• 한국해양공학회지
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• 제27권6호
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• pp.32-42
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• 2013

In this paper, a wave-induced swirl water chamber (SWC) for breakwater and wave power generation is introduced and its applicability to wave power generation in the coastal waters of Korea is investigated. The SWC type of wave power generation is a way to drive a turbine using the unidirectional swirl flow that is induced in the back of a curtain wall of a breakwater due to incident waves. The typical wave characteristics are obtained by analyzing the annual statistical wave data from KHOA (Korea Hydrographic and Oceanographic Administration). A numerical analysis is carried out on the variations in the SWC entrance height, wave height, and different installation conditions. For the numerical analysis, a commercial code, Fluent based on FVM, is used. As the entrance height decreases, the mass flow rate through the entrance is rarely changed, whereas the magnitude of the flow velocity of the smaller entrance height is greater than the other ones, which is better for the formation of an SWC swirl flow inside and the flow kinetic energy at the entrance. In cases of installation conditions where a wall is place behind and under SWC, it has been shown that the mass flow rate through the entrance is greater than that in the open condition, and sufficient flow kinetic energy is generated in the entrance for wave power generation. However, the swirl flow kinetic energy is relatively small. Thus, in the future, it is necessary to study the swirl flow generation, which is affected by the SWC shape.

• 한국해안·해양공학회논문집
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• 제34권4호
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• pp.119-127
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• 2022

본 연구에서는 테트라포드로 피복된 경사식 마운드 위의 직립벽에 작용하는 충격쇄파압을 쇄파 형태에 따라 구분하기 위해 규칙파를 생성하고 충돌 직전의 유속장과 기포분율을 측정하였다. 유속장 측정을 위해 쇄파 중 발생하는 기포의 움직임을 추적하는 기포영상유속계를 사용하고 기포분율 측정을 위해 복합파고계 기법(Na and Son, 2021)을 활용하였다. 측정된 입사파의 주기가 짧을수록 최대평균유속은 사면에서 파속에 비해 적은 감소율을 보였지만 파랑이 사면을 따라 진행하며 쇄파가 더 빨리 발생하여 기포분율이 증가하였고 결과적으로 중복파압형태의 파압이 작용하였다. 주기가 큰 실험파의 경우 충돌 전 유입되는 공기가 적어 flip-through 형태(Cooker and Peregrine, 1991)의 흐름양상을 보였고, 파압이 급격하게 증가함을 확인할 수 있었다.

• 한국항공운항학회:학술대회논문집
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• 한국항공운항학회 2004년도 추계학술발표대회 논문집
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• pp.240-246
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• 2004

For the prediction on the onset of oblique shock wave-induced vortex breakdown, computational studies on the Oblique Shock wave/Vortex Interaction (OSVI) are conducted and compared with both experimental results and analytic model. A Shock-stable numerical scheme, the Roe scheme with Mach number-based function (RoeM), and a two-equation eddy viscosity-transport approach are used for three-dimensional turbulent flow computations. The computational configuration is identical to available experiment, and we attempt to ascertain the effect of parameters such as a vertex strength, streamwise velocity deficit, and shock strength at a freestream Mach number of 2.49. Numerical simulations using the ${\kappa}-{\omega}SST$ turbulence model and suitably modeled vortex profiles are able to accurately reproduce many fine features through a direct comparison with experimental observations. The present computational approach to determine the criterion on the onset of oblique shock wave-induced vortex breakdown is found to be in good agreement with both the experimental result and the analytic prediction.

• 자원환경지질
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• 제30권4호
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• pp.353-362
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• 1997

P wave velocities of core samples from the Pocheon granite were measured before and after applications of cyclic loading. Then. distribution of the pre-existing microcracks and microcracks developed due to the cyclic loading was investigated by analyzing P wave velocity anisotropies and microscopic observations from thin sections. Anisotropy constants were calculated with three different ways: (1) $C_A$ between the maximum and the minimum velocities, (2) $C_AI$ between velocities measured along the axial direction and the average of six velocities measured in the planes perpendicular to the loading axis (rift plane) and (3) $C_AII$ between the maximum and the minimum velocities measured in the plane perpendicular to the loading axis. Among anisotropy constants. $C_AI$ was the most effective anisotropy constant to identify the rift plane whose orientation is parallel to the pre-existing microcracks as well as the distribution of stress induced microcracks. $C_AI$ decreased after cyclic loading and the relationship between $C_AI$ and number of cycles shows comparatively coherent negative trends. indicating that stress induced microcracks are aligned perpendicular to the orientation of pre-existing microcracks and that the amounts are proportional to the number of loading cycles. The difference of anisotropy constants before and after cyclic loading was effective in delineating the level of cracks and we called it Induced Crack Index. Velocity measurements and microscopic observations show that anisotropy was caused mainly due to microcracks aligned to a particular direction.

• 전기학회논문지
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• 제60권3호
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• pp.594-599
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• 2011

A SAW strain sensor based on Shear Horizontal wave with an 92 MHz central frequency was developed. It consists of SAW sensor, PCB substrate and bonding material (Loctite 401). External force applied to PCB substrate bonded to a piezoelectric substrate induces strain at the substrate surface, which causes changes in the elastic constant and density of the substrate and hence the propagation velocity of the SAW. The change in the velocity of the SAW result in a frequency shift of the sensor and by measuring a frequency shift, we can extract the strain induced by the external force. The $41^{\circ}$ YX $LiNbO_3$ was used because it has a Leaky shear horizontal(SH) wave propagation mode and a high electromechanical coupling coefficient ($K^2$=17.2%). And to compare with Rayleigh wave mode, $128^{\circ}$ YX $LiNbO_3$ was used. And to make a stable and low insert loss, Split IDT structure was used. The obtained sensitivity and linearity of the SAW strain sensor in the case of Split IDT were measured to be 17.2 kHz / % and 0.99, respectively.

• Ocean and Polar Research
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• 제37권1호
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• pp.11-22
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• 2015

In-situ measurements are labor-intensive, time-consuming, and limited in their ability to observe currents with spatial variations in the surf zone. This paper proposes an optical image-based method of measurement of currents in the surf zone. This method measures nearshore currents by tracking in time wave breaking-induced foam patches from sequential images. Foam patches in images tend to be arrayed with irregular pixel intensity values, which are likely to remain consistent for a short period of time. This irregular intensity feature of a foam patch is characterized and represented as a keypoint using an image-based object recognition method, i.e., Scale Invariant Feature Transform (SIFT). The keypoints identified by the SIFT method are traced from time sequential images to produce instantaneous velocity fields. In order to remove erroneous velocities, the instantaneous velocity fields are filtered by binding them within upper and lower limits, and averaging the velocity data in time and space with a certain interval. The measurements that are obtained by this method are comparable to the results estimated by an existing image-based method of observing currents, named the Optical Current Meter (OCM).

• Journal of Mechanical Science and Technology
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• 제17권9호
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• pp.1339-1348
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• 2003

Several factors, including peak dp/dt of the ventricular pressure and maximum closing velocity of leaflet have been studied as indices of the cavitation threshold. In the present study, just before closing velocity of the leaflet has been studied as indices of the cavitation threshold, and cavitation erosion on the surface of a mechanical valve was examined by focusing on squeeze flow and the water hammer phenomenon during the closing period of the valve. A simple solenoid-actuated test device that can directly control the valve closing velocity was developed, and opening-closing tests of 3,000 and 40,000 cycles were performed at various closing velocities. There was a closing velocity threshold to occur erosion pitting of valve surface, and its value was about 0.4 m/s in this study. Cavitation-induced erosion pits were observed only in regions where squeeze flow occurred immediately before valve closure On the other hand, the number of the pits was found to be closely related to an area of water hammer-induced pressure wave below the critical pressure defined by water vapor pressure. Therefore, it was concluded that cavitation is initiated and augmented by the two pressure drops due to squeeze flow and water hammer phenomenon, respectively.

• 비파괴검사학회지
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• 제29권4호
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• pp.308-315
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• 2009

구조물 손상시 방출되는 고주파 음향방출(AE)신호는 실시간 손상 감지에 유리하므로 지능형 구조물의 구조건전성 모니터 링에 널리 응용되고 있다. 하지만 복합 적층 구조물에서 방출되는 AE신호의 경우에는 복합재 구조의 특성상 전달 경로에 따른 신호의 감쇄정도가 일반 금속 구조물에 비해 상대적으로 크므로 손상 전후 신호진폭의 크기만을 비교하여 손상을 예측하는 방법은 손상 판단에 오류를 야기할 수 있는 여지가 있다. 따라서 복합적층 구조에서의 정확한 충격손상 예측을 위해서는 신호감쇄의 영향을 덜 받는 신호의 분석 방법이 필요하고 손상 전후 신호특성을 정량적으로 구분할 수 여는 인자의 선택이 필요하다. 본 연구에서는 구조물의 경계조건에 따른 신호특성 변화의 영향을 줄이기 위해 충격손상 발생후 선행적으로 전파되는 선행파(leading wave)의 특성변화를 신호처리에 이용하였으며, 웨이블릿 변환을 이용하여 신호를 분해하였다. 최종적으로 복합적층판의 저속충격 시험에서 압전 센서로 취득된 충간분리 신호를 처리하였으며, 분해된 웨이블릿 고주파 세부성분 사이의 점유율 분포를 이용하여 복합적층판의 손상을 판단하기 위한 신호처리 방법을 제시하였다.

• 한국지반환경공학회 논문집
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• 제16권3호
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• pp.5-13
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• 2015

해일에 의한 바닷물의 월류는 방조제 구조물에 피해를 유발시킨다. 지금까지 발생된 마운드 형태 방조제의 파괴 유형 조사 결과, 육지 쪽 마루부 파손과 선단부의 세굴이 대표적인 파괴 사례로 지목되었다. 이와 같은 파괴를 유발하는 가장 큰 원인은 월류에 의해 발생된 압력과 유속이다. 본 연구에서는 에너지 관점에서 마운드 형태 방조제에서 압력과 유속을 산정할 수 있는 이론해를 제안하고 검증하였다. 이를 위해 방조제 마루부와 선단부에 흐름을 유사정적비회전류로 보고 동심원유선이 형성된다고 가정하였다. 한계흐름조건과 베르누이정리를 이용하여 방조제 마루부와 배면 선단부에서의 작용하는 압력 및 유속 산정식을 유도하였다. 이들 식을 이용하여 동심원유선 및 월류고를 가정하여 마루부와 선단부에서 압력과 유속을 산정하였다. 그 결과 마루부에서는 부의 압력이 선단부에서는 양의 압력이 각각 크게 작용하는 것으로 나타났으며 유속에 의한 전단응력도 작용하는 것을 확인하였다. 또한 제안된 이론해의 적용 한계에 대한 고찰도 이루어졌다.

• 대한토목학회논문집
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• 제33권2호
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• pp.619-629
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• 2013

본 연구의 목적은 인공 발파진동실험을 이용하여 흙댐 축조재료의 전단파속도를 추정하고 산정 방법의 실효성을 확인하는 것이다. 이를 위하여 운영 중인 성덕댐에 대하여 국내 최초로 실대규모 근접 발파진동 실험을 수행하였다. 장약량과 발파 시추공심도를 4가지 유형으로 달리한 발파진동을 유발시키고, 각 유형별 발파 시에 폭원에 인접한 기반암노두와 댐 정상부에서 가속도를 각각 계측하였다. 발파진동실험으로부터 댐 정상부에서 얻어진 계측기록을 주파수 분석하여 대상댐의 고유진동수를 산정하고, 계측된 가속도기록으로 산정한 고유진동수와 기반암에서 계측한 발파파 가속도를 입력하중으로 한 반복적인 동적수치해석을 수행하여 계산한 고유진동수를 일치시키는 방법으로, 흙댐 성토재료의 심도별 전단파속도를 추정하였다. 산정된 대상댐 성토재료의 전단파속도는 발파유형에 영향을 받지 않고 일관성 있는 결과를 산정함을 확인하였고, 기존의 경험적 연구결과와 비교하여 그 실효성도 확인하였다. 이로부터 지진계가 설치되지 않아 실지진 계측기록을 이용할 수 없는 중소규모 댐의 경우, 인접발파에 따른 발파진동계측기록에 대한 분석으로도 댐 축조재료의 전단파속도를 실효성 있게 추정할 수 있음을 확인하였다.