• 한국유체기계학회 논문집
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• 제13권5호
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• pp.43-53
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• 2010

This study investigates a design optimization of a rotating two-pass rectangular cooling channel with staggered arrays of pin-fins. The radial basis neural network method is used as an optimization technique with Reynolds-averaged Navier-Stokes analysis of fluid flow and heat transfer with shear stress transport turbulent model. The ratio of the diameter to height of the pin-fins and the ratio of the streamwise spacing between the pin-fins to height of the pin-fin are selected as design variables. The optimization problem has been defined as a minimization of the objective function, which is defined as a linear combination of heat transfer related term and friction loss related term with a weighting factor. Results are presented for streamlines, velocity vector fields, and contours of Nusselt numbers, friction coefficients, and turbulent kinetic energy. These results show how fluid flow in a two-pass square cooling channel evolves a converted secondary flows due to Coriolis force, staggered arrays of pin-fins, and a $180^{\circ}$ turn region. These results describe how the fluid flow affects surface heat transfer. The Coriolis force induces heat transfer discrepancy between leading and trailing surfaces, having higher Nusselt number on the leading surface in the second pass while having lower Nusselt number on the trailing surface. Dean vortices generated in $180^{\circ}$ turn region augment heat transfer in the turning region and in the upstream region of the second pass. As the result of optimization, in comparison with the reference geometry, thermal performance of the optimum geometry shows the improvement by 30.5%. Through the optimization, the diameter of pin-fin increased by 14.9% and the streamwise distance between pin-fins increased by 32.1%. And, the value of objective function decreased by 18.1%.

• International Journal of Precision Engineering and Manufacturing
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• 제9권2호
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• pp.18-22
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• 2008

The pitch and orthogonality of two-dimensional (2-D) gratings were measured using a metrological atomic force microscope (MAFM), and the measurement uncertainty was analyzed. Gratings are typical standard devices for the calibration of precision microscopes, Since the magnification and orthogonality in two perpendicular axes of microscopes can be calibrated simultaneously using 2-D gratings, it is important to certify the pitch and orthogonality of such gratings accurately for nanometrology. In the measurement of 2-D gratings, the MAFM can be used effectively for its nanometric resolution and uncertainty, but a new measurement scheme is required to overcome limitations such as thermal drift and slow scan speed. Two types of 2-D gratings with nominal pitches of 300 and 1000 nm were measured using line scans to determine the pitch measurement in each direction. The expanded uncertainties (k = 2) of the measured pitch values were less than 0.2 and 0.4 nm for each specimen, and the measured orthogonality values were less than $0.09^{\circ}$ and $0.05^{\circ}$, respectively. The experimental results measured using the MAFM and optical diffractometer agreed closely within the expanded uncertainty of the MAFM. We also propose an additional scheme for measuring 2-D gratings to increase the accuracy of calculated peak positions, which will be the subject of future study.

• 한국정밀공학회지
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• 제21권11호
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• pp.75-82
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• 2004

A metrological atomic force microscope (M-AFM) was developed fur the length measurements of nanometer range, through the modification of a commercial AFM. To eliminate nonlinearity and crosstalk of the PZT tube scanner of the commercial AFM, a two-axis flexure hinge scanner employing built-in capacitive sensors is used for X-Y motion instead of PZT tube scanner. Then two-dimensional displacement of the scanner is measured using two-axis heterodyne laser interferometer to ensure the meter-traceability. Through the measurements of several specimens, we could verify the elimination of nonlinearity and crosstalk. The uncertainty of length measurements was estimated according to the Guide to the Expression of Uncertainty in Measurement. Among several sources of uncertainty, the primary one is the drift of laser interferometer output, which occurs mainly from the variation of refractive index of air and the thermal stability. The Abbe error, which is proportional to the measured length, is another primary uncertainty source coming from the parasitic motion of the scanner. The expanded uncertainty (k =2) of length measurements using the M-AFM is √(4.26)$^2$+(2.84${\times}$10$^{-4}$ ${\times}$L)$^2$(nm), where f is the measured length in nm. We also measured the pitch of one-dimensional grating and compared the results with those obtained by optical diffractometry. The relative difference between these results is less than 0.01 %.

• 대한기계학회논문집B
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• 제26권2호
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• pp.227-237
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• 2002

A numerical technique for simulating the aggregation of charged particles was presented with a Brownian dynamic simulation in the free molecular regime. The Langevin equation was used for tracking each particle making up an aggregate. A periodic boundary condition was used for calculation of the aggregation process in each cell with 500 primary particles of 16 nm in diameter. We considered the thermal force and the electrostatic force for the calculation of the particle motion. The electrostatic force on a particle in the simulation cell was considered as a sum of electrostatic forces from other particles in the original cell and its replicate cells. We assumed that the electric charges accumulated on an aggregate were located on its center of mass, and aggregates were only charged with pre-charged primary particles. The morphological shape of aggregates was described in terms of the fractal dimension. In the simulation, the fractal dimension for the uncharged aggregate was D$\_$f/ = 1.761. The fractal dimension changed slightly for the various amounts of bipolar charge. However, in case of unipolar charge, the fractal dimension decreased from 1.641 to 1.537 with the increase of the average number of charges on the particles from 0.2 to 0.3 in initial states. In the bipolar charge state, the average sizes of aggregates were larger than that of the uncharged state in the early and middle stages of aggregation process, but were almost the same as the case of the uncharged state in the final stage. On the other hand, in the unipolar charge state, the average size of aggregates and the dispersion of particle volume decreased with the increasing of the charge quantities.

• Tribology and Lubricants
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• 제38권6호
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• pp.235-240
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• 2022

Atomically-thin 2D nanomaterials can be easily deformed and have surface corrugations which can influence the frictional characteristics of the 2D nanomaterials. Chemical vapor deposition (CVD) graphene can be grown in a wafer scale, which is suitable as a large-area surface coating film. The CVD growth involves cooling process to room temperature, and the thermal expansion coefficients mismatch between graphene and the metallic substrate induces a compressive strain in graphene, resulting in the surface corrugations such as wrinkles and atomic ripples. Such corrugations can induce the friction anisotropy of graphene, and therefore, accurate imaging of the surface corrugation is significant for better understanding about the friction anisotropy of CVD graphene. In this work, the combinatorial analysis using friction force microscopy (FFM) and transverse shear microscopy (TSM) was implemented to unveil the friction anisotropy of CVD bi-layer graphene. The periodic friction anisotropy of the wrinkles was measured following a sinusoidal curve depending on the angles between the wrinkles and the scanning tip, and the two domains were observed to have the different friction signals due to the different directions of the atomic ripples, which was confirmed by the high-resolution FFM and TSM imaging. In addition, we revealed that the atomic ripples can be easily suppressed by ironing the surface during AFM scans with an appropriate normal force. This work demonstrates that the friction anisotropy of CVD bilayer graphene is well-correlated with the corrugated structures and the local friction anisotropy induced by the atomic ripples can be controllably removed by simple AFM scans.

• The Journal of Advanced Prosthodontics
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• 제15권2호
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• pp.63-71
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• 2023

PURPOSE. The aim of this study was to assess the effect of hemispherical dimple structures on the retention of cobalt-chromium (Co-Cr) crowns cemented to titanium abutments, with different heights and numbers of dimples on the axial walls. MATERIALS AND METHODS. 3.0-mm and 6.0-mm abutments (N = 180) and Co-Cr crowns were prepared. The experimental groups were divided into two and four dimple groups. The crowns were cemented by TempBond and PANAVIA F 2.0 cements. The retention forces were measured after thermal treatments. A two-way Analysis of Variance (ANOVA) and post-hoc Tukey HSD test were conducted to analyze change in retention forces by use of dimples between groups, as well as t test for the effect of abutment height change (α = .05). RESULTS. Results of the two-way ANOVA showed a statistically significant difference in retention force due to the use of dimples, regardless of the types of cements used (P < .001). A significantly higher mean retention forces were observed in the groups with dimples than in the control group, using the post hoc Tukey HSD test (P < .001). Results of t test displayed a statistically significant increase in the retention force with 6.0-mm abutments compared with 3.0-mm abutments (P < .001). The groups without dimples revealed adhesive failure of cements, while the groups with dimples showed mixed failure of cements. CONCLUSION. Use of hemispherical dimples was effective for increasing retention forces of cemented crowns.

• 한국전산구조공학회논문집
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• 제28권3호
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• pp.325-333
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• 2015

최근들어 철근콘크리트 슬래브와 벽의 연결부에 적용 가능한 열교차단장치에 대한 연구가 다수 수행되고 있다. 이에 본 연구에서는 열교차단장치가 적용된 슬래브의 균열 전 탄성거동, 균열 후 항복거동 및 극한강도까지 적용이 가능한 해석모델을 제안하고, 실험결과와의 비교를 통하여 제안 모델의 정확도를 검증하고자 하였다. 해석모델은 변형률 적합조건과 힘의 평형개념을 적용하였으며, 이 때 구성 재료의 응력-변형률 관계는 재료실험 결과를 적용하였다. 해석모델의 신뢰성 검증을 위해 모멘트-곡률 관계, 하중단계에 따른 중립축을 실험결과와 비교하였으며, 제안된 해석모델은 실험결과로 획득한 전체적인 휨거동 양상과 거의 일치함을 확인할 수 있었다.

• 한국전기전자재료학회:학술대회논문집
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• 한국전기전자재료학회 2009년도 추계학술대회 논문집
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• pp.112-112
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• 2009

In this study, the effect of a long post-deposition thermal annealing(600 and 1000 $^{\circ}C$) on the surface acoustic wave (SAW) properties of polycrystalline (poly) aluminum-nitride (AlN) thin films grown on a 3C-SiC buffer layer was investigates. The poly-AlN thin films with a (0002) preferred orientation were deposited on the substrates by using a pulsed reactive magnetron sputtering system. Experimental results show that the texture degree of AlN thin film was reduced along the increase in annealing temperature, which caused the decrease in the electromechanical coupling coefficient ($k^2$). The SAW velocity also was decreased slightly by the increase in root mean square (RMS) roughness over annealing temperature. However, the residual stress in films almost was not affect by thermal annealing process due to small lattice mismatch different and similar coefficient temperature expansion (CTE) between AlN and 3C-SiC. After the AlN film annealed at 1000 $^{\circ}C$, the insertion loss of an $IDT/AlN/3C-SiC/SiO_2/Si$ structure (-16.44 dB) was reduced by 8.79 dB in comparison with that of the as-deposited film (-25.23 dB). The improvement in the insertion loss of the film was fined according to the decrease in the grain size. The characteristics of AlN thin films were also evaluated using Fourier transform-infrared spectroscopy (FT-IR) spectra and X-ray diffraction (XRD), scanning electron microscopy (SEM), and atomic force microscopy (AFM) images.

• 대한기계학회논문집A
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• 제38권11호
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• pp.1207-1214
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• 2014

균열이 존재하는 구조물의 균열 선단에서 과도한 소성 상태에 도달하게 되면 파괴역학 매개변수에 의하여 예측되는 균열 선단의 응력장이 실제와는 매우 다르게 나타난다. 구조물의 크기와 형상 그리고 하중의 형태에 따라 균열 선단의 응력장이 유일하지 않다. 이는 균열 선단의 구속 효과가 단일 매개변수 파괴역학으로 표현 될 수 없기 때문이다. 따라서 전통적으로 사용되었던 K, J를 이용한 단일 매개변수 파괴역학을 보완하기 위하여 다양한 시도가 있었고, 대표적으로 Q-응력이 있다. 본 논문에서는 Q-응력을 이용하여 원주방향으로 완전히 발달한 표면 균열이 있을 경우의 균열 선단의 구속 효과를 확인하였다. 고려된 하중은 일차하중으로 축 방향 인장하중과 반경방향 열구배에 의한 이차하중의 복합하중이다. 이차하중이 일차하중보다 구속효과가 더 심하며 기계하중보다 열하중이 뒤에 가해지는 열충격조건에서 구속효과가 더 심하게 나타남을 확인하였다.

• 한국해양학회지
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• 제20권2호
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• pp.28-39
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• 1985

저층 jet와 야간 최대 연안표층풍의 유도 mechanism을 텍사스주의 Port Aransas 와 Victoria지역에서 1976년 6월부터 1980년 8월까지 관측된 바람자료를 사용하여 경압모델에 의해 연구되었다. 이모델은 forcing function인 종관 및 중기압 경도, 마찰력과 대기안정도를 고려하여 개발된다. 연구결과는 야간최대 연안표층풍 에 대해 저층 jet가 95%의 출현빈도를 보여준다. 여름철에는 경사지형에 의한 경압 성이 thermal field의 일변화를 초래한다. 이 온도풍이 낮에는 탁월한 남풍의 풍속을 감소시키고 밤에는 증가시킨 다. 야간대기안정도는 마찰력 유리를 유도하므로 야간 저층 jet를 강화한다. 해안에서는 중립안정도가 지배적이고 또 저층 jet로부터 운동 량의 연직전이는 야간 최대연안표층풍을 형성한다. 저층 jet의 출현고도는 역전층과 의 뚜렷한 관계를 보여주지 않으며, 이 모델에 의한 계산결과는 관측치와 잘 일치 한다.