• 한국해양공학회지
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• 제23권4호
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• pp.6-11
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• 2009

In this study, the internal waves in two-density layered fluids were analyzed using the Numerical Wave Tank (NWT) technique in the frequency domain. The NWT is based on a two-domain Boundary Element Method with the potential fluids using the whole-domain matrix scheme. From the mathematical solution of the two-domain boundary integral equation, two different wave modes could be classified: a surface wave mode and an internal wave mode, and each mode were shown to have a wave number determined by a respective dispersion relation. The magnitudes of the internal waves against surface waves were investigated for various fluid densities and water depths. The calculated results are compared with available theoretical data.

• 대한기계학회논문집B
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• 제27권4호
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• pp.415-423
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• 2003

The hydrodynamic instability of the three-dimensional boundary-layer over a rotating disk has been numerically investigated for these flows; Ro = -1, -0.5, and 0, using linear stability theory. Detailed numerical values of the disturbance wave number. wave frequency. azimuth angle. radius (Reynolds number, Re) and other characteristics have been calculated for the pre-swirl flows. On the basis of Ekman and Karman boundary layer theory, the instability of the pre-swirl flows have been investigated for the unstable criteria. The disturbance will be relatively fast amplified at small fe and within wide bands of wave number compared with previously known Karman boundary-layer results. The flow (Ro =-0.5) is found to be always stable for a disturbance whose dimensionless wave number is greater than 0.9. It has a larger range of unstable interval than Karman boundary layer and can be unstable at smaller Re.

• 한국음향학회지
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• 제25권1호
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• pp.43-48
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• 2006

압전식 충격파 쇄석기를 사용하여 대상물을 파쇄하는 과정에서 발생하는 음의 특성을 조사한 결과, 대상물이 파쇄됨에 따라 방사음이 더욱 저주파대로 이동한다는 실험적인 결과가 발표되었고(2), 이 결과를 입증하기 위하여 실제의 대상물이 파쇄되어가는 과정을 모델링하여 제작한 대상물을 사용한 실험에서도 동일한 결과가 관측되었다. 본 논문에서는 이와 같이 실험적으로 밝혀진 연구결과를 이론적으로 확인하고자 한다. 따라서, 유한요소법을 이용한 수치 시뮬레이션을 실험과 유사하게 모델링한 대상물에 실행하였고, 시뮬레이션에서 관측된 결과를 제시하여 실험적으로 관측된 결과가 이론적으로도 유효하다는 것을 입증하였다.

• Geomechanics and Engineering
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• 제13권5호
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• pp.775-791
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• 2017

Surface wave techniques are widely used as non-invasive method for geotechnical site characterization. Field surface wave data are collected and analyzed using different processing techniques to generate the dispersion curves, which are further used to extract the shear wave velocity profile by inverse problem solution. Characteristics of a dispersion curve depend on the subsurface layering information of a vertically heterogeneous medium. Sometimes soft layer can be found between two stiff layers in the vertically heterogeneous media, and it can affect the wave propagation dramatically. Now most of the surface wave techniques use the fundamental mode Rayleigh wave propagation during the inversion, but this may not be the actual scenario when a soft layer is present in a vertically layered medium. This paper presents a detailed and comprehensive study using finite element method to examine the effect of soft layers which sometimes get trapped between two high velocity layers. Determination of the presence of a soft layer is quite important for proper mechanical characterization of a soil deposit. Present analysis shows that the thickness and position of the trapped soft layer highly influence the dispersion of Rayleigh waves while the higher modes also contribute in the resulting wave propagation.

• Structural Engineering and Mechanics
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• 제69권5호
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• pp.487-497
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• 2019

Rapid advances in the engineering applications can bring further areas to provide the opportunity to manipulate anisotropic structures for direct productivity in design of micro/nano-structures. For the first time, magnetic affected wave characteristics of nanosize plates made of anisotropic material is investigated via the three-dimensional bi-Helmholtz nonlocal strain gradient theory. Three small scale parameters are used to predict the size-dependent behavior of the nanoplates more accurately. After owing governing equations of wave motion, an analytical approach based harmonic series is utilized to fine the wave frequency as well as phase velocity. It is observed that the small scale parameters, magnetic field and wave number have considerable influence on the wave characteristics of anisotropic nanoplates. Due to the lack of any study on the mechanics of three-dimensional bi-Helmholtz gradient plates made of anisotropic materials, it is hoped that the present exact model may be used as a benchmark for future works of such nanostructures.

• 한국추진공학회지
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• 제2권3호
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• pp.36-46
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• 1998

초음속 유동에서 발생하는 충격파는 일반적으로 충격파 상류와 하류의 압력비에 의해 일반적으로 결정된다. 본 연구에서는 충격파의 진동현상을 규명하기 위한 연구의 일환으로, 미소교란법(small perturbation method)을 이용하여 이론해석을 수행하였으며, 충격파 하류의 유동장에 에너지 손실을 적용하여 충격파의 안정성을 이론적으로 해석하였다. 이론해석에서는 충격파 상류에 경계층 유동에서 발생하는 난류변동이나 주류에서의 압력변동이 충격파의 진동과 관련된다고 생각하여, 충격파 상류의 유동에 미소압력변동을 적용하여 충격파의 진동특성을 해석하였다. 본 연구의 결과들을 타 연구의 결과와 비교하였으며, 열선풍속계(hot wire)를 이용한 실험적 연구결과와 비교하였다.

• 전기학회논문지
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• 제63권7호
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• pp.968-975
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• 2014

This paper proposed a evaluation method for individual's subjective preferred location using EEG and emotional assessment. Visual stimulus were sequentially presented a total six points(the top and the bottom of the left, the top and the bottom of the center and the top and the bottom of the right on the screen). EEG were measured from twenty subjects according to each six points. At the same time, we were executed evaluation of subjects preferred location from emotional assessment. Alpha and beta wave were measured in Fp1, Fp2, F7 and F8 location, followed by ten to twenty electrode system. Correlations and variations of alpha and beta wave from each channel were calculated and analyzed. Because of the number of subjects population under 30, we used Speareman test for a correlation analysis between alpha and beta wave. Also, emotional assessments which compose of visual sense harmony, visual sense stability, stability of position and the visibility were performed and were analyzed by average and frequency. After visual stimulus, emotional assessments were performed. From the variance analysis of EEG, beta wave from F7 was appeared statistically significant as significance probability of 0.006. Also, between alpha wave and beta wave appeared a negative correlation(r=-0.190). From the post-hoc test of F7 beta wave, location 1, 5 and 6 appeared to difference statistically significant. Emotional assessment result according to six positions showed 0.00 significance probability. Thus, location and emotional assessment appeared to influence on each other. From the average and frequency analysis of emotional assessment, location 2 showed obtained of best emotional assessment score and appeared lower beta wave than other locations. Finally, most subjects showed a preference for location 2. Through obtained results in this paper, will be helpful to about human emotional assessment and EEG research.

• 물과 미래
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• 제29권6호
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• pp.155-166
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• 1996

개수로내의 검변 및 급변 부정류 해석을 위해서 dynamic wave식을 기본방정식으로 하고 이를 불연속 보간함수와 upstream weighting 을 도입한 Petrov-Galerkin 기법에 의해 해석하는 유한요소모형을 개발하였다. 매트릭스 안정성 해석 결과 Petrov-Galerkin기법은 단파장에서의 선택적 감쇠능력과 위상오차에 있어 우수한 것으로 나타났다. 반면에 Preissmann기법은 단파장에서의 선택적 감쇠능력과 위상오차에 있어 열등한 것으로 나타났고, Bubnov-Galerkin 기법은 비감쇠특성을 나타내고 있어 단파장 영역에서 발산해를 일으키는 주요원인임을 확인할 수 있었다. Petrov-Galerkin 방법은 Courant수의 넓은 범위에서 높은 주파수를 가진 진행파에 대한 선택적인 감쇠와 작은 Courant 수의 범위에서 양호한 위상정도를 가지는 이상적인 조합을 나타내고 있어 점변 및 급변 부정류 해석에 있어 이상적인 기법으로 활용될 수 있을 것으로 판단되었다.

• 한국소음진동공학회논문집
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• 제20권9호
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• pp.816-821
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• 2010

The paper presents a theoretical analysis on the propagation characteristics of the high-frequency wave in an elastic waveguide whose diameter is less than or similar to the wavelength. The theoretical results were verified by comparing them with the numerical results obtained by the boundary-element method. The ratio of the waveguide diameter to the wavelength affects the number of the existing wavenumber, and thus it affects the propagation characteristics. In the media with attenuation, the trend is similar to that in the media without attenuation except the decreasing amplitude.

• 한국정밀공학회:학술대회논문집
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• 한국정밀공학회 2009년도 춘계학술대회 논문집
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• pp.225-226
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• 2009