• Journal of the Korean Society of Propulsion Engineers
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• v.16 no.6
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• pp.41-47
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• 2012

It is very important to predict the nonlinear behavior of combustion instability such as transition phenomena and limit cycle amplitude for fully understanding and controlling the instabilities. These nonlinear instability characteristics are highly dependent upon the flames' nonlinear dynamics in a gas turbine premixed combustor. In this study, nonlinear instability TA(Thermo-acoustic) models were introduced by applying the concept of flame describing function to the thermoacoustic analysis method. As a result of model development, for a given combustor length, the growth rate of instability was greatly affected by the change in amplitude, although the instability frequency was not. Further researches under various operating conditions and model validation on limit cycle amplitude are required.

• The Transactions of the Korea Information Processing Society
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• v.2 no.5
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• pp.691-700
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• 1995

Most fractal image codings using fractal concept require long encoding time because a large amount of computation is needed to find an optimal affine transformation point. Such a problem can be solved by designing a block classifier fitted to characteristics of image blocks. In general, it is possible to predict more precise and various types of blocks in frequency domain than in spatial domain. In this paper, we propose a block classifier to predict the block type using characteristics of DCT(Discrete Cosine Transform). This classifier has merits to enhance the quality of decoded images as well as to reduce the encoding time meeting fractal features. AC coefficient values in frequency domain make it possible to predict various types of blocks. As the results, the number of comparisons between a range block and the correspoding domain blocks to reach an optimal affine transformation point can be reduced. Specially, signs of DCT coefficients help to find the optimal affine transformation point with only two isometric transformations by eliminating unnecessary isometric transformations among eight isometric transformations used in traditional fractal codings.

• Proceedings of the Korea Water Resources Association Conference
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• 2023.05a
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• pp.511-511
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• 2023

기후변화로 동절기 기온 저하에 따른 수도계량기의 동파는 지속적으로 심화되고 있으며, 이는 계량기 교체 비용, 누수, 누수량 동결에 의한 2차 피해, 단수 등 사회적 문제를 야기한다. 이와같은 문제를 해결하고자 구조적 대책으로 개별 가정에서 동파 방지형 계량기를 설치할 수 있으나 이를 위한 비용발생이 상당하고, 비구조적 대책으로는 기상청의 동파 지도 알림 서비스를 활용하여 사전적으로 대응하고자 하나, 기상청자료는 대기 온도를 중심으로 제공하고 있기 때문에 해당서비스만으로는 계량기의 동파를 예측하는데 필요한 추가적인 다양한 변수를 활용하는데 한계가 있다. 최근 정부와 공공부문에서 22개 지역, 110개소 이상의 수도계량기함내 IoT 온도센서를 시범 설치하여 계량기 함내의 상태 등을 확인할 수 있는 사업을 수행했다. 전국적인 계량기 상태의 예측과 진단을 위해서는 추가적인 센서 설치가 필요할 것이나, IoT센서 설치 비용 등의 문제로 추가 설치가 더딘 실정이다. 본 연구에서는 겨울 동파 예방을 위해 실제 온도센서를 기반으로 가상센서를 구축하고, 이를 혼합한 하이브리드 방식으로 동파위험 기준에 따라 전국 동파위험 지도를 구축하였다. 가상센서 개발을 위해 독립변수로 위경도, 고도, 음·양지, 보온재 여부 및 기상정보(기온, 강수량, 풍속, 습도)를 활용하고, 종속변수로 실제 센서의 온도를 사용하여 기계학습 모델을 개발하였다. 지역 특성에 따라 정확한 모델을 구축하기 위해 위치정보 및 보온재여부 등의 변수를 활용하여 K-means 방법으로 군집화 하였으며, 각 군집별로 3가지의 기계학습 회귀모델을 적용하였다. 최적의 군집 수를 검토한 결과 4개가 적정한 것으로 판단되었다. 군집의 특성은 지역별 구분과 유사한 패턴을 보이며, 모든 군집에서 Gradient Boosting 회귀모델을 적용하는 것이 적합한 것으로 나타났다. 본 연구에서 개발한 모델을 바탕으로 조건에 따라 동파 예측 알람서비스에 실무적으로 활용할 수 있도록 양호·주의·위험·매우위험 총 4개의 기준을 설정하였다. 실제 본 연구에서 개발된 알고리즘을 국가상수도정보 시스템에 반영하여 테스트 수행중에 있으며, 향후 지속 검증을 할 예정에 있다. 이를 통해 동파 예방 및 피해 최소화, 물절약 등 직간접적 편익이 기대된다.

• Journal of the Korean Society of Propulsion Engineers
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• v.20 no.3
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• pp.103-110
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• 2016

A performance analysis program of a disk type fluidic valve was developed to predict a chamber pressure and a response time. A parametric study of this device was performed by using scattering plot method. A sensitivity of Mach number at a nozzle outlet showed the highest value about a outlet diameter of nozzle. An inlet flow rate is the most important parameter to design the fluidic valve because it has high sensitivity value both a outlet velocity and a response time.

• Journal of the Korean Society of Groundwater Environment
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• v.4 no.1
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• pp.5-13
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• 1997

The Island of Cheju plans massive ground water development to meet predicted water demand. Effective management of ground water resources requires impact assessment study. Due to the nature of the island, effects of sea water must be considered. In this work, salt water intrusion, due to hypothetical ground water development in Eastern and Central Cheju Watersheds, is predicted using a sharp-interface model. The model considers simultaneously hydrodynamics of both freshwater and saltwater. The hypothetical ground water development was designed such that it follows closely the regional ground water development plan. The numerical model predicted that the saltwater wedge may intude over 1km depending on the location. This observation leaves doubt on impact assement studies based on freshwater-flow only modeling.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.24 no.6
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• pp.483-492
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• 2014

Uncomfortable feelings of occupants by indoor floor impact noise in a residential building are not accurately represented by the floor impact noise from a standard impact source. It is due to the characteristics of standard impact sources, which are different from the impact forces produced by occupants. It varies significantly by impact source, and it is not easy to be replicated for testing. As a result, the indoor floor impact noise under different acoustic conditions cannot be directly compared. Using frequency response function(FRF), which represents the input-output relationships of a dynamic system, it is possible to examine the characteristics of the system. Especially, FRF can predict the response of a linear dynamic system subjected to various excitation. To determine the relationship between impact force and the corresponding response of dynamic system in residential building, the acceleration response of a concrete slab and the floor impact noise in the living room, produced by bang-machine and rubber-ball excitation, were measured. The test results are compared to the estimates based on FRF and impact force spectrum.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.3 no.4
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• pp.280-284
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• 2002

This paper presents computer simulations of the stacking and transferring system for the fast freeform fabrication system. The stacking and transferring system is essential for the fast freeform fabrication system and its stable motion is very important for consistent stacking of the cut slices. The stacking and transferring system can be modeled as a pneumatic system. The system consists of air compressor, the control valve, and the cylinder. The governing parameters have been changed and the simulation results are shown to predict the time response of the system. The results show some parameters should be correctly tuned to obtain stable system response.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.30 no.11C
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• pp.1091-1097
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• 2005

In this paper, we propose fast motion estimation algorithm. Local statistics of a motion vector is highly correlated to motion vectors of its neighboring blocks. According to the property, block-based motion search range is adaptively determined in order to reduce unnecessary search points. Based on the determined search range, motion vector is obtained by variable step search motion estimation. Experimental results show that comparing to Full search motion estimation, the motion searching points of proposed algorithm is reduced as much as $98\%$. Moreover, PSNR and Bit Rate are almost same to Full search method.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.33 no.11
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• pp.1244-1248
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• 2009

Although rubber components are extensively used in mechanic parts. There are still a lot of difficulties in designing the rubber components applied in complex shapes and preloaded states because of the complicated material properties. One of the most important parameters for more detailed and accurate mechanical analysis during the development stages is the dynamic characteristics of the rubber components. It is well known that the dynamic properties of rubber are dependent on frequency as well as static preload. Consequently, a large number of experiments have to be conducted to identify the dynamic stiffness of a rubber bush considering the various applied conditions. In this paper, an efficient experimental method is suggested, which estimates the dynamic stiffness of a rubber bush using rubber material test and static stiffness of the bush. This method is capable of predicting the dynamic stiffness of a rubber bush under various load conditions from minimized test data.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2001.05a
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• pp.540-545
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• 2001

In case of small motors, coil bundle occupies a large portion of stator in view of mass and volume as well as dynamics. It is observed through modal test on the stator of an IPM BLDC (interior permanent magnet brushless direct current) motor that coil bundle wound on the stator core causes the first and second natural frequencies to decrease by about 20-30% compared with those of bare stator. Especially the third natural frequency is newly observed below 3 KHz, which is not observed on the bare stator. It is found that at the third mode the end-coil and the core vibrate out of phase in radial direction. In this paper, the stator is dynamically modeled in terms of the core and the coil bundle consisting of the end-coil and the slot coil based on the above observations for the prediction of dynamic properties. The core can easily be modeled using finite element method with its actual material properties and geometric shape. The concept of equivalent bending stiffness is used for modeling of the end-coil so that predictions may match with the measured natural frequencies for the end-coil cut out of the stator. Although the same concept can be applied to the slot coil, separation of the slot coil from the stator is impractical. Therefore, equivalent bending stiffness of the slot coil is determined through iterative comparisons with the measurements of natural frequencies of the stator with the slot coil in it.