• Journal of Korean Society of Coastal and Ocean Engineers
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• v.11 no.1
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• pp.20-33
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• 1999

The evolution of deep-sea waves is driven by energy input from wind, nonlinear energy transfer between wave components, and dissipation through whitecaps. A comparative study was implemented by the use of two wave models in which only the computation methods of nonlinear wave-wave interactions are different from each other. It was reaffirmed that the nonlinear interaction plays a central role in such phenomena that occurred during the spectral growth of wind-seas as down-shift of the spectral peak frequency, overshoot, undershoot, and formation of self-similar spectrum. Specifically, the directional distribution at high frequencies develops into bimodal form, which is attributed to the nonlinear interactions. As saturation stage is reached, spectral density at high frequencies becomes proportional to negative 4 power to the frequency. Perturbations introduced into the spectrum quickly vanished through the actions of the self-similar mechanism. Thus, the nonlinear transfer has important contribution to the stability of numerical ocean wave models.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.19 no.1
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• pp.29-37
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• 2007

In this study, transmission and reflection of multi-directional random waves propagating over impermeable submerged breakwaters are calculated by using eigenfunction expansion method. A series of mutiderectional random waves is generated by using the Bretschneider-Mitsuyasu frequency and Mitsuyasu type directional spectrum. Strong reflection is occurred at the Bragg reflection condition of the peak frequency. If the row of breakwaters is fixed at 3 and the relative height of breakwater is fixed at 0.6, more than 25% of incident wave energy is reflected to offshore. It is also found that the reflection of directionally spreading random waves increases as the maximum spreading parameter $s_{max}$ increases.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.30 no.2
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• pp.39-50
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• 2018

In this study, the nonlinear interaction of irregular waves with wave breaking and currents around permeable submerged breakwater was investigated with the aid of olaFlow model which is open source CFD software published under the GPL license. The irregular wave performance of olaFlow applied in this study was verified by comparing and evaluating the target frequency spectrum and the generated frequency spectrum for applicability to irregular waves. Based on the applicability of this numerical model to irregular wave fields, in the coexistence fields of irregular waves and currents, the characteristics of wave height, frequency spectrum, breaking waves, averaged velocity and turbulent kinetic energy around porous submerged breakwater with the respect to the beach type and current direction versus wave propagation were carefully investigated. The numerical results revealed that the shape of wave breaking on the crown of the submerged breakwater and the formation of the mean flow velocity around the structure depend greatly on the current directions and the type of the beach. In addition, it was found that the wave height fluctuation due to the current direction with respect to the wave propagation is closely related to the turbulent kinetic energy.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.2 no.2
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• pp.83-95
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• 1990

It can be assumed that the ocean waves consist of many independent pure sinusoidal components which progress in arbitrary directions. To analyze irregular sea waves, both the spectrum method and the individual wave method have been used. The spectral approach is valid in the region where the water depth is deep and the linear property of velocity distribution is predominent, while the individual wave analysis method in the region where the water depth is shallow and the wave nonlinearity is significant. Therefore, to investigate the irregular wave transformation from the deep water to the shallow water region, it is necessary to relate the frequency spectrum which is estimated by the spectrum analysis method to the i oint probability distribution of wave height, period and direction affected by the boundary condition of the individual wave analysis method. It also becomes important to define the region where both methods can be applied. This study is a part of investigation to establish a systematic approach for analyzing the irregular wave transformation. The region where the spectral approach can be applied is discussed by earring out the experiments on the irregular wave transformation in the two-dimensional wave tank together with the numerical simulation. The applicability of the individual wave analysis method for predicting irregular wave transformation including wave shoaling and breaking and the relation between frequency spectrum and joint probability distribution of wave height and period are also investigated through the laboratory experiment and numerical simualtion.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.21 no.4
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• pp.308-315
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• 2009

Wave-current interaction due to strong ambient currents causes to alter wave properties such as wave height, wave profile and wave spectrum. In this study we first examined the SWAN model's applicability by comparing with an analytical solution of Suh et al. (1994) for wave-current interaction in finite water-depth. Numerical experiments using SWAN model have been conducted for Garolim Bay to estimate the design waveheights influenced by strong tidal currents. For the design wave periods of 8~10 sec, the design wave height of 3 m in NNW direction was increased by up to 40% when the incident waves encounter the opposing currents of 1.4 m/s while the wave height was reduced by 26% due to the following currents of 1.1 m at the bay mouth. This result indicates that the effect of wave-current interaction must be included to determine the design wave height if there exists a strong current.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2006.11a
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• pp.498-501
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• 2006

When an object or objects, rigid or flexible, presents in incident sound field, the sound wave is scattered. This, we call, is scattered sound field. It, of course, depends on the amplitude and the direction of the incident sound field as well as the geometry and the surface impedance of the scatterer(object). This paper addresses the way to measure scattered sound field by using arbitrary incident sound wave. This means that the method can decompose the scattered field from measured sound field with respect to any magnitudes and directions of incident plane-waves.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.35 no.1
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• pp.27-32
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• 2007

this paper the acoustic signature from a supersonic nozzle is measured and compared to the result of a program developed for a gas turbine noise prediction. In order to measure the jet Mach Number, the pressure and temperature at the settling chamber was measured along with pressures from a pitot-tube placed near the exit. The results are also compared to the ones obtained with a shadow graph technique. Jet noise produced by an imperfectly expanded jet contains shock associated noise, which consist of broadband noise and screech tone noise. For subsonic condition, the directivity is dominant to the downstream direction due to turbulence mixing noise. For supersonic conditions, however, the directivity is dominant toward upstream direction due to shock associated noise. The comparison with a jet exhaust noise prediction code shows good agreement at supersonic conditions but needs to be improved at subsonic speeds.

• Journal of the Society of Disaster Information
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• v.18 no.3
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• pp.646-659
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• 2022

Purpose: In this study, we developed a 4-way sway prevention brace that efficiently reduces the installation area and has excellent stability and seismic performance compared to the conventionl sway prevention brace used in existing firefighting facilities. The performance and reliability of the developed 4-way way prevention brace were analyzed by the tensile, compression tests and seismic tests. Method: As the static test, 4-way sway prevention braces were installed on the horizontal and vertical pipes to perform the tensile and compression tests based on the KFI certification standard and the maximum movement was measured at the rated load. As a dynamic test, 4-way sway prevention braces were installed in the pipes filled with water, and the test response spectrum to the input excitation wave were measured through the acceleration sensors. After the seismic tests, separation, failure, and local deformation of the pipes, and 4-way sway prevention braces were not observed. Result: The results of the tensile and compression tests indicated that the maximum movement of the pipe during tension and compression was 50% to 70% or less compared to the certification values, indicating that the performances of the 4-way sway prevention braces were very excellent. The results of the the seismic tests indicated that the test response spectrum of the 4-way sway prevention braces is within the required response spectrum. Conclusion: In this study, it was found that the 4-way sway prevention braces satisfied the KFI certification standard and were superior compared to the existing sway prevention brace in terms of the stability, cost, and installation area.

• 유체기계공업학회:학술대회논문집
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• 2000.12a
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• pp.200-210
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• 2000

본 연구에서는 고속의 원심압축기에서 스톨 발단에 관한 연구를 수행하였다. 스톨을 일으키는 요인과, 스톨이 발생하기 전에 이를 미리 경고할 수 있는 방법을 주된 연구 주제로 삼았다. 원주방향으로 균일하게 분포된 8개의 고속응답 압력변환기를 사용하여 순간압력을 측정하였으며, 이 신호를 공간 푸리에 변환(space Fourier transform)을 사용하여 스톨의 발단을 알리는 신호를 측정하였고, 회전하는 파의 에너지(Traveling Wave Energy) 방법을 사용하여 스톨을 미리 경고하는 방법에 대하여 연구하였다. 회전하는 파의 에너지 방법은 스톨을 경고하는 데 좋은 성능을 보였으며, 저속에서는 약 임펠러 100회전, 중간속도에서는 약 200회전, 그리고 고속에서는 약 임펠러 1000회전의 경고시간을 보였다. 그리고 스톨 발단 근처에서 공간 푸리에 계수의 위상이 임펠러 주파수의 속도로 선형적인 증가를 보이는 구간이 나타났으며, 또한 임펠러 주파수의 스펙트럼이 스톨로 접근하면서 증가하는 것으로부터, 임펠러 주파수가 스톨을 일으키는 중요한 요인으로 작용함을 알 수 있었다. 또한 임펠러의 회전속도에 관계없이 스톨로 접근하면서 임펠러 주파수의 스펙트럼이 증가하므로, 이 값이 스톨을 경고하는 방법으로 사용될 수 있음을 보였으며, 약 임펠러 2n회전의 경고시간을 얻을 수 있었고, 임펠러의 속도가 빠를수록 긴 경고시간을 얻었다. 이 방법의 개발로 하나의 센서의 측정만으로도 효과적으로 스톨을 경고할 수 있는 기반을 마련하였다.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.25 no.1
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• pp.28-33
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• 2013

Multidirectional random waves that obliquely approach the shore were found to become directionally asymmetric due to refraction. The directional asymmetry was expressed in terms of the asymmetry parameter which is related to the maximum spreading parameter ($s_{max}$). In this study, we calculate variation of both the asymmetry and maximum spreading parameters at different water depths for various cases of incident wave angles and maximum spreading parameters in deep water. These values are different from Goda and Suzuki (1975) who neglected directional asymmetry of waves. In calculating directional asymmetry and maximum spreading parameters, we use the JONSWAP spectrum (Hasselmann et al., 1973) and Lee et al.'s (2010) directional distribution function. The processes and results are nondimensionalized with significant wave height, peak frequency and peak wave length in deep water.