• 대한기계학회:학술대회논문집
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• 대한기계학회 2004년도 춘계학술대회
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• pp.798-803
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• 2004

Leak noise is a good source to identify the exact location of a leak point of underground water pipelines. Water leak generates broadband sound from a leak location and this sound propagation due to leak in water pipelines is not a non-dispersive wave any more because of the surrounding pipes and soil. However, the necessity of long-range detection of this leak location makes to identify low-frequency acoustic waves rather than high frequency ones. Acoustic wave propagation coupled with surrounding boundaries including cast iron pipes is theoretically analyzed and the wave velocity was confirmed with experiment. The leak locations were identified both by the acoustic emission (AE) method and the cross-correlation method. In a short-range distance, both the AE method and cross-correlation method are effective to detect leak position. However, the detection for a long-range distance required a lower frequency range accelerometers only because higher frequency waves were attenuated very quickly with the increase of propagation paths. Two algorithms for the cross-correlation function were suggested, and a long-range detection has been achieved at real underground water pipelines longer than 300m.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2004년도 추계학술대회논문집
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• pp.327-332
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• 2004

Leak noise is a good source to identify the exact location of a leak point of underground water pipelines. Water leak generates broadband noise from a leak location and can be propagated to both directions of water pipes. This sound propagation due to leak in water pipelines is not a non-dispersive wave any more because of the surrounding pipes and soil. However, the necessity of long-range detection of this leak location makes to identify low-frequency acoustic waves rather than high frequency ones. Acoustic wave propagation coupled with surrounding boundaries including cast iron pipes is theoretically analyzed and the wave velocity was confirmed with experiment. The leak locations were identified both by the acoustic emission (AE) method and the cross-correlation method. In a short-range distance, both the AE method and cross-correlation method are effective to detect leak position. However, the detection for a long-range distance required a lower frequency range accelerometers only because higher frequency waves were attenuated very quickly with the increase of propagation paths. Two algorithms for the cross-correlation function were suggested, and a long-range detection has been achieved at real underground water pipelines longer than loom.

• 한국전자파학회논문지
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• 제14권3호
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• pp.290-295
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• 2003

Sheet형 전파흡수체를 제작하여 Mn-Zn페라이트의 입자크기에 따른 전파흡수능의 관계를 조사하였다. 입자 크기가 증가함에 따라 정합주파수는 낮은 쪽으로 이동하였으며, 정합주파수에서 전파흡수능은 감소하였다. 하지만 휴대폰의 사용주파수인 0.8 ㎓와 1.8 ㎓에서는 입자의 크기가 큰 것으로 구성되어 있는 전파흡수체자 작은 입자로 구성되어 있는 전파흡수체보다 우수한 전파흡수능을 나타내었다. 따라서 휴대폰용 전파흡수체를 Mn-Zn 페라이트 파우더로 제작할 경우에는 입자의 크기가 큰 것이 바람직하다고 생각된다.

• 한국해양공학회지
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• 제23권5호
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• pp.15-24
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• 2009

The results of a simulation study of variable liquid column oscillations in U-tanks with a novel control scheme are presented. The configuration under investigation is analogous to that of the tuned liquid-column damper used to suppress oscillatory motion in large structures like tall buildings and cargo ships. However, by virtue of an adequate controller, the response of amplitude of the U-tanks becomes larger in a desired frequency range. The motion of wave energy conversion system equipped with a variable liquid column oscillator is described by a series of nonlinear differential equations. The equations describe the motion of body under ocean wave excitation, and the motion of liquid with an air-spring effect caused by the compression and expansion of air in vertical liquid columns and air chambers. It is shown that the effect of the air-spring has a vital role to maintain the natural frequency of oscillation in the system to synchronize with the frequency of the ocean wave, thus the system provides the most effective mode for energy extraction from the ocean.

• 한국해양공학회지
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• 제20권2호
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• pp.36-40
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• 2006

As large waves enter a harbor, during their propagation, the motions a floating body are large and if may even be damaged by waves. This phenomenon may be caused by harbor resonance, resulting from large motion at low wave frequency, which is close to the natural frequency of a vessel. In order to calculate the motion of a floating body in a harbor, it is necessary to use the wave forces containing the body-harbor interference. The simulation program to predict the motions of a floating body by waves in a harbor is developed, and this program is based on the method of velocity potential contiuation method proposed by Ijima and Yoshida The calculated results are shown by the variation of wave frequency, wave angle, and the position of a floating body.

• Journal of Electrical Engineering and Technology
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• 제13권4호
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• pp.1515-1527
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• 2018

This paper presents a sensorless speed control of IPMSM (Interior Permanent Magnet Synchronous Motor) using the high-frequency (HF) square wave injection method. In the proposed HF pulsating square wave injection method, injection voltage is applied into the estimated d-axis of rotor and high-frequency induced q-axis current is considered to estimate the rotor position. Conventional square wave injection methods may need complex demodulation process to find rotor position, while in the proposed method, an easy demodulation process based on the rising-falling edge of the injected voltage and carrier induced q-axis current is implemented, which needs less processing time and improves control bandwidth. Unlike some saliency-based sensorless methods, the proposed method uses maximum torque per ampere (MTPA) strategy, instead of zero d-axis command current strategy, to improve control performance. Furthermore, this paper directly uses resultant d-axis current to detect the magnet polarity and eliminates the need to add an extra pulse injection for magnet polarity detection. As experimental results show, the proposed method can quickly find initial rotor position and MTPA strategy helps to improve the control performance. The effectiveness of the proposed method and all theoretical concepts are verified by mathematical equations, simulation, and experimental tests.

• Steel and Composite Structures
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• 제19권1호
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• pp.131-152
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• 2015

Easy detection and evaluation of defect in the tube structure is a continuous problem and remains a significant demand in tube inspection technologies. This study is aimed to automate defect detection using the pattern recognition approach based on the classification of high frequency stress wave signals. The stress wave signals from vibrational impact excitation on several tube conditions were captured to identify the defect in ASTM A179 seamless steel tubes. The variation in stress wave propagation was captured by a high frequency sensor. Stress wave signals from four tubes with artificial defects of different depths and one reference tube were classified using the autoregressive (AR) algorithm. The results were demonstrated using a dendrogram. The preliminary research revealed the natural arrangement of stress wave signals were grouped into two clusters. The stress wave signals from the healthy tube were grouped together in one cluster and the signals from the defective tubes were classified in another cluster. This approach was effective in separating different stress wave signals and allowed quicker and easier defect identification and interpretation in steel tubes.

• 한국신재생에너지학회:학술대회논문집
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• 한국신재생에너지학회 2010년도 추계학술대회 초록집
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• pp.184.1-184.1
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• 2010

Floating wind turbines have been suggested as a feasible solution for going further offshore into deeper waters. However, floating platforms cause additional unsteady motions induced by wind and wave conditions, so that it is difficult to predict annual energy output of wind turbines by using conventional power prediction method. That is because sectional inflow condition on a rotor plane is varied by unsteady motion of floating platforms. Therefore, aerodynamic simulation using Vortex Lattice Method(VLM) were used to investigate the influence of motion on the aerodynamic performance of a floating offshore wind turbine. Simulation with individual motion of offshore platform were compared to the case of onshore platform and carried out according to the wave height and the wave angular frequency.

• 산업기술연구
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• 제25권B호
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• pp.115-120
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• 2005

Impact seismic wave method is a method for non-destructive testing of concrete structure using of stress wave which is propagate and reflected from internal flaws within concrete structure and external surface. In this study, we performed frequency domain method using impact seismic wave test for safety diagnosis of civil engineering structure. And reflection method which is used for one-dimensional target such as tunnel lining and transmission method are compared with each other.

• 한국압력기기공학회 논문집
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• 제7권2호
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• pp.21-25
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• 2011

Although many defects in PZR heater sleeve have been reported continually from operating experiences in oversea nuclear power plant, utilities get into difficulties in finding appropriate methods for diagnostics of the components due to the limited access or high radiation problems. Recently, as an alternative, diagnostics using Guided Wave Testing(GWT) are proposed and the attention of the methods has been growing gradually because of their long range inspection capability. This study is to investigate the effectiveness of GWT to detect PWSCC in welding points of PZR heater sleeve. Moreover, mode sensitivity analysis of GWT and optimal frequency for the diagnostics of PWSCC are presented by testing the mock-ups specimens that contain artificial flaws.