고, TOFD 법의 단점이라 할 수 있는 표면 근처에 존재하는 결함을 보다 쉽게 검출하기 위하여 deconvolution 법을 사용하여 근거리 분해능을 향상시켰다.

• Geomechanics and Engineering
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• 제23권1호
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• pp.1-13
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• 2020

To give a solution for seismic stability of tunnel faces subjected to earthquake ground shakings, the pseudo-dynamic approach is originally introduced to analyze tunnel face stability in this study. In the light of the upper-bound theorem of limit analysis, an advanced three-dimensional mechanism combined with pseudo-dynamic approach is proposed. Based on this mechanism, the required support pressure on tunnel face can be obtained by equaling external work rates to the internal energy dissipation and implementing an optimization searching procedure related to time. Both time and space feature of seismic waves are properly accounted for in the proposed mechanism. For this reason, the proposed mechanism can better represent the actual influence of seismic motion and has a remarkable advantage in evaluating the effects of vertical seismic acceleration, soil amplification factor, seismic wave period and initial phase difference on tunnel face stability. Furthermore, the pseudo-dynamic approach is compared with the pseudo-static approach. The difference between them is illustrated from a new but understandable perspective. The comparison demonstrates that the pseudo-static approach is a conservative method but still could provide precise enough results as the pseudo-dynamic approach if the value of seismic wavelengths is large or the height of soil structures is small.

• International Journal of Aeronautical and Space Sciences
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• 제14권3호
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• pp.201-209
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• 2013

Much numerical and experimental research has been done for the flow around an oscillating airfoil. The main research topics are vortex shedding, dynamic stall phenomenon, MAV's lift and thrust generation. Until now, researches mainly have been concentrated on analyzing the wake flow for the variation of frequency and amplitude at a low angle of attack. In this study, wake structures and acoustic wave propagation characteristics were studied for a plunging airfoil at high angle of attack. The governing equations are the Navier-Stokes equation with LES turbulence model. OHOC (Optimized High-Order Compact) scheme and 4th order Runge-Kutta method were used. The Mach number is 0.3, the Reynolds number is, and the angle of attack is from $20^{\circ}$ to $50^{\circ}$. The plunging frequency and the amplitude are from 0.05 to 0.15, and from 0.1 to 0.2, respectively. Due to the high resolution numerical method, wake vortex shedding and pressure wave propagation process, as well as the propagation characteristics of acoustic waves can be simulated. The results of frequency analysis show that the flow has the mixed characteristics of the forced plunging frequency and the vortex shedding frequency at high angle of attack.

• 대한화학회지
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• 제30권1호
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• pp.105-108
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• 1986

초음파(50KHz)가 상온 상압하에서 히드라진 철 활성탄을 이용한 방향족 니트로기를 방향족 아미노기로의 환원반응을 크게 가속시켜 3시간만에 100% 전환율을 얻을 수 있었다. 활성탄은 탈색이나 반응성이 큰 물질의 분말을 섞는데 사용되어 왔으나 히드라진 철을 이용한 방향족 니트로기의 환원반응에서는 활성탄 없이는 반응이 전혀 일어나지 않으며 활성탄의 증가에 따라 환원율이 높아지는 결과를 얻었다.

• 대한조선학회논문집
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• 제45권4호
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• pp.345-352
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• 2008

The motion of a projectile initiated by the release of highly pressurized air is simulated presuming the flow field as a two dimensional one. The effects of water compressibility on projectile movements are investigated, comparing results based on the Fluent VOF model where water is treated as an incompressible medium with those from the presently developed VOF scheme. The present model considers compressibility of both air and water. The Fluent results show that the body moves farther and at higher speeds than the present ones. As time proceeds, the relative difference of speed and displacement between the two results drops substantially, after acoustic waves in water traverse and return the full length of the tube several times. To estimate instantaneous accelerations, however, requires implementation of the water compressibility effect as discrepancies between them do not decrease even after several pressure wave cycles.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2003년도 춘계학술대회논문집
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• pp.168-173
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• 2003

This experimental study describes the propagation characteristics of the impulse noise emitted from the exit of a perforated pipe attached to the open end of a simple shock tube. The pressure amplitudes and directivities of the impulse wave propagating from the exit of perforated pipe with several different configurations are measured and analyzed fur the range of the incident shock wave Mach number between 1.02 and 1.2. In the experiments, the impulse waves are visualized by a Schlieren optical system for the purpose of investigating their propagation pattern. The results obtained show that for the near sound field the impulse noise strongly propagates toward to the pipe axis, but for the far sound field the impulse noise uniformly propagates toward to the all directions, indicating that the directivity pattern is almost same regardless of the pipe type. Moreover, it is shown that for the far sound field the perforated pipe has little performance to suppress the impulse noise.

• 한국해양공학회지
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• 제30권6호
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• pp.484-497
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• 2016

In order to understand the characteristics of the topography change in front of an impermeable breakwater, a coupled model for a two-way analysis of the existing LES-WASS-2D and newly developed morphodynamic model was suggested. A comparison to existing experimental results revealed that the results computed using the 2-D hydro-morphodynamic model were in good agreement with the experimental results for the wave form, pore water pressure in the seabed, and topographical change in front of a submerged breakwater. It was shown that the two-way model suggested in this study is applicable to a morphological change in the seabed around a submerged breakwater. Then, using the numerical results, the topographical changes in front of an impermeable submerged breakwater were examined in relation to partial standing waves. Moreover, the characteristics of the local scour depths in front of them are also discussed in relation to incident wave conditions, sediment qualities, and submerged breakwater shapes.