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.

NUMERICAL SIMULATION OF VERTICAL WATER-EXIT AND WATER-ENTRY OF CAVITATING PROJECTILES

Van-Tu Nguyen,Cong-Tu Ha,Wam-Gyu Park 한국전산유체공학회 2012 한국전산유체공학회 학술대회논문집 Vol.2012 No.11

The present study focuses on the simulation of unsteady ventilated-cavitating flow during the vertical water-exit and water-entry of projectiles with and without exhaust plume. The propulsion exhaust plume gas embedded within the surrounding liquid strongly affects on the wake of flow. For simulation of water-exit and water-entry behavior of the cavitating projectiles which presents aspects of the flow physics such as cavity shape and phase topography of the flow during the processes, a numerical multi-phase homogeneous mixture model is utilized. The model is developed based on the unsteady incompressible Reynolds-averaged Navier-Stokes equations. Preconditioning pseudo-time stepping method is employed to improve the computational efficiency of the model. The results indicate that characteristics of cavity flow induced during water-exit and water-entry primarily depend on the geometry of projectiles and the velocities of flow. In addition, the gas exhaust delays the surface closure of the water-entry cavity that is one of the most important events in later cavity growth.

NUMERICAL SIMULATION OF THREE-PHASE FLOWS USING A SHARP INTERFACE CAPTURING METHOD

Van-Tu Nguyen,Warn-Gyu Park 한국전산유체공학회 2018 한국전산유체공학회 학술대회논문집 Vol.2018 No.5

Switching Voltage Modeling and PWM Control in Multilevel Neutral-Point-Clamped Inverter under DC Voltage Imbalance

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.

Efficient Forest Fire Detection using Rule-Based Multi-color Space and Correlation Coefficient for Application in Unmanned Aerial Vehicles

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.

Optimal Procedure for Determining Constitutive Parameters of Giuffrè–Menegotto–Pinto Model for Steel Based on Experimental Results

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.

3-D Numerical Simulation of Water Entry of Free Falling Objects Using Navier-Stokes Computations and Moving Chimera Grid Scheme

Van-Tu Nguyen,Duc-Thanh Vu,Warn-Gyu Park 한국전산유체공학회 2014 한국전산유체공학회 학술대회논문집 Vol.2014 No.10

An implicit algorithm based on the unsteady three-dimensional (3-D) incompressible Navier-Stokes equations is developed to simulate water-entry flows. The numerical algorithm employs a dual-time preconditioned method with multi-block and parallel computing to improve computational productivity. To handle the motion of an object in the water-entry problem, the Navier-Stokes solver is dynamically integrated to a six-degree-of-freedom (6DOF) rigid body motion model and a moving Chimera grid scheme. The Chimera domain decomposition scheme uses an 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. Calculations were performed first for vertical water-entry problems of a hemisphere and a cone. The numerical scheme was then employed for the simulation of inclined cylinder free-falling into the water. The present results are compared with available experimental data and other numerical and theoretical results. The good agreement between these results demonstrates the capability of the methodology.

New Generalized PWM Schemes for Multilevel Inverters Providing Zero Common-Mode Voltage and Low Current Distortion

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.

Enhancement of Navier–Stokes solver based on an improved volume-of-fluid method for complex interfacial-flow simulations

Nguyen, Van-Tu,Park, Warn-Gyu Elsevier 2018 Applied ocean research Vol.72 No.-

<P><B>Abstract</B></P> <P>In this study, an extended validation of an improved volume-of-fluid (VOF)-based method for 3D highly nonlinear, complex, and breaking free-surface problems in engineering is presented. The VOF interface-tracking scheme is implemented with a Navier–Stokes solver in a curvilinear coordinate system. The numerical procedure is advanced by implementing the scheme on moving Chimera (overset) grids for application in complex engineering problems. The overset grids are used to facilitate the flow simulations of complex geometries and arbitrary motions of objects and improve computational productivity by taking advantage of their flexibility. To solve the six degrees of freedom (6DOF) motions of a rigid body simultaneously, the nonlinear 6DOF motion equations in a vector form system are strongly combined with a flow solver. A collection of problems is carefully selected such that a large density ratio and complex cases under a wide variety of Froude numbers are presented with the aim of demonstrating the capabilities of the new enhanced method on a complicated moving overset grid system. The method is validated for a wide range of parameters. The results of different free-surface problems are presented. The numerical results are compared with numerical alternatives and experimental measurements, and accurate approximations and good agreement are obtained for complex flows.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Advances of accurate and efficient Navier–Stokes solver based on improved VOF method for interfacial-flows. </LI> <LI> Chimera grid method and solid motions model are integrated into the numerical solver for complex problems. </LI> <LI> Extended validation of the numerical method and examples for complex 3D problems are presented. </LI> <LI> Reasonable agreement of simulation with the experimental data is obtained. </LI> </UL> </P>

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.