• International Journal of Naval Architecture and Ocean Engineering
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• 제9권2호
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• pp.160-176
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• 2017

The interaction of focused wave groups with a vertical wall is investigated based on the second order potential theory. The NewWave theory, which represents the most probable surface elevation under a large crest, is adopted. The analytical solutions of the surface elevation, velocity potential and wave force exerted on the vertical wall are derived, up to the second order. Then, a parametric study is made on the interaction between nonlinear focused wave groups and a vertical wall by considering the effects of angles of incidence, wave steepness, focal positions, water depth, frequency bandwidth and the peak lifting factor. Results show that the wave force on the vertical wall for obliquely-incident wave groups is larger than that for normally-incident waves. The normalized peak crest of wave forces reduces with the increase of wave steepness. With the increase of the distance of focal positions from the vertical wall, the peak crest of surface elevation, although fluctuates, decreases gradually. Both the normalized peak crest and adjacent crest and trough of wave forces become larger for shallower water depth. For focused wave groups reflected by a vertical wall, the frequency bandwidth has little effects on the peak crest of wave elevation or forces, but the adjacent crest and trough become smaller for larger frequency bandwidth. There is no significant change of the peak crest and adjacent trough of surface elevation and wave forces for variation of the peak lifting factor. However, the adjacent crest increases with the increase of the peak lifting factor.

• 한국해안해양공학회지
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• 제15권1호
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• pp.11-20
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• 2003

본 논문에서는 규칙파 모델을 이용하여 유공케이슨 방파제로부터의 불규칙파의 반사를 계산하는 몇 가지 방법들을 검토한다. 첫 번째 방법은 불규칙파를 파고 및 주기가 각각 불규칙파의 제곱평균제곱근 파고 및 유의주기와 같은 규칙파로 근사하는 것이다. 두 번째는 불규칙파 스펙트럼의 각 주파수 성분에 대하여 규칙파 모델을 반복적으로 사용하는 것이다. 파의 주기는 성분파의 주파수에 따라 결정되며, 모든 주파수에 대해서 제곱평균제곱근 파고를 사용한다. 세 번째 방법은 두 번째 방법과 같으나, 각 성분파의 에너지에 해당하는 파고를 사용한다. 이전 연구자들의 실험 결과와 비교해 본 결과, 두 번째 방법이 가장 타당하며, 주파수 평균 반사율과 반사파 스펙트럼 모두 실험치와 양호한 일치를 보였다.

• Journal of Mechanical Science and Technology
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• 제19권8호
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• pp.1682-1691
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• 2005

Direct numerical simulation database of an axial turbulent boundary layer is used to compute frequency and wave number spectra of the wall shear-stress fluctuations in a low-Reynolds number axial turbulent boundary layer. One-dimensional and two-dimensional power spectra of flow variables are calculated and compared. At low wave numbers and frequencies, the power of streamwise shear stress is larger than that of spanwise shear stress, while the powers of both stresses are almost the same at high wave numbers and frequencies. The frequency/streamwise wave number spectra of the wall flow variables show that large-scale fluctuations to the rms value is largest for the stream wise shear stress, while that of small-scale fluctuations to the rms value is largest for pressure. In the two-point auto-correlations, negative correlation occurs in streamwise separations for pressure, and in span wise correlation for both shear stresses.

• 설비공학논문집
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• 제15권11호
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• pp.895-901
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• 2003

Direct numerical simulation database of an axial turbulent boundary layer is used to compute frequency and wave number spectra of the wall shear-stress fluctuations in a low-Reynolds number axial turbulent boundary layer. One-dimensional and two-dimensional power spectra of flow variables are calculated and compared. At low wave numbers and frequencies, the power of streamwise shear stress is larger than that of spanwise shear stress, while the powers of both stresses are almost the same at high wave numbers and frequencies. The frequency/streamwise wave number spectra of the wall flow variables show that large-scale fluctuations to the ms value is largest for the streamwise shear stress, while that of small-scale fluctuations to the rms value is largest for pressure. In the two-point auto-correlations, negative correlation occurs in streamwise separations for pressure and spanwise shear stress, and in spanwise correlation for both shear stresses.

• 대한기계학회논문집B
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• 제25권10호
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• pp.1327-1336
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• 2001

The structural modes driven by the low wave-number components of smooth elastic wall pressure provide a relatively weak coupling between the flow and the wall motion. If the elastic thin plate has any resonant mode whose wave-number of resonance coincides with $\omega$/U$\sub$c/, the power will be transmitted to those modes of vibration by the flows. We examine the problem in which the elastic thin plate is subject to pressure fluctuations under turbulent boundary layer. Measurements are presented of the frequency spectra of the near- and far-field pressures and radiated sound contributed by the various wave modes of the thin elastic plate. Dispersion equation for wave motions of elastic plate is used to investigate the effect of bending waves of relatively low wave number on radiated sound. The low wave-number motion of elastic plate is observed to have much less influence on the low-frequency energy of wall pressure fluctuations than that of the rediated sound. High amplitude events of the wall pressure are observed to weakly couple with high-frequency energy of radiated sound for case of low tension applied to the plate. The sound source localization is applied to the measurement of radiated sound by using acoustic mirror system.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2002년도 학술대회지
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• pp.205-208
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• 2002

Numerous studies have been performed to develop an analytical model that can predict the reflection of regular or irregular waves from a perforated-wall caisson breakwater. Though such irregular wave models as Suh et at. (2001) become available, regular wave models are still in extensive use because of their simplicity. In the present study, using the regular wave model of Fuggazza and Natale(1992), the reflection of irregular waves from a perforated-wall caisson breakwater was calculated in several different methods. First, the regular wave model was re-validated by the hydraulic model tests. Though the model somewhat over-predicted the reflection coefficients at larger values and under-predicted them at smaller values, overall agreement was pretty good between calculation and measurement. Then, the regular wave model was applied to calculate the irregular wave reflection in the experiments of Suh et at.(2001) and Bennett et al. (1992). In applying the regular wave model to irregular wave reflection, several different methods were used. The results showed that it is the most reasonable to use the regular wave model repeatedly for each frequency component of the irregular wave specuum with the root-mean-squared wave height for all the frequencies .

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 1996년도 추계학술대회논문집; 한국과학기술회관, 8 Nov. 1996
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• pp.148-152
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• 1996

A method of deminishing low frequency noise by acoustic acoupling with compliant wall is described. The coupled governing equations and boundary conditions are derived and solved. The coupled system shows very interesting behavior in the low frequency region; in the low frequency, acoustic wave doesn't propagate, but decay to satisfy the boundary condition with the compliant wall. Henceforth using this mechanism, we propose a method of reducing low frequency noise, which is infact related with the physical properties of compliant wall. The method has been experimentally verified.

• 한국항해학회지
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• 제22권1호
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• pp.55-62
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• 1998

본 논문에서는 TV 주파수에 있어서 건조물에 의해 발생하는 전자파장해를 평가하기 위해 건축물 주위의 산란전자계를 수치계산하였다. 건축물 외벽에 전파흡수체를 부분적으로 부착한 경우 콘크리트면의 노출면적과 반사전계강도와의 관계를 조사하였다. 계산된 산란 전자계로부터 DU 비를 구해 TV 고스트방지를 위하여 건축물 외벽에 부착할 전파흡수체에 요구되는 반사 감쇠량을 구하였다.

• 한국가스학회지
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• 제9권3호
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• pp.1-8
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• 2005

배관은 사용기간 중에 여러 가지 열화에 의해 손상을 받으며, 그 중 감육(두께감소)이 발생하게 되면 심각한 결과를 초래할 수 있다. 따라서, 배관시스템의 보전유지를 위해서는 감육되어진 배관의 강도를 평가하는 것이 중요하다. 어떤 재료가 외부 응력 등에 의해 파괴 혹은 변형된다면 그 특징적인 탄성파가 변형메카니즘에 따라 여러 가지로 다르게 나타날 수 있다. 본 연구에서는 감육을 가지는 배관에서 감육결함의 길이 및 깊이를 다르게 상정하였을 때 나타나는 AE신호들에 대해 Wavelet Transform시스템을 이용하여 평가하였다. 또한,실배관의 굽힘시험을 통하여 기계적인 강도를 평가하였다. 결과로부터, 감육의 깊이비(d/t)의 정도에 따라 주파수 성분을 구별하는 것이 가능하였다.

• 한국산업융합학회 논문집
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• 제3권3호
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• pp.209-217
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• 2000

An attempt has been made to examine the highest liquid surface by means of measuring the movement of liquid free surface wave in the agitated vessel wall. Also, Relationship has been investigated between transition frequency, $N_z$ for behavior of liquid free surface and transition frequency for behavior of mass transfer coefficient in the agitated vessel wall. A surface wave is occured within agitated vessel, the highest height of wetted vessel wall, $h_w$ is coincided with the highest liquid surface, h. However the lowest height of wetted vessel, $h^*{_w}$ is not descended to lowest liquid free surface, $h^*$. The proposed model has been tested sucessfully for the behavior of liquid free surface.