• 대한기계학회:학술대회논문집
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• 대한기계학회 2000년도 춘계학술대회논문집B
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• pp.661-666
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• 2000

Near-wake flow field of a circular cylinder is studied by means of a cinematic PIV system with high sampling rate and large internal memory block. Experiments are conducted in a closed-cycle water tunnel system and a cross-correlation algorithm in conjunction with FFT (Fast Fourier Transform) analysis and an offset correlation technique is used for vector processing. With the help of very high sampling frequency compared to the shedding frequency, it is possible to obtain phase-averaged information of the three-dimensional wake, even though the shedding is not forced but natural. Phase-locked vortical structures observed simultaneously from the spanwise and cross-stream planes are displayed in the wake-transition regime where fine-scale secondary vortices have a spanwise wavelength or around one diameter. Spatial relations and temporal evolutions of the primary Karman vortex and the secondary vortex are also discussed schematically.

• Journal of Astronomy and Space Sciences
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• 제30권4호
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• pp.345-353
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• 2013

A W-band Orthomode Transducer (OMT) has been developed for Korean VLBI Network (KVN) polarization observation. The OMT design was based on E-plane split-block technique using septum structure. 3-dimensional electromagnetic simulation was fully employed to optimize the performance of the OMT. Measurements of the fabricated OMT show that the return losses for the vertically and horizontally polarized modes are better than -20 dB across 80 ~ 108 GHz and the insertion losses for the both modes are less than 0.47 dB. The cross-polarization level of the OMT is less than -30 dB. The bandwidth of the developed OMT is estimated as around 30%.

• The Korean Journal of Physiology
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• 제27권2호
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• pp.139-150
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• 1993

We have reported that dopamine potentiates spontaneous contractions dose-dependently in guinea-pig antral circular muscle strips (Hwang et al, 1991). To clarify the underlying excitatory mechanism of dopamine on the gastric smooth muscle, the effects of dopamine on voltage-dependent $Ca^{2+}\;currents\;and\;Ca^{2+}\;-dependent\;K^+\;currents$ were observed in enzymatically dispersed guinea-pig gastric myocytes using the whole-cell voltage-clamp technique. Experiments were also done using isometric tension recording and conventional intracellular microelectrode techniques. 1) The effect of dopamine on the spontaneous contraction of antral circular muscle strips of the guinea-pig was excitatory in a dose-dependent manner, and was blocked by phentolamine, an ${\alpha}-adrenoceptor$ blocker. 2) The slow waves were not changed by dopamine. 3) The voltage-operated inward $Ca^{2+}$ current was not influenced by dopamine. 4) The $Ca^{2+}\;-dependent\;K^+$ outward current, which might reflect the changes of intracellular calcium concentration, was enhanced by dopamine. This effect was abolished by phentolamine. 5) The enhancing effect of dopamine on the $Ca^{2+}\;-dependent\;K^+$ current disappeared with heparin which is known to block the action of $InsP_3$. From these results, it is suggested that dopamine acts via $InsP_3-mediated\;Ca^{2+}$ mobilization from intracellular stores and such action potentiates the spontaneous contraction of guinea-pig gastric smooth muscle.

• 수산해양기술연구
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• 제47권1호
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• pp.1-9
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• 2011

In order to investigate the cause and magnitude of scale effect occurring in the model experiments of fishing nets, five pairs of Nylon pyramid nets and one pair of PE ones in which all the two nets paired were equal each other in the factors determining their flow resistance, i. e., the ratio d/l of diameter d to length l of bars, the angle f between two adjacent bars, the attack angle q of nettings to the water flow, and the wall area S of nets, and different in the values of d and l were prepared. Then, the nets were attached to the circular steel frame alternately and their flow resistances with shapes in water were measured on the sea ascribing no turbulent flows by using the tension meter made of a block bearing for the experiment. All the Nylon nets were spreads out easily in water to form a circular cone at relatively low velocity of water and showed the resistance smaller a little in the nets with larger d and l than them with smaller d and l, because the filtration of water through meshes become easier in nets especially with larger l. But PE nettings were not spread out sufficiently on account of their small flexibility and showed higher resistance especially in them with thicker twines. Therefore, the difference in bar length or mesh size and flexibility of nettings between prototype and model nets are regarded to become factors ascribing scale effect. Especially the influence of the difference in mesh size may become large significantly in actual model experiments because the mesh size of model nets is decided at much larger value than that given by scale ratio and so the difference of mesh size between the two nets become much larger than that between nets used in this experiment.

• 한국통신학회논문지
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• 제24권12A호
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• pp.1974-1984
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• 1999

본 논문은 정확한 전파 모델과 신속한 전파 예측 모델을 필요로 하는 다중 경로 현상을 효과적으로 획득하기 위해 효율적인 데이터 표현 구조를 제안한다. 본 논문은 ray tracing에서 사용되고 있는 VR(Vector Representation)을 향상시키기 위한 데이터 표현 구조로서, 빌딩과 같은 오브젝트를 표현하기 위해 CR(Circular Representation)을 이용하는 오브젝트 표현 방법을 제안한다. 제안된 CR은 건물의 중심에서 건물을 둘러싸도록 원을 그리는 형태이다. 본 구성에서 CR은 기하학적 구조를 위한 기본 빌딩 구조로서의 기능을 가지며, VR이 단독으로 사용되어 졌을 때 보다 더 많은 효율성을 증진시킨다. VR은 건물을 표현하기 위해 여러 개의 벽면 벡터를 필요로 하는 반면, CR은 하나의 원으로 표현된다. 결과적으로 제안된 방법에 의해 ray tracing에서 상당한 양의 계산 비용을 줄일 수 있다. 본 논문의 목표는 효율적인 ray tracing 예측 모델을 위해 데이터 구조화 시에 효율성을 얻기 위한 해결책으로서 CR을 제안하는 것이다. 본 논문은 제안된 방법에 의해 계산 부하량이 현저하게 줄어듬을 보인다. 또한 deterministic ray tracing 모델에서 CR의 계층? 구조의 수반 가능한 사용을 보인다. 시뮬레이션 결과는 계층적 octree 구조가 약 50% 계산 부하를 감소시키고 있는 반면 본 논문에서 제안된 CR은 분산된 물체의 수에 비례하여 계산 부하량을 현저하게 감소시킨다는 것을 나타낸다.

• Journal of Multimedia Information System
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• 제7권4호
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• pp.239-248
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• 2020

The performance of the facial recognition system is determined by many technical factors. Further, most of the technical factors have been realized or are still in continued research. The recognition rate has a great influence on performance not only by technical factors but also by other factors. However, researchers are trying to improve the recognition rate by focusing only on technical factors. The mechanism of recognizing is to compare a face image obtained by photography to an already stored face image and determine the score of the similarity. However, if the photographed image is damaged by external light, even a system with a good algorithm will fail to recognize it. Therefore, it is important to prevent the disturbance of light entering from the outside, so it should be blocked, but the camera will not work without light. Thus, it is proposed that a method to secure the external light but block the disturbance of light that affects photography. A method of blocking disturbance light is to use a polarization filter. There are three polarization methods: circular polarization, linear polarization, and elliptical polarization. In this paper, an experiment was performed to overcome disturbance of light using only a circularly polarized filter. In addition, a lighting system that reproduces disturbance light was provided for the experiment, and light of varying intensities and angles was installed to affect the face recognition camera. As a result of actual application, it was determined that a very improved recognition performance in various disturbance light environments.

• Journal of Computing Science and Engineering
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• 제8권4호
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• pp.187-198
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• 2014

In this paper an efficient image encryption scheme based on cyclic rotations and multiple blockwise diffusions with two chaotic maps is proposed. A Sin map is used to generate round keys for the encryption/decryption process. A Pomeau-Manneville map is used to generate chaotic values for permutation, pixel value rotation and diffusion operations. The encryption scheme is composed of three stages: permutation, pixel value rotation and diffusion. The permutation stage performs four operations on the image: row shuffling, column shuffling, cyclic rotation of all the rows and cyclic rotation of all the columns. This stage reduces the correlation significantly among neighboring pixels. The second stage performs circular rotation of pixel values twice by scanning the image horizontally and vertically. The amount of rotation is based on $M{\times}N$ chaotic values. The last stage performs the diffusion four times by scanning the image in four different ways: block of $8{\times}8$ pixels, block of $16{\times}16$ pixels, principal diagonally, and secondary diagonally. Each of the above four diffusions performs the diffusion in two directions (forwards and backwards) with two previously diffused pixels and two chaotic values. This stage makes the scheme resistant to differential attacks. The security and performance of the proposed method is analyzed systematically by using the key space, entropy, statistical, differential and performance analysis. The experimental results confirm that the proposed method is computationally efficient with high security.

• 터널과지하공간
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• 제27권2호
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• pp.100-108
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• 2017

균열텐서 파라미터는 절리의 기하학적 속성이 결합된 효과를 지시하는 척도로 사용할 수 있으며 불연속절리망(DFN)에서 유체 유동통로의 연결 상태를 정량화할 수 있다. 본 연구는 이차원 DFN의 균열텐서 파라미터와 수리적 특성 간의 상관성 분석을 수행하였다. DFN에서 임의 방향으로의 수리전도도는 이에 직교하는 방향으로 산정된 균열텐서성분과 강한 비선형 관계를 갖는 것으로 평가되었다. 서로 다른 규모의 이차원 DFN 블록에서 방향에 따른 균열텐서의 일차불변량($F_0$)을 방사형 도표로 나타내었을 때, 방향에 따른 $F_0$ 값의 유의미한 변화가 없는 원형의 플롯은 절리성 암반을 대표요소체적으로 취급하기 위한 필요조건이 될 수 있다. DFN 블록에서 임의 방향으로 개별체 해석기법으로 산정한 수리전도도와 이론적 수리전도도 사이의 상대오차(ER)는 $F_0$의 증가에 따라 감소한다. $F_0$는 평균 블록수리전도도와 강한 함수관계를 갖는 것으로 분석되었다.

• 대한기계학회논문집
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• 제13권3호
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• pp.345-352
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• 1989

본 연구에서는 3차원 변형형상을 갖는 소재에서 상하금형면의 마찰이 다른 경우에, 작각주 소재와 비직각주 소재(non-prismatic block)에 대해서 적용할 수 있는 동적 가용 속도장(kinematically admissible velocity field)을 유도해 내고자 한다.이론계산에 있어서 가공경화를 고려하기 위해 소성변형 영역을 유한개의 요소들로 나누고 이들 각 요소에 대해서 유효변형도, 유효응력을 계산하여 에너지 소비율을 계산하였다.

• 한국산학기술학회논문지
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• 제22권4호
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• pp.124-130
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• 2021

치과보철물의 심미적 결과를 조절하는 중요한 요인은 반투명도이며, 지르코니아 착색제의 건조 정도가 반투명도에 어떠한 영향을 미치는지 알아보고자 한다. 실험을 위해서 지르코니아 블록별로 원형 시편을 총 90개 제작하였다. 그 다음 지르코니아 시편을 착색제로 180초 침적 후 미건조(0초), 중간건조(30초), 완전건조(30분)의 건조시간(Dry time)을 부여한 후 지르코니아 소결로를 이용하여 원형 시편을 소결하였다. 분광광도계(Spectrophotometer CM-3600A, Konica Minolta, Japan)를 이용하여 흑색표준타일과 백색표준타일 위에 시편을 놓고, 표준광원 D65의 조건에서 정반사광 제거방식으로 지르코니아 원형 시편을 각각 측정하였다. 시편당 총 3번씩 반복측정하였다. 지르코니아의 건조시간에 따른 반투명도를 비교 분석하기 위하여 One-way ANOVA를 시행하였다. 지르코니아 종류별, 착색제에 따라 반투명도는 통계적으로 차이가 있었고(P < 0.001), 건조시간에 따라서는 통계적으로 차이는 없었다(P > 0.922). 그러나, 완전건조인 상태일 때, 반투명도 값이 작은 것으로 나타나 건조가 잘 될수록 지르코니아 색상이 투명하다는 것을 알 수 있었다.