• Computers and Concrete
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• 제25권2호
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• pp.133-144
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• 2020

This paper aims to study vibration characteristics of chiral and zigzag double-walled carbon nanotubes entrenched on Donnell shell model. The Eringen's nonlocal elastic equations are being combined with Donnell shell theory to observe small scale response. Wave propagation is proposed technique to establish field equations of model subjected to four distinct end supports. A nonlocal model has been formulated to explore the frequency spectrum of both chiral and zigzag double-walled CNTs along with diversity of indices and nonlocal parameter. The significance of scale effect in relevance of length-to-diameter and thickness- to- radius ratios are discussed and displayed in detail. The numerical solution based on this nonlocal Donnell shell model can be further used to predict other frequency phenomena of double-walled and multi-walled CNTs.

• 제어로봇시스템학회:학술대회논문집
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• 제어로봇시스템학회 2004년도 ICCAS
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• pp.690-693
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• 2004

This paper proposes a multi-resolution electrostatic potential field (MREPF) based solution to the mobile robot path planning and collision avoidance problem in 2D dynamic environment. The MREPF is an environment method in calculation time and updating field map. The large scale resolution map is added to EPF and this resolution map interacts with the small scale resolution map to find an optimal solution in real time. This approach can be interpreted with Atlantis model. The simulation studies show the efficiency of the proposed algorithm.

• 천문학회지
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• 제29권2호
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• pp.83-91
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• 1996

We present a model that rotating primordial blackholes(PBHs) produced at the end of inflation generate the random, non-oriented primordial magnetic field. PBHs are copiously produced as the Universe completes the cosmic phase transition via bubble nucleation and tunneling processes in the extended inflation hypothesis. The PBHs produced acquire angular momentum through the mutual tidal gravitational interaction. For PBHs of mass less than 1013g, one can show that the evaporation (photon) luminosity of PBHs exceeds the Eddington limit. Thus throughout the lifetime of the rotating PBH, radiation flow from the central blackhole along the Kerr-geodesic exerts torque to ambient plasma. In the process similar to the Bierman's battery mechanism electron current reaching up to the horizon scale is induced. For PBHs of Grand Unified Theories extended inflation with the symmetry breaking temperature of $T_{GUT}\;\~\;10^{10}$ GeV, which evaporate near decoupling, we find that they generate random, non-oriented magnetic fields of $\~10^{-11}G$ on the last-scattering surface on (the present comoving) scales of $\~O(10)Mpc$.

• Journal of Information Display
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• 제5권1호
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• pp.34-40
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• 2004

Time resolved infrared IR absorption spectroscopy is carried out to investigate the dynamics of electric field induced reorientation of the biphenyl molecular core and alkyl tail sub-fragments of the nematic liquid crystal 5CB (4-pentyl-4-cyano-biphenyl). The planar to homeotropic transition for high pre-tilt planar aligned cells, is studied for switching times ranging from 200 ${\mu}sec$ down to 80 ${\mu}sec$, the latter a factor of 1000 times faster than any previous nematic IR study. The reorientation rates of the core and tail are found to be the same to within experimental error and scale inversely with applied field squared, as expected for the balance of field and viscous torques. Thus any molecular conformation change during switching must relax on a shorter time scale. A simple model shows that no substantial differences exist between the reorientational dynamics of the tails and cores on the time scales longer than on the order of 10 ${\mu}s$.

• ETRI Journal
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• 제39권2호
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• pp.284-291
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• 2017

Drain-induced barrier lowering (DIBL) is one of the main parameters employed to indicate the short-channel effect for nano metal-oxide semiconductor field-effect transistors (MOSFETs). We propose a new physical model of the DIBL effect under two-dimensional approximations based on the energy-conservation equation for channel electrons in FETs, which is different from the former field-penetration model. The DIBL is caused by lowering of the effective potential barrier height seen by the channel electrons because a lateral channel electric field results in an increase in the average kinetic energy of the channel electrons. The channel length, temperature, and doping concentration-dependent DIBL effects predicted by the proposed physical model agree well with the experimental data and simulation results reported in Nature and other journals.

• Journal of the Korean Society for Industrial and Applied Mathematics
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• 제20권1호
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• pp.37-50
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• 2016

A large eddy simulation (LES) of a turbulent channel flow is performed by using the third order low-storage Runge-Kutta method in time and second order finite difference formulation in space with staggered grid at a Reynolds number, $Re_{\tau}=590$ based on the channel half width, ${\delta}$ and wall shear velocity, $u_{\tau}$. To reduce the calculation cost of LES, algebraic wall model (AWM) is applied to approximate the near-wall region. The computation is performed in a domain of $2{\pi}{\delta}{\times}2{\delta}{\times}{\pi}{\delta}$ with $32{\times}20{\times}32$ grid points. Standard Smagorinsky model is used for subgrid-scale (SGS) modeling. Essential turbulence statistics of the flow field are computed and compared with Direct Numerical Simulation (DNS) data and LES data using no wall model. Agreements as well as discrepancies are discussed. The flow structures in the computed flow field have also been discussed and compared with LES data using no wall model.

• 지구물리와물리탐사
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• 제26권2호
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• pp.84-93
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• 2023

지도 학습 기반 딥러닝 탄성파 역산은 소규모 영역을 대상으로 하는 합성 자료 예제에서 성공적인 역산 성능을 보여주었다. 지도 학습 기반 딥러닝 탄성파 역산은 시간 영역 파동장을 입력, 지하 속도 모델을 출력으로 사용하는데, 시간 영역 파동장은 다양한 파동 정보를 포함하고 있어 자료의 크기가 상당히 크다. 따라서 대량의 데이터로 훈련하는 지도 학습 기반 딥러닝 탄성파 역산을 현장 규모의 자료에 적용하는 연구는 아직까지 수행되지 못하고 있다. 본 연구에서는 지도 학습 기반 딥러닝 탄성파 역산 기법을 현장 규모의 자료에 적용하기 위해 시간 영역 파동장 대신 라플라스 영역 파동장을 입력으로 사용하여 지하 속도 모델을 예측하였다. 시간 영역 파동장 대신 라플라스 영역 파동장을 사용하면 결과의 해상도는 다소 떨어지지만 입력 자료의 크기가 크게 감소하여 신경망 훈련이 빨라지게 된다. 또한, 큰 격자 간격을 사용할 수 있어 현장 자료 크기의 속도 모델을 효율적으로 예측할 수 있으며 이를 통해 얻은 결과는 후속 역산의 초기 모델로 사용될 수 있다. 신경망 훈련을 위해 현장 자료 크기를 가지는 대량의 합성 속도 모델과 라플라스 영역 파동장을 생성한 후 인공 합성 자료만으로 신경망을 훈련시켰다. 또한, 해양 탄성파 탐사를 시뮬레이션하기 위해 견인 스트리머 취득 조건을 채택하였다. 테스트 자료와 벤치마크 모델을 이용한 수치 예제에서 훈련된 신경망을 테스트한 결과, 적절한 배경 속도 모델들을 얻을 수 있었다.

• 한국수자원학회:학술대회논문집
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• 한국수자원학회 2019년도 학술발표회
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• pp.154-154
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• 2019

Characterization of reliable field-scale root-zone soil moisture (RZSM) variability contribute to effective hydro-meterological monitoring. Although a promising cosmic-ray neutron probe (CRNP) holds the pontential for field-scale RZSM measurement, it is often restricted at deeper depths due to the non-unique sensitivity of CRNP-measured fast neutron signal to other hydrogen pools. In this study, a merging framework relied on coupling cosmic-ray soil moisture with a representative additional RZSM, was introduced to scale shallower CRNP effective depth to represent root-zone layer. We tested our proposed framework over a densely vegetated region in South Korea covering a network of one CRNP and nine in-situ point measurements. In particular, cosmic-ray soil moisture and ancillary RZSM retrieved from the most time stable location were considered as input datasets; whereas the remaining point locations were used to generate a reference RZSM product. The errors between these two input datasets and the reference were forecasted by a linear autoregressive model. A linear combination of forecasts was then employed to compute a suitable weight for merging two input products from the predicted errors. The performance of merging framework was evaluated against reference RZSM in comparison to the two original products and a commonly used exponential filter technique. The results of this study showed that merging framework outperformed other products, demonstrating its robustness in improving field-scale RZSM. Moreover, a strong relationship between the quality of input data and the performance merging framework in light of CRNP effective depth variation has been also underlined via the merging framework.

• Wind and Structures
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• 제13권6호
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• pp.557-580
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• 2010

A new inflow turbulence generation method and a combined dynamic SGS model recently developed by the authors were applied to evaluate the wind effects on 508 m high Taipei 101 Tower. Unlike the majority of the past studies on large eddy simulation (LES) of wind effects on tall buildings, the present numerical simulations were conducted for the full-scale tall building with Reynolds number greater than $10^8$. The inflow turbulent flow field was generated based on the new method called discretizing and synthesizing of random flow generation technique (DSRFG) with a prominent feature that the generated wind velocity fluctuations satisfy any target spectrum and target profiles of turbulence intensity and turbulence integral length scale. The new dynamic SGS model takes both advantages of one-equation SGS model and a dynamic production term without test-filtering operation, which is particular suitable to relative coarse grid situations and high Reynolds number flows. The results of comparative investigations with and without generation of inflow turbulence show that: (1) proper simulation of an inflow turbulent field is essential in accurate evaluation of dynamic wind loads on a tall building and the prescribed inflow turbulence characteristics can be adequately imposed on the inflow boundary by the DSRFG method; (2) the DSRFG can generate a large number of random vortex-like patterns in oncoming flow, leading to good agreements of both mean and dynamic forces with wind tunnel test results; (3) The dynamic mechanism of the adopted SGS model behaves adequately in the present LES and its integration with the DSRFG technique can provide satisfactory predictions of the wind effects on the super-tall building.

• International Journal of Fluid Machinery and Systems
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• 제2권4호
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• pp.353-362
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• 2009

In the process of turbine modernizations, the investigation of the influences of water passage roughness on radial flow machine performance is crucial and validates the efficiency step up between reduced scale model and prototype. This study presents the specific losses per component of a Francis turbine, which are estimated by CFD simulation. Simulations are performed for different water passage surface roughness heights, which represents the equivalent sand grain roughness height. As a result, the boundary layer logarithmic velocity profile still exists for rough walls, but moves closer to the wall. Consequently, the wall friction depends not only on roughness height but also on its shape and distribution. The specific losses are determined by CFD numerical simulations for each component of the prototype, taking into account its own specific sand grain roughness height. The model efficiency step up between reduced scale model and prototype value is finally computed by the assessment of specific losses on prototype and by evaluating specific losses for a reduced scale model with smooth walls. Furthermore, surveys of rough walls of each component were performed during the geometry recovery on the prototype and comparisons are made with experimental data from the EPFL Laboratory for Hydraulic Machines reduced scale model measurements. This study underlines that if rough walls are considered, the CFD approach estimates well the local friction loss coefficient. It is clear that by considering sand grain roughness heights in CFD simulations, its forms a significant part of the global performance estimation. The availability of the efficiency field measurements provides an unique opportunity to assess the CFD method in view of a systematic approach for turbine modernization step up evaluation. Moreover, this paper states that CFD is a very promising tool for future evaluation of turbine performance transposition from the scale model to the prototype.