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APPLICATION OF MOVING CHIMERA GRID TECHNOLOGY IN NUMERICAL SIMULATIONOF WATER IMPACT PROBLEM
Van-Tu Nguyen,Duc-Thanh Vu,Warn-Gyu Park 한국전산유체공학회 2014 한국전산유체공학회 학술대회논문집 Vol.2014 No.5
A numerical scheme based on unsteady three-dimensional (3D), incompressible Navier-Stokes equations is applied to simulate water impact flows. The implicit method employs a dual-time, preconditioned algorithm with multi-block and parallel computing to improve our productivity in doing CFD analysis in a very effective way. A moving Chimera grid scheme were integrated with the Navier-Stokes solver to allows to assemble the computational grid by a collection of subgrids. The chimera domain decomposition scheme uses overlapping, embedding, and moving approach to facilitate the flow simulations of arbitrary translational and rotational complex geometries among various computational blocks. The domain-communication module is fully parallel and is ideally suited for handling moving body problems. The numerical scheme was validated by comparing present results of three free falling bodies including a hemisphere, a cone and a wedge entering the water with available experimental data and others numerical/theoretical results. A good obtained agreement between those results demonstrated the effectiveness of the methodology.
Influence of incident angles of earthquakes on inelastic responses of asymmetric-plan structures
Nguyen, Van Tu,Kim, Dookie Techno-Press 2013 Structural Engineering and Mechanics, An Int'l Jou Vol.45 No.3
This paper presents the influence of incident angles of earthquakes on inelastic dynamic responses of asymmetry single story buildings under seismic ground motions. The dynamic responses such as internal forces and rotational ductility factor are used to evaluate the importance of the incident angles of ground motions in the inelastic range of structural behavior. The base shear and torque (BST) response histories of the resisting elements and of the building are used to prove that the shape of the BST surface of the building can be a practical tool to represent those of all resisting elements. This paper also shows that the different global forces which produce the maximum demands in the resisting elements tend to converge toward a single distribution in a definable intensity range, and this single distribution is related to the resistance distribution of the building.
Nguyen, Nho-Van,Nguyen, Tam-Khanh Tu,Lee, Hong-Hee The Korean Institute of Power Electronics 2015 JOURNAL OF POWER ELECTRONICS Vol.15 No.2
This paper presents a novel switching voltage model and an offset-based pulse width modulation (PWM) scheme for multilevel inverters with unbalanced DC sources. The switching voltage model under a DC voltage imbalance will be formulated in general form for multilevel neutral-point-clamped topologies. Analysis of the reference switching voltages from active and non-active switching voltage components in abc coordinates can enable voltage implementation for an unbalanced DC-source condition. Offset voltage is introduced as an indispensable variable in the switching voltage model for multilevel voltage-source inverters. The PWM performance is controlled through the design of two offset components in a subsequence. One main offset may refer to the common mode voltage, and the other offset restricts its effect on the quality of PWM control in related DC levels. The PWM quality can be improved as the switching loss is reduced in a discontinuous PWM mode by setting the local offset, which is related to the load currents. The validity of the proposed algorithm is verified by experimental results.
Nguyen Duc Anh,Pham Van Thanh,Doan Tu Lap,Nguyen Tuan Khai,Tran Van An,Tran Duc Tan,Nguyen Huu An,Dang Nhu Dinh 한국인터넷정보학회 2022 KSII Transactions on Internet and Information Syst Vol.16 No.2
Forest fires inflict great losses of human lives and serious damages to ecological systems. Hence, numerous fire detection methods have been proposed, one of which is fire detection based on sensors. However, these methods reveal several limitations when applied in large spaces like forests such as high cost, high level of false alarm, limited battery capacity, and other problems. In this research, we propose a novel forest fire detection method based on image processing and correlation coefficient. Firstly, two fire detection conditions are applied in RGB color space to distinguish between fire pixels and the background. Secondly, the image is converted from RGB to YCbCr color space with two fire detection conditions being applied in this color space. Finally, the correlation coefficient is used to distinguish between fires and objects with fire-like colors. Our proposed algorithm is tested and evaluated on eleven fire and non-fire videos collected from the internet and achieves up to 95.87% and 97.89% of F-score and accuracy respectively in performance evaluation.
Van Tu Nguyen,Xuan Dai Nguyen 한국강구조학회 2022 International Journal of Steel Structures Vol.22 No.3
Describing the nonlinear behaviour of constitutive materials plays an important role in structural analysis. The Giuff rè– Menegotto–Pinto (GMP) model is widely used in the nonlinear modelling of steel structures, with its constituent parameters often calibrated from tests. However, the experimental results obtained require intermediate identifi cation procedures before being used directly, meanwhile, the calibration of model parameters based on experimental data is complicated due to the many interrelated constituent variables. This paper aims to propose a method that calibrates the GMP model parameters optimally based on the experimental data. An available set of test results of high-strength steels subjected to cyclic strain is employed to perform an optimal analysis. The obtained results are then compared to numerical and experimental results to evaluate the eff ectiveness of the proposed method. An extensive study was carried out to evaluate the applicability of the optimal parameters obtained and those suggested by OpenSees. The fi ndings reveal that the proposed procedure is highly effi cient, making it a useful option for developing OpenSees applications that automatically calibrate model parameters. A typical 3D steel frame structure subjected to an earthquake is analyzed to evaluate the applicability of the results obtained.
Nguyen, Van Tu,Ha, Seongju,Yeom, Dong-Il,Ahn, Yeong Hwan,Lee, Soonil,Park, Ji-Yong Elsevier 2019 CURRENT APPLIED PHYSICS Vol.19 No.10
<P><B>Abstract</B></P> <P>Large-scale growth of mostly monolayer molybdenum disulfide (MoS<SUB>2</SUB>) on quartz, sapphire, SiO<SUB>2</SUB>/Si, and waveguide substrates is demonstrated by chemical vapor deposition with the same growth parameters. Centimeter-scale areas with large flakes and films of MoS<SUB>2</SUB> on all the growth substrates are observed. The atomic force microscopy and Raman measurements indicate the synthesized MoS<SUB>2</SUB> is monolayer with high quality and uniformity. The MoS<SUB>2</SUB> field effect transistors based on the as-grown MoS<SUB>2</SUB> exhibit carrier mobility of 1–2 cm<SUP>2</SUP>V<SUP>−1</SUP>s<SUP>−1</SUP> and On/Off ratio of ~10<SUP>4</SUP> while showing large photoresponse. Our results provide a simple approach to realize MoS<SUB>2</SUB> on various substrates for electronics and optoelectronics applications.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Mostly monolayer MoS<SUB>2</SUB> can be grown in the same CVD setup with the same procedure on various substrates such as quartz, sapphire, SiO<SUB>2</SUB>/Si, and waveguide substrates. </LI> <LI> The growth region extends almost cm-scale, which shows that scale-up production can be possible with CVD. </LI> <LI> Devices based on as-grown MoS2 show large albeit slow photoresponse under visible light illuminations. </LI> </UL> </P>
Numerical investigation of ventilated supercavitation of a projectile by hot exhaust gas
Van-Tu Nguyen,Thanh-Hoang Phan,Dong-Hyun Kim,Warn-Gyu Park 대한기계학회 2021 대한기계학회 춘추학술대회 Vol.2021 No.11
In this study, physical characteristics of supercavitation generated by high-temperature exhaust gas were numerically investigated. The hot gas in a range from 400 C° to 1000 C° was exhausted from the base of a shkval-shape projectile with a high velocity (approximately Ma = 2 in the air), while the velocity of the projectile is 120 m/s. The hot gas was also ventilated near the nose of the projectile with the same velocity as the projectile. With a very high temperature and high pressure, it is difficult to implement not only experiments but also CFD. In particular, since boiling due to temperature and cavity due to pressure must be considered together, a very high level of numerical analysis is required. In this study, the analysis of the multiphase flow was performed using a numerical program based on a fully compressible multiphase Navier-Stokes system with a phase change model. The numerical results provide detailed the temperature profile and shock structure that occurred in the wake flow.
Nguyen, Nho-Van,Nguyen, Tam-Khanh Tu The Korean Institute of Power Electronics 2019 JOURNAL OF POWER ELECTRONICS Vol.19 No.4
This paper presents two advanced hybrid pulse-width modulation (PWM) strategies for multilevel inverters (MLIs) that provide both common-mode voltage (CMV) elimination and current ripple reduction. The first PWM utilizes sequences that apply one switching state at the double ends of a half-carrier cycle. The second PWM combines the advantages of the former and an existing four-state PWM. Analyses of the harmonic characteristics of the two groups of switching sequences based on a general switching voltage model are carried out, and algorithms to optimize the current ripple are proposed. These methods are simple and can be implemented online for general n-level inverters. Using a three-level NPC inverter and a five-level CHB inverter, good performances in terms of the root mean square current ripple are obtained with the proposed PWM schemes as indicated through improved harmonic distortion factors when compared to existing schemes in almost the entire region of the modulation index. This also leads to a significant reduction in the current total harmonic distortion. Simulation and experimental results are provided to verify the effectiveness of the proposed PWM methods.
Van Tu Nguyen,하성주,염동일,안영환,이순일,박지용 한국물리학회 2019 Current Applied Physics Vol.19 No.10
Large-scale growth of mostly monolayer molybdenum disulfide (MoS2) on quartz, sapphire, SiO2/Si, and waveguide substrates is demonstrated by chemical vapor deposition with the same growth parameters. Centimeterscale areas with large flakes and films of MoS2 on all the growth substrates are observed. The atomic force microscopy and Raman measurements indicate the synthesized MoS2 is monolayer with high quality and uniformity. The MoS2 field effect transistors based on the as-grown MoS2 exhibit carrier mobility of 1–2 cm2V−1s−1 and On/Off ratio of ~104 while showing large photoresponse. Our results provide a simple approach to realize MoS2 on various substrates for electronics and optoelectronics applications.