| 1. |
- Phu, Vu Dinh, et al.
(författare)
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Ventilator-associated respiratory infection in a resource-restricted setting: impact and etiology
- 2017
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Ingår i: Journal of Intensive Care. - : BioMed Central (BMC). - 2052-0492. ; 5
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Tidskriftsartikel (refereegranskat)abstract
- Ventilator-associated respiratory infection (VARI) is a significant problem in resource-restricted intensive care units (ICUs), but differences in casemix and etiology means VARI in resource-restricted ICUs may be different from that found in resource-rich units. Data from these settings are vital to plan preventative interventions and assess their cost-effectiveness, but few are available.
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| 2. |
- Dao, Tuan Anh, et al.
(författare)
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A multi-criteria optimization model for emission-concerned multi-depot vehicle routing problem with heterogeneous fleet
- 2018
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Ingår i: 2018 International Conference on Applied Smart Systems (ICASS). - : Institute of Electrical and Electronics Engineers (IEEE). - 9781538668665 - 9781538668658 - 9781538668672
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Konferensbidrag (refereegranskat)abstract
- Not only greenhouse gases but also other air pollutant emissions from transportation have direct impacts on the environment and human health. The challenge of solving the conflict between profit and environmental consequences in logistics has motivated many studies on the vehicle routing problem. In this paper, a multiobjective mixed integer linear programming model is proposed to minimize transportation expenses and pollutant emissions for the multi-depot heterogeneous vehicle routing problem. A metaheuristic is adapted to obtain the Pareto optimal solutions. After a search procedure, decision makers are able to choose among best transportation plans that balance many objectives at once including economic benefits and environmental impacts. Computational experiments are performed on seven well-known benchmark problem sets. The results demonstrate the existence of greener transportation plans, which are illustrated alongside the best solutions previously reported. The study shows that, in return for a minimal economic tradeoff, a substantial amount of pollution could be avoided.
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| 3. |
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| 4. |
- Dao, Tuan Anh, et al.
(författare)
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A High-Order Residual-Based Viscosity Finite Element Method for the Ideal MHD Equations
- 2022
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Ingår i: Journal of Scientific Computing. - : Springer Nature. - 0885-7474 .- 1573-7691. ; 92:3
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Tidskriftsartikel (refereegranskat)abstract
- We present a high order, robust, and stable shock-capturing technique for finite element approximations of ideal MHD. The method uses continuous Lagrange polynomials in space and explicit Runge-Kutta schemes in time. The shock-capturing term is based on the residual of MHD which tracks the shock and discontinuity positions, and adds sufficient amount of viscosity to stabilize them. The method is tested up to third order polynomial spaces and an expected fourth-order convergence rate is obtained for smooth problems. Several discontinuous benchmarks such as Orszag-Tang, MHD rotor, Brio-Wu problems are solved in one, two, and three spacial dimensions. Sharp shocks and discontinuity resolutions are obtained.
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| 5. |
- Dao, Tuan Anh, et al.
(författare)
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A nodal based high order nonlinear stabilization for finite element approximation of Magnetohydrodynamics
- 2024
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Ingår i: Journal of Computational Physics. - 0021-9991 .- 1090-2716. ; 512, s. 113146-113146
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Tidskriftsartikel (refereegranskat)abstract
- We present a novel high-order nodal artificial viscosity approach designed for solving Magnetohydrodynamics (MHD) equations. Unlike conventional methods, our approach eliminates the need for ad hoc parameters. The viscosity is mesh-dependent, yet explicit definition of the mesh size is unnecessary. Our method employs a multimesh strategy: the viscosity coefficient is constructed from a linear polynomial space constructed on the fine mesh, corresponding to the nodal values of the finite element approximation space. The residual of MHD is utilized to introduce high-order viscosity in a localized fashion near shocks and discontinuities. This approach is designed to precisely capture and resolve shocks. Then, high-order Runge-Kutta methods are employed to discretize the temporal domain. Through a comprehensive set of challenging test problems, we validate the robustness and high-order accuracy of our proposed approach for solving MHD equations.
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| 6. |
- Dao, Tuan Anh, et al.
(författare)
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A structure preserving numerical method for the ideal compressible MHD system
- 2024
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Ingår i: Journal of Computational Physics. - 0021-9991 .- 1090-2716. ; 508
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Tidskriftsartikel (refereegranskat)abstract
- We introduce a novel structure-preserving method in order to approximate the compressible ideal Magnetohydrodynamics (MHD) equations. This technique addresses the MHD equations using a non-divergence formulation, where the contributions of the magnetic field to the momentum and total mechanical energy are treated as source terms. Our approach uses the Marchuk-Strang splitting technique and involves three distinct components: a compressible Euler solver, a source-system solver, and an update procedure for the total mechanical energy. The scheme allows for significant freedom on the choice of Euler's equation solver, while the magnetic field is discretized using a curl-conforming finite element space, yielding exact preservation of the involution constraints. We prove that the method preserves invariant domain properties, including positivity of density, positivity of internal energy, and the minimum principle of the specific entropy. If the scheme used to solve Euler's equation conserves total energy, then the resulting MHD scheme can be proven to preserve total energy. Similarly, if the scheme used to solve Euler's equation is entropy-stable, then the resulting MHD scheme is entropy stable as well. In our approach, the CFL condition does not depend on magnetosonic wave-speeds, but only on the usual maximum wavespeed from Euler's system. To validate the effectiveness of our method, we solve a variety of ideal MHD problems, showing that the method is capable of delivering second-order accuracy in space for smooth problems, while also offering unconditional robustness in the shock hydrodynamics regime as well.
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| 7. |
- Dao, Tuan Anh, et al.
(författare)
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Energy stable and accurate coupling of finite element methods and finite difference methods
- 2022
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Ingår i: Journal of Computational Physics. - : Elsevier. - 0021-9991 .- 1090-2716. ; 449
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Tidskriftsartikel (refereegranskat)abstract
- We introduce a hybrid method to couple continuous Galerkin finite element methods and high-order finite difference methods in a nonconforming multiblock fashion. The aim is to optimize computational efficiency when complex geometries are present. The proposed coupling technique requires minimal changes in the existing schemes while maintaining strict stability, accuracy, and energy conservation. Results are demonstrated on linear and nonlinear scalar conservation laws in two spatial dimensions.
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| 8. |
- Dao, Tuan Anh, 1994-
(författare)
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Invariant domain preserving schemes for magnetohydrodynamics
- 2024
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Magnetohydrodynamics (MHD) studies the behaviors of ionized gases, such as plasmas, in the presence of a magnetic field. MHD is used in many applications, such as geophysics, space physics, and nuclear fusion.Despite intensive research in recent decades, many physical and numerical aspects of MHD are not well understood. The challenges inherent in solving MHD stem from the obstacles encountered in ordinary hydrodynamics, such as those described by the compressible Euler/Navier-Stokes equations, along with the intricacies arising from electromagnetism. A characteristic of compressible flows is their tendency to develop shocks/discontinuities over time. This often leads to unphysical traits in numerical approximations if the capturing scheme is not constructed properly. By physical laws, the magnetic field is solenoidal. However, in practice, numerical schemes seldom ensure this property precisely, which may lead to instability and convergence to wrong solutions. In numerical simulation of many applications, positive physical quantities such as density and pressure can easily become negative. On the whole, preserving the physical relevance of the numerical solutions poses a significant challenge in MHD.This thesis presents several numerical schemes based on Galerkin approximations to solve MHD. The schemes rely on viscous regularization, a technique to remove mathematical singularities by adding a vanishing viscosity term to the MHD equations. At the continuous level, we propose several choices of viscous regularization and rigorously show that they are consistent with thermodynamics. Based on these choices, we construct numerical schemes of which robustness is confirmed through many challenging benchmarks. Finally, we propose a nonconventional algorithm that simultaneously preserves many desirable physical properties, including positivity of density and internal energy, conservation of total energy, minimum entropy principle, and zero magnetic divergence.
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| 9. |
- Dao, Tuan Anh, et al.
(författare)
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Monolithic parabolic regularization of the MHD equations and entropy principles
- 2022
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Ingår i: Computer Methods in Applied Mechanics and Engineering. - : Elsevier. - 0045-7825 .- 1879-2138. ; 398
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Tidskriftsartikel (refereegranskat)abstract
- We show at the PDE level that the monolithic parabolic regularization of the equations of ideal magnetohydrodynamics (MHD) is compatible with all the generalized entropies, fulfills the minimum entropy principle, and preserves the positivity of density and internal energy. We then numerically investigate this regularization for the MHD equations using continuous finite elements in space and explicit strong stability preserving Runge–Kutta methods in time. The artificial viscosity coefficient of the regularization term is constructed to be proportional to the entropy residual of MHD. It is shown that the method has a high order of accuracy for smooth problems and captures strong shocks and discontinuities accurately for non-smooth problems.
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| 10. |
- Dao, Tuan Anh, et al.
(författare)
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Viscous Regularization of the MHD Equations
- 2024
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Ingår i: SIAM Journal on Applied Mathematics. - : Society for Industrial and Applied Mathematics. - 0036-1399 .- 1095-712X. ; 84:4, s. 1439-1459
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Tidskriftsartikel (refereegranskat)abstract
- Nonlinear conservation laws such as the system of ideal magnetohydrodynamics (MHD) equations may develop singularities over time. In these situations, viscous regularization is a common approach to regain regularity of the solution. In this paper, we present a new viscous flux to regularize the MHD equations that holds many attractive properties. In particular, we prove that the proposed viscous flux preserves positivity of density and internal energy, satisfies the minimum entropy principle, is consistent with all generalized entropies, and is Galilean and rotationally invariant. We also provide a variation of the viscous flux that conserves angular momentum. To make the analysis more useful for numerical schemes, the divergence of the magnetic field is not assumed to be zero. Using continuous finite elements, we show several numerical experiments, including contact waves and magnetic reconnection.
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