• Smart Structures and Systems
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• 제20권3호
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• pp.329-342
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• 2017

In this paper, the dispersion characteristics of elastic waves propagation in sandwich nano-beams with functionally graded (FG) face-sheets reinforced with carbon nanotubes (CNTs) is investigated based on various high order shear deformation beam theories (HOSDBTs) as well as nonlocal strain gradient theory (NSGT). In order to align CNTs as symmetric and asymmetric in top and bottom face-sheets with respect to neutral geometric axis of the sandwich nano-beam, various patterns are employed in this analysis. The sandwich nano-beam resting on Pasternak foundation is subjected to thermal, magnetic and electrical fields. In order to involve small scale parameter in governing equations, the NSGT is employed for this analysis. The governing equations of motion are derived using Hamilton's principle based on various HSDBTs. Then the governing equations are solved using analytical method. A detailed parametric study is conducted to study the effects of length scale parameter, different HSDBTs, the nonlocal parameter, various aligning of CNTs in thickness direction of face-sheets, different volume fraction of CNTs, foundation stiffness, applied voltage, magnetic intensity field and temperature change on the wave propagation characteristics of sandwich nano-beam. Also cut-off frequency and phase velocity are investigated in detail. According to results obtained, UU and VA patterns have the same cut-off frequency value but AV pattern has the lower value with respect to them.

• 한국의학물리학회지:의학물리
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• 제30권4호
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• pp.139-149
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• 2019

Purpose: With neurodegeneration, the signal intensity of the cerebrospinal fluid (CSF) in the brain increases. The objective of this study was to evaluate chemical exchange saturation transfer (CEST) signals with and without the contribution of CSF signals in elderly human brains using two different 3T magnetic resonance imaging (MRI) sequences Methods: Full CEST signals were acquired in ten subjects (Group I) with a three-dimensional (3D)-segmented gradient-echo echo-planar imaging (EPI) sequence and in ten other subjects (Group II) with a 3D gradient and spin-echo (GRASE) sequence using two different 3T MRI systems. The segmented tissue compartments of gray and white matter were used to mask the CSF signals in the full CEST images. Two sets of magnetization transfer ratio asymmetry (MTR_asym) maps were obtained for each offset frequency in each subject with and without masking the CSF signals (masked and unmasked conditions, respectively) and later compared using paired t-tests. Results: The region-of-interest (ROI)-based analyses showed that the MTR_asym values for both the 3D-segmented gradient-echo EPI and 3D GRASE sequences were altered under the masked condition compared with the unmasked condition at several ROIs and offset frequencies. Conclusions: Depending on the imaging sequence, the MTR_asym values can be overestimated for some areas of the elderly human brain when CSF signals are unmasked. Therefore, it is necessary to develop a method to minimize this overestimation in the case of elderly patients.

• 환경생물
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• 제22권1호
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• pp.213-219
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• 2004

동굴 내 정점별 세균 군집 구조를 분석하기 위하여 PCf amplified 16S rDNA denaturing gradient gel electrophoresis(DGGE)를 적용하였다. DGGE는 동일한 분자량을 갖는 dsDNA band라고 할지라도, 각각의 염기서열 차이에 따라 전기영동 상에서 고유한 band양상을 나타낼 수 있다. eubacteria의 16S rDNA V3region을 증폭하기 위해 GC341F와 PRUN518r을 primer로 사용하여 지하수내에 미생물 군집의 다양성과 유사성을 분석하였다. DGGE band 양상을 통해 동굴내의 세균 군집 구조는 외부 환경에 비해 상대적으로 종다양성이 낮으며 동굴내 에서 특이적으로 서식하는 종이 있음을 확인하였다. 또한 유기 영양물질의 공급이 제한되어 있는 동굴에서 구아노가 주요 유기 영양물질의 공급원으로서 큰 영향을 미치고 있는 것으로 파악되었다. DGGE 상의 일부 band의 염기서열분석 결과 Pseudomonas sp. NZ060과 Pseudomonas pseudoalcaligenes, uncultured Variovorax sp., soil bacterium NS7로 동정되었다.

• 한국컴퓨터정보학회논문지
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• 제16권8호
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• pp.57-66
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• 2011

본 논문에서는 단일 입력 영상에서 특징을 추출하여 실시간으로 에지 대칭과 기울기의 방향성 특징을 이용하여 효과적으로 사람을 검출하는 알고리즘을 제안한다. 제안된 알고리즘은 전처리, 사람 후보 영역 분할, 후보 영역 검증인 3단계로 구성되었다. 여기서 전처리 단계는 주변 조도 환경과 밝기에 강인하고, 사람의 특징인 모양 특징 크기, 사람의 조건을 고려한 사람의 특성을 가진 윤곽선을 검출한다. 그리고 사람 후보 영역 분할 단계는 검출된 윤곽선에서 사람의 에지 대칭성과 크기를 가지고 영역을 분리하고, 에이타부스트 알고리즘을 적용하여 1차 후보 영역을 분할한다. 마지막으로 후보 영역 검증 단계는 분할된 국소 영역에 대한 기울기의 특징 벡터 및 분류기를 이용하여 후보 영역을 검증하여 오검출의 성능을 우수하게 한다. 제안된 알고리즘을 적용하여 모의실험을 한 결과, 제안된 알고리즘은 단일 알고리즘을 적용한 기존 알고리즘 보다 처리 속도가 약 1.7배 정도 개선되었으며, FNR(False Negative Rate)은 3% 정도 우수함을 확인하였다.

• Geomechanics and Engineering
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• 제37권5호
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• pp.475-498
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• 2024

The prediction of the susceptibility of soil to liquefaction using a limited set of parameters, particularly when dealing with highly unbalanced databases is a challenging problem. The current study focuses on different ensemble learning classification algorithms using highly unbalanced databases of results from in-situ tests; standard penetration test (SPT), shear wave velocity (V_s) test, and cone penetration test (CPT). The input parameters for these datasets consist of earthquake intensity parameters, strong ground motion parameters, and in-situ soil testing parameters. liquefaction index serving as the binary output parameter. After a rigorous comparison with existing literature, extreme gradient boosting (XGBoost), bagging, and random forest (RF) emerge as the most efficient models for liquefaction instance classification across different datasets. Notably, for SPT and V_s-based models, XGBoost exhibits superior performance, followed by Light gradient boosting machine (LightGBM) and Bagging, while for CPT-based models, Bagging ranks highest, followed by Gradient boosting and random forest, with CPT-based models demonstrating lower G_mean(error), rendering them preferable for soil liquefaction susceptibility prediction. Key parameters influencing model performance include internal friction angle of soil (ϕ) and percentage of fines less than 75 µ (F₇₅) for SPT and V_s data and normalized average cone tip resistance (q_c) and peak horizontal ground acceleration (a_max) for CPT data. It was also observed that the addition of V_s measurement to SPT data increased the efficiency of the prediction in comparison to only SPT data. Furthermore, to enhance usability, a graphical user interface (GUI) for seamless classification operations based on provided input parameters was proposed.

• Investigative Magnetic Resonance Imaging
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• 제3권2호
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• pp.167-172
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• 1999

목적 : Echo planar image (EPn와 다론 MR 영상기법에서 2시간 이내의 초급성 출혈의 발견율의 차이를 비교해 보고자 한다. 대상 및 방법 : 인위적으로 뇌출혈을 유발시킨 백서 10마리를 대상으로 EP1, 급속 스핀에코 (FSE) T2 강조영상, Fluid attenuated inversion recovery (FLAIR) 영상, 스펀에코 (SE) Tl 강조영상과 Tl 강조 Gradient echo (GE) 영상을 이용하여, 뇌출혈을 일으킨 2시간 후에 MR 촬영을 하였다. 추가적으로 3마리는 출혈 30분 후에, 6마리는 1시간 후에 각각 EPI와 FSE T2 강조영상을 얻었다. 얻어진 영상에 대한 주관적인 시각적 평가를 하기위하여 세명의 방사선과 의사가 출혈 병변이 어느 MR 영상 기법에서 제일 잘 보이는지를 평가하였다. 정량적인 평가 로는 병변-정상 뇌설질의 대조잡음비(contrast-to-noise ratio, CNR)를 측정하였다. 통계처리는 Wilcoxon-Ranks test를 이용하였다. 결과 : 시각적 평가에서 출혈 병변이 높은 신호강도로 나타난 EP1, FLAIR, FSE T2 영상 중 EPI 영상이 특히 높은 신호강도로 나타나 제일 쉽게 병변이 구별되었다. 정량적인 평가에서 출혈 병 변-정상 뇌설질의 CNR은 EPI와 FLAIR 영상에서 다른 세 영상기법에 비해 통계적으로 유의하게 우월하였다 (p<0.01). 그러나 EPI와 FLAIR 영상간의 CNR은 동계적으로 유의한 차이가 없었다 (p>0.10). 추가적으로 출혈 30분과 1l시간 후 촬영한 모두에서 EPI영상과 FSE T2 강조 영상은 정상에 비해 높은 신호강도를 보여 병변의 구별이 가능하였다. 결론 : 백서에서 유발시킨 초급성기 뇌출혈의 여러 기법의 MR 영상 중 EPI 영상이 영상획득 시간이 가장 빠르면서 시각적 평가와 정량적 평가에서 출혈의 발견율이 가장 높아 출혈의 선속한 진단에 유용할 것으로 생각된다.

• Investigative Magnetic Resonance Imaging
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• 제19권2호
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• pp.76-87
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• 2015

Purpose: There is an ongoing search for a stent material that produces a reduced susceptibility artifact. This study evaluated the effect of manganese (Mn) content on the MRI susceptibility artifact of ferrous-manganese (Fe-Mn) alloys, and investigated the correlation between MRI findings and measurements of Fe-Mn microstructure on X-ray diffraction (XRD). Materials and Methods: Fe-Mn binary alloys were prepared with Mn contents varying from 10% to 35% by weight (i.e., 10%, 15%, 20%, 25%, 30%, and 35%; designated as Fe-10Mn, Fe-15Mn, Fe-20Mn, Fe-25Mn, Fe-30Mn, and Fe-35Mn, respectively), and their microstructure was evaluated using XRD. Three-dimensional spoiled gradient echo sequences of cylindrical specimens were obtained in parallel and perpendicular to the static magnetic field (B0). In addition, T1-weighted spin echo, T2-weighted fast spin echo, and $T2^*$weighted gradient echo images were obtained. The size of the low-intensity area on MRI was measured for each of the Fe-Mn binary alloys prepared. Results: Three phases of ${\alpha}^{\prime}$-martensite, ${\gamma}$-austenite, and ${\varepsilon}$-martensite were seen on XRD, and their composition changed from ${\alpha}^{\prime}$-martensite to ${\gamma}$-austenite and/or ${\varepsilon}$-martensite, with increasing Mn content. The Fe-10Mn and Fe-15Mn specimens comprised ${\alpha}^{\prime}$-martensite, the Fe-20Mn and Fe-25Mn specimens comprised ${\gamma}+{\varepsilon}$ phases, and the Fe-30Mn and Fe-35Mn specimens exhibited a single ${\gamma}$ phase. The size of the low-intensity areas of Fe-Mn on MRI decreased relative to its microstructure on XRD with increasing Mn content. Conclusion: Based on these findings, proper conditioning of the Mn content in Fe-Mn alloys will improve its visibility on MR angiography, and a Mn content of more than 25% is recommended to reduce the magnetic susceptibility artifacts on MRI. A reduced artifact of Fe-Mn alloys on MRI is closely related to the paramagnetic constitution of ${\gamma}$-austenite and/or ${\varepsilon}$-martensite.

• Journal of Mechanical Science and Technology
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• 제16권8호
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• pp.1135-1143
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• 2002

The intermittent spray characteristics of the single-hole diesel nozzle (d$\sub$n/=0.32 mm) used in the fuel injection system of heavy-duty diesel engines were experimentally investigated. The mean velocity and turbulent characteristics of the diesel spray injected intermittently into the still ambient were measured by using a 2-D PDPA (phase Doppler particle analyzer) . The gradient of spray half-width linearly increased with time from the start of injection, and it approximated to 0.04 at the end of the injection. The axial mean velocity of the fuel spray measured along the radial direction was similar to that of the free air jet within R/b= 1.0-1.5 regardless of elapsing time, and its non-dimensional distribution corresponds to the theoretical velocity distributions suggested by Hinze in the downstream of the spray flow fields. The turbulent intensity of the axial velocity components measured along the radial direction represented the 20-30% of the U$\sub$cι/ and tended to decrease in the outer region. The turbulent intensity in the trailing edge was higher than that in the leading edge.

• 산업기술연구
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• 제20권A호
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• pp.169-178
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• 2000

The 4-point bending tests have been performed In order to estimate the effect of TIG-dressing on fatigue strength and fatigue characteristics quantitatively for non load-carrying fillet welded joints subjected to pure bending. As a result of fatigue tests, fatigue strengths of as-welded specimens have satisfied the grade of fatigue strength prescribed in specifications of korea, AASHTO and JSSC. Fatigue strength at 2 million cycles of TIG-dressing specimens have increased compared with as-welded specimens. As the result of beachmark tests, fatigue cracks occurred at several points, where the radius of curvature and flank angle in the weld bead toes were low, and grew as semi-elliptical cracks, then approached to fracture. As a result of finite element analysis, stress concentration factor in weld bead toes has closely related to the flank angle and radius of curvature, and between these, the radius of curvature has more largely affected in stress concentration factor than flank angle. As a result of fracture mechanics approaches, the crack correction factor of test specimens has largely affected on stress gradient correction factor in case a/t is below 0.4. From the relations between stress intensity factor range estimated from FEM analysis and fatigue crack growth rate, fatigue life has been correctly calculated.

• 한국의학물리학회:학술대회논문집
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• 한국의학물리학회 2003년도 제27회 추계학술대회
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• pp.41-41
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• 2003

Purpose: Even if the wedge filter is widely used for the radiation therapy to modify the photon beam intensity, the wedged photon beam dose calculation is not so easy. Radiation therapy planning systems (RTPS) have been used the empirical or semi-analytical methods such as attenuation method using wedge filter parameters or wedge filter factor obtained from measurement. However, these methods can cause serious error in penumbra region as well as in edge region. In this study, we propose the dose calculation algorithm for wedged field to minimize the error especially in the outer beam region. Materials and Method: Modified intensity by wedge filter was calculated using tissue-maximum ratio (TMR) and scatter-maximum ratio (SMR) of wedged field. Profiles of wedged and non-wedged direction was also used. The result of new dose calculation was compared with measurement and the result from attenuation method. Results: Proposed algorithm showed the good agreement with measurement in the high dose-gradient region as well as in the inner beam region. The error was decreased comparing to attenuation method. Conclusion: Although necessary beam data for the RTPS commissioning was increased, new algorithm would guarantee the improved dose calculation accuracy for wedged field. In future, this algorithm could be adopted in RTPS.