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APPLICATION OF VOLUME OF FLUID INTERFACE TRACKING METHOD TO SIMULATION OFWATER IMPACT
Van-Tu Nguyen,Duc-Thanh Vu,Cong-Tu Ha,Warn-Gyu Park 한국전산유체공학회 2013 한국전산유체공학회 학술대회논문집 Vol.2013 No.5
In this paper, numerical simulations of water-entry of bodies are presented. Homogeneous flow model based on unsteady incompressible Reynolds-averaged Navier-Stokes equations was used to solve the flow field. Dual-time pseudo-compressibility method was employed to improve the computational efficiency of the solver. To investigate behavior of the water impact phenomenon, a VOF interface tracking algorithm known as the least squares volume-of-fluid interface reconstruction algorithm (LVIRA) is utilized to track the water-air interface. This approach uses the geometric information from the reconstructed interface and the velocity fields in each physical time step to update the volume fractions by solving the advection equation for water phase. The application to the computations of water impact of a hemisphere, two cones, and a wedge through free fall in one degree of freedom are performed. The free surface deformation, pressure coefficients, impact velocities and vertical accelerations during the impact are compared with available experiments and theoretical results. Good agreements with those results are obtained.
Van Tu Vu,Quang Huy Tran,Thanh Loc Pham,Phuong Nam Dao 제어·로봇·시스템학회 2022 International Journal of Control, Automation, and Vol.20 No.3
This article addresses a trajectory tracking control approach for an uncertain surface vessel using the new cascade structure of adaptive reinforcement learning (ARL) algorithm and kinematic controller, feed-forward term. Since a surface vessel is decoupled by kinematic sub-system and dynamic sub-system, the cascade control system is an ideal method for obtaining the tracking problem. In the proposed control structure, the dynamic control loop is designed to be the optimized method of the corresponding dynamic sub-system and the kinematic control loop is implemented by a nonlinear controller combining with feed-forward term. The online actor-critic architecture is considered in ARL algorithm to overcome the challenge of solving the Hamilton-Jacobi-Bellman (HJB) equation. Additionally, the proposed controller is able to handle the difficulty of the non-autonomous optimal control problem by designing the ARL technique for the corresponding system with a small number of state variables. Based on theoretical analysis, the ARL based control design has been made to guarantee the uniformly ultimately bounded (UUB) stability of the closed system. Finally, the simulation results are illustrated to verify the effectiveness of the proposed control scheme.
Thang Phan,Ha Phan Ai Nguyen,Cao Khoa Dang,Minh Tri Phan,Vu Thanh Nguyen,Van Tuan Le,Binh Thang Tran,Chinh Van Dang,Tinh Huu Ho,Minh Tu Nguyen,Thang Van Dinh,Van Trong Phan,Binh Thai Dang,Huynh Ho Ngo The Korean Society for Preventive Medicine 2023 예방의학회지 Vol.56 No.4
Objectives: The coronavirus disease 2019 (COVID-19) pandemic has increased the workload of healthcare workers (HCWs), impacting their health. This study aimed to assess sleep quality using the Pittsburgh Sleep Quality Index (PSQI) and identify factors associated with poor sleep among HCWs in Vietnam during the COVID-19 pandemic. Methods: In this cross-sectional study, 1000 frontline HCWs were recruited from various healthcare facilities in Vietnam between October 2021 and November 2021. Data were collected using a 3-part self-administered questionnaire, which covered demographics, sleep quality, and factors related to poor sleep. Poor sleep quality was defined as a total PSQI score of 5 or higher. Results: Participants' mean age was 33.20±6.81 years (range, 20.0-61.0), and 63.0% were women. The median work experience was 8.54±6.30 years. Approximately 6.3% had chronic comorbidities, such as hypertension and diabetes mellitus. About 59.5% were directly responsible for patient care and treatment, while 7.1% worked in tracing and sampling. A total of 73.8% reported poor sleep quality. Multivariate logistic regression revealed significant associations between poor sleep quality and the presence of chronic comorbidities (odds ratio [OR], 2.34; 95% confidence interval [CI], 1.17 to 5.24), being a frontline HCW directly involved in patient care and treatment (OR, 1.59; 95% CI, 1.16 to 2.16), increased working hours (OR, 1.84; 95% CI,1.37 to 2.48), and a higher frequency of encountering critically ill and dying patients (OR, 1.42; 95% CI, 1.03 to 1.95). Conclusions: The high prevalence of poor sleep among HCWs in Vietnam during the COVID-19 pandemic was similar to that in other countries. Working conditions should be adjusted to improve sleep quality among this population.
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.
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.
Determinants of Liquidity in Manufacturing Firms
VU, Thu Minh Thi,TRUONG, Tu Van,DINH, Dung Thuy Korea Distribution Science Association 2020 The Journal of Asian Finance, Economics and Busine Vol.7 No.12
This study examines the factors that affect firm's liquidity in manufacturing companies listed in Vietnam. Factors studied include the board size, the board independence, the firm size, the firm age, and its return. We use different metrics to measure firm's solvency status, including the cash ratio, the quick ratio, and the cash conversion cycle. Accordingly, three econometric models are built to test hypotheses proposed by researchers in order to explain the relationship between the five factors above and liquidity's measures. The study used the data set of manufacturing companies listed on the Ho Chi Minh City Stock Exchange in the period from 2015 to 2019. The final sample group comprises 139 firms with 633 observations. The results show that in manufacturing firms, while the cash ratio and the quick ratio are positively associated to the board size, the board independence, and the firm's profitability, the net operating cycle is negatively correlated to the board size, the firm size, the board independence, and the profitability. Therefore, larger firms with larger board size and more independent members can help to improve capital management efficiency.There is no evidence for the relationship between the firm age and solvency measurements, between cash conversion cycle and firm's profitability.
Hieu Van Ha,Vu Anh Le,Tu Thi Cam Nguyen,Hoa Duong Quang 대한수학회 2023 대한수학회지 Vol.60 No.4
We give a classification of real solvable Lie algebras whose non-trivial coadjoint orbits of corresponding to simply connected Lie groups are all of codimension 2. These Lie algebras belong to a well-known class, called the class of MD-algebras.