• Journal of Astronomy and Space Sciences
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• 제40권3호
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• pp.101-111
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• 2023

We conducted a statistical study of polar mesospheric summer echoes (PMSEs) in relation to magnetic local time (MLT), considering the geomagnetic conditions using the K-index (or K). Additionally, we performed a case study to examine the velocity profile, specifically for high velocities (≥ ~100 m/s) varying with high temporal resolution at high K-index values. This study utilized the PMSE data obtained from the mesosphere-stratosphere-troposphere radar located in Esrange, Sweden (63.7°N, 21°E). The change in K-index in terms of MLT was high (K ≥ 4) from 23 to 04 MLT, estimated for the time PMSE was present. During the near-midnight period (0-4 MLT), both PMSE occurrence and signal-to-noise ratio (SNR) displayed an asymmetric structure with upper curves for K ≥ 3 and lower curves for K < 3. Furthermore, the occurrence of high velocities peaked at 3-4 MLT for K ≥ 3. From case studies focusing on the 0-3 MLT period, we observed persistent eastward-biased high velocities (≥ 200 m/s) prevailing for ~18 min. These high velocities were accompanied with the systematic motion of profiles at 85-88 km, including large shear formation. Importantly, the rapid variations observed in velocity could not be attributed to neutral wind effects. The present findings suggest a strong substorm influence on PMSE, especially in the midnight and early dawn sectors. The large zonal drift observed in PMSE were potentially energized by local electromagnetic fields or the global convection field induced by the electron precipitation during substorms.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2005년도 춘계학술대회논문집
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• pp.59-68
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• 2005

In this study, we establish a theory for dynamic behaviors of an automatic ball balancer, analyze its dynamic characteristics, and provide its design guide line. Equations of motion are derived by using the polar coordinate system instead of the rectangular coordinate system which was previously used in other researches. After non-dimensionalization of the equations, the perturbation method is applied to locate the equilibrium positions and to obtain the linearized equations of motion around the equilibrium positions. The Eigenvalue problem is used to verify the dynamic stability around the equilibrium positions. On the other hand, the time responses are computed from the nonlinear equations of motion by using a time integration method.

• 소음진동
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• 제9권2호
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• pp.363-370
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• 1999

In this study, we establish a theory for dynamic behaviors of an automatic ball balancer, analyze its dynamic characteristics, and provide its design guide line. Equations of motion are derived by using the polar coordinate system instead of the rectangular coordinate system which was previously used in other researches. After nondimensionalization of the equations, the perturbation method is applied to locate the equilibrium positions and to obtain the linearized equations of motion around the equilibrium positions. The Eigenvalue problem is used to verify the dynamic stability around the equilibrium positions. On the other hand, the time responses are computed from the nonlinear equations of motion by using a time integration method.

• Journal of Astronomy and Space Sciences
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• 제35권2호
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• pp.93-103
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• 2018

Non-specular, vertically upward transit, fast-moving radar echoes are observed in the summer polar upper mesosphere near 90 km using 52 MHz VHF radar at Esrange, Sweden. By resolving maximum echo power movement, the unusual meteor trails propagate vertically upward with taking horizontal displacements at an initial speed of 10 km/s exponentially decreasing with increasing height from 85-89 km, lasting for 3.5 sec. Another upward transit is observed as following a downward transit echo target in about ~1 sec, lasting over 5 sec. The upward motion cannot be explained with the dynamics of penetrating meteors or by atmospheric dynamics. The observation proposes that secondary produced plasma jets occurring from meteor trail are possibly responsible for upward fast moving echoes. The long-lasting (3-5 sec), ascending meteor trails at speeds of a few $10^4m/s$ are distinctive from any previous occurrences of meteors or upper atmospheric electrical discharges in the aspect of long-lasting upward/downward motions. This result possibly suggests a new type of meteor-trail leader discharge occurring in the summer polar upper mesosphere and lower thermosphere.

• 한국해양학회지
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• 제29권3호
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• pp.296-303
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• 1994

회전하는 원통형 이층 유체에 하층 유체와 동일한 밀도의 유체를 경사진 하층 바 닥의 둘레를 통하여 주입시 하층 내부는 일정한 수직 운동을 하게 되며 이러한 수직운 동은 결국 하층 유체에 Sverdrup type의 운동인 남향의 서안 경계류와 북향의 내부 운 동을 발생시킨다. 이때 $\beta$-효과와 동시에 하층 유체가 상층 경계면과 만나게 하므로써 소위 극전선 효과를 갖고 있는 상층 유체의 운동 특성을 관찰하였다. 서안에서 의 순 압성, 동안에서 의 경압성 운동이 뚜렷하게 나타나고 특히 동안 경계류의 남향과 서안 경계류의 남향은 서안에서 의 이안 현상을 야기시킨다. 상층에 $\beta$-효과를 증가시 서안 경계류의 폭이 감소하는 이안 현상후 내부에 "eddy" 모양의 흐름이 존재한다. 한편 경압성 Rossby wave는 동안 부근에서 뚜렷하게 관측되나 서쪽으로 진행하면서 소멸되 는 현상이 관측되었다. 하층 유체에 국지적 지형 효과서 준 경우 상층수의 서안 경계 류 및 동안 경계류가 민감하게 반응하여 동안에서는 정상파 형태의 운동이 나타나고 서안에서는 이안 현상 없이 경계류의 폭이 증가함을 보인다. 이는 하층의 순압성 운동 이 지형 효과를 느끼는 반면, 그 영향은 상층에 직접전달되지 못하고 있음을 보여주고 있는 것이라 사료된다. 상층의 $\beta$-효과 증가시 internal radius보다 훨씬 큰 규모의 파형이 내부에 존재하는 특성을 보여 주고 있다. 보여 주고 있다.

• 한국소음진동공학회논문집
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• 제15권3호
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• pp.364-370
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• 2005

The active magnetic bearing (AMB) systems mounted in moving vehicles are exposed to the disturbances due to the base motion, often leading to malfunction or damage as well as inaccurate positioning of the systems. Thus, in the controller design of such AMB systems, robustness to base disturbances becomes an essential requirement. In this study, effective control schemes are proposed for the homo-polar AMB system, which uses permanent magnets for generation of bias magnetic flux, when it is subject to base motion, and its control performance is experimentally evaluated. The base motion of AMB system is modeled as the dynamic disturbances in the gravity and base excitation forces. To effectively compensate for the disturbances, the angle feed-forward controller based on the inverse dynamic model and the acceleration feed-forward controller based on the normalized filtered-X LMS algorithm are proposed. The performance test of the prototype AMB system is carried out, when the system is mounted on rate table. The experimental results show that the performance of the proposed controllers for the AMB system is satisfactory in compensating for the disturbances due to the base motion.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2004년도 추계학술대회논문집
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• pp.486-489
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• 2004

The active magnetic bearing (AMB) systems mounted in moving vehicles are exposed to the disturbances due to the base motion, often leading to malfunction or damage as well as inaccurate positioning of the systems. Thus, in the controller design of such AMB systems, robustness to base disturbances becomes an essential requirement. In this study, effective control schemes are proposed for the homo-polar AMB system, which uses permanent magnets for generation of bias magnetic flux, when it is subject to base motion, and its control performance is experimentally evaluated. The base motion of AMB system is modeled as the dynamic disturbances in the gravity and base excitation forces. To effectively compensate for the disturbances, the angle feed-forward controller based on the inverse dynamic model and the acceleration feed-forward controller based on the normalized filtered-X LMS algorithm are proposed. The performance test of the prototype AMB system is carried out, when the system is mounted on rate table. The experimental results show that the performance of the proposed controllers for the AMB system is satisfactory in compensating for the disturbances due to the base motion.

• 대한원격탐사학회지
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• 제34권6_2호
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• pp.1215-1227
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• 2018

해빙의 이동은 지역적 분포뿐만 아니라 해빙의 생성 및 두께에도 영향을 미치기 때문에 해빙의 변화를 평가하는 데에 중요한 정보가 된다. 이 연구에서는 북극해 해빙의 이동 특성 탐지를 위해 Korea Multi-Purpose Satellite-2(KOMPSAT-2)와 Korea Multi-Purpose Satellite-3(KOMPSAT-3)의 두 위성 센서로부터 다중 시기 고해상도 광학 위성 영상을 획득하고, SIFT(Scale-Invariant Feature Transform), SURF(Speeded Up Robust Features) 및 ORB(Oriented FAST and Rotated BRIEF)의 특징점 추적 기법을 적용하였다. 두 위성 센서에서 획득된 영상의 활용을 위해 전처리 단계에서 공간해상도와 방사해상도를 일치시킨 후 특징점 추적 기법을 적용한 결과 SIFT의 경우 영상 전반에 걸쳐 특징점의 고른 공간 분포가 나타났고, SURF의 경우 해빙과 해양의 경계 부분에 특징점이 주요하게 분포하는 경향이 관찰되었으며 이러한 경향은 ORB에서 가장 현저하게 나타났다. 특징점 추적 기법별 연산 시간 측정 결과 SIFT, SURF 및 ORB의 순서로 연산 시간이 감소하였다. ORB의 경우 SIFT 기법 대비 추적된 특징점 수가 평균 59.8%로 줄어들었지만 연산 시간은 평균 8.7%에 해당하는 시간이 소요되어 해빙 이동 특성의 고속 탐지에 적합한 기법으로 판단된다.

• 지질공학
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• 제31권3호
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• pp.257-267
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• 2021

Constraints on the structure and composition of the active layer are important for understanding permafrost evolution. Soil convection owing to repeated moisture-induced freeze-thaw cycles within the active layer promotes the formation of self-organized patterned ground. Here we present the results of ground penetrating radar (GPR) surveys across a selected sorted circle near King Sejong Station, Antarctica, to better delineate the active layer and its relation to the observed patterned ground structure. We acquire GPR data in both bistatic mode (common mid-points) for precise velocity constraints and monostatic mode (common-offset) for subsurface imaging. Reflections are derived from the active layer-permafrost boundary, organic layer-weathered soil boundary within the active layer, and frozen rock-fracture-filled ice boundary within the permafrost. The base of the imaged sorted circle possesses a convex-down shape in the central silty zone, which is typical for the pattern associated with convection-like soil motion within the active layer. The boundary between the central fine-silty domain and coarse-grained stone border is effectively identified in a radar amplitude contour at the assumed active layer depth, and is further examined in the frequency spectra of the near- and far-offset traces. The far-offset traces and the traces from the lower frequency components dominant on the far-offset traces would be associated with rapid absorption of higher frequency radiowave due to the voids in gravel-rich zone. The presented correlation strategies for analyzing very shallow, thin-layered GPR reflection data can potentially be applied to the various types of patterned ground, particularly for acquiring time-lapse imaging, when electric resistivity tomography is incorporated into the analysis.

• 한국음향학회지
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• 제43권1호
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• pp.49-59
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• 2024

최근 환경규제가 강화되고 고유가 문제로 인하여 전기차 시장이 점차 커지고 있으며 또한 내연기관 자동차에서도 엔진의 Noise, Vibration, Harshness(NVH) 관련 소음이 저감되고 외부에서 유입되는 소음의 차폐 기술이 발전됨에 따라 Buzz, Squeak, Rattle(BSR) 이음의 민감도가 증가하는 추세이다. 본 연구에서는 자동차의 Panoramic Curved Display(PCD)에서 발생하는 Stick-slip 이음과 고분자 플라스틱의 표면에너지 및 극성 성분과의 상관관계에 대하여 분석하였다. 극성 고분자 소재인 Acrylonitrile Butadiene Styrene(ABS)와 PolyCarbonate-Acrylonitrile Butadiene Styrene(PC-ABS)를 대상으로 컴파운드 소재를 제작하여 평가하였다. 결과적으로 고분자 플라스틱의 극성성분이 3.86 mN/m 이상일 때 Stick-slip 거동이 발생하였으며, 시간에 따른 마찰 거동에서 absolute transition slope가 증가할수록 Stick-slip의 이음 가능성이 증가하고 마찰계수의 값 차이가 클수록 Stick-slip 이음의 세기가 증가하였다.