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• Coupling of CFD and CSD methodologies for modeling blast and structural response

  J. Baum, E. Mestreau, H. Luo, D. Pelessone, C. Charman, M. Giltrud, Y. Sohn

  WIT Transactions on The Built Environment (2003). Vol. 71 (10), pp. 329-340

  
  

  Abstract

  This paper describes recent algorithm developments and applications of a program that couples parallel Computational Fluid Dynamics (CFD) and Computational Structural Dynamics (CSD) methodologies. FEFL098 is the CFD code used while DYNA3D handles the CSD portion. FEFL098 solves the time-dependent compressible Euler and Reynolds-Averaged Navier-Stokes equations on an unstructured mesh of tetrahedral elements. DYNA3D solves explicitly the large deformation, large strain formulation equations on an unstructured grid composed of bricks and hexahedral elements. While the initial coupled algorithm used the so-called "glued-mesh" approach, where the CFD and CSD faces match identically, failure of this approach to model severe structural deformation, as well as crack propagation in steel and concrete, led us to the development of the so-called "embedded-mesh" approach. Here, the CSD objects float through the CFD domain. While each approach has its own advantages, limitations, and deficiencies, the embedded approach was proven to be superior for the class of problems modeled here. Critical application of both approaches are described, including weapon detonation and fragmentation, airblast interaction with a reinforced concrete wall, and fragment/airblast interaction with a steel wall. The final application models the interaction of an external airblast with a generic steel ship.

  Abstract
  This paper describes recent algorithm developments and applications of a program that couples parallel Computational Fluid Dynamics (CFD) and Computational Structural Dynamics [...]

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• Fluid-structure interaction using adaptive embedded unstructured grids

  R. Lohner, J. Baum, E. Mestreau, C. Charman, D. Pelessone

  WIT Transactions on The Built Environment (2003). Vol. 71 (10), pp. 283-292

  
  

  Abstract

  Fluid-structure interaction cases with severe topological change due to fragmentation or rupture in the structure have prompted the development of flow solvers using a so-called embedded mesh approach. A simple embedded domain method for node-based unstructured grid solvers is presented. Edges crossing embedded surface faces are either removed or duplicated. Several techniques to improve the treatment of boundary points close to the immersed surfaces are explored. Adaptive mesh refinement based on proximity to the curvature or corners of the embedded CSD surfaces is used to enhance the accuracy of the solution. User-defined or automatic deactivation for the regions inside immersed solid bodies is employed to avoid unnecessary work. Several examples are included that show the viability of this approach for coupled fluid-structure problems.

  Abstract
  Fluid-structure interaction cases with severe topological change due to fragmentation or rupture in the structure have prompted the development of flow solvers using a so-called [...]

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• On The Modeling Of Boundary Conditions For Embedded Schemes

  E. Mestreau, J. Baum, R. Lohner, C. Charman

  WIT Transtactions on The Built Environment (2003). Vol. 71 (12), pp. 229-239

  
  

  Abstract

  A simple embedded method for an unstructured grid is presented. An enhancement of the boundary treatments using ghost points is then described. Several examples are used to demonstrate the viability of the approach for inviscid flow simulations.

  Abstract
  A simple embedded method for an unstructured grid is presented. An enhancement of the boundary treatments using ghost points is then described. Several examples are used to [...]

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• Adaptive Embedded Unstructured Grid Methods

  R. Lohner, J. Baum, E. Mestreau, D. Sharov, C. Charman, D. Pelessone

  International Journal for Numerical Methods in Engineering (2004). Vol. 60 (3), pp. 641-660

  
  

  Abstract

  A simple embedded domain method for node-based unstructured grid solvers is presented. The key modification of the original, edge-based solver is to remove all geometry-parameters (essentially the normals) belonging to edges cut by embedded surface faces. Several techniques to improve the treatment of boundary points close to the immersed surfaces are explored. Alternatively, higher-order boundary conditions are achieved by duplicating crossed edges and their endpoints. Adaptive mesh refinement based on proximity to or the curvature of the embedded CSD surfaces is used to enhance the accuracy of the solution. User-defined or automatic deactivation for the regions inside immersed solid bodies is employed to avoid unnecessary work. Several examples are included that show the viability of this approach for inviscid and viscous, compressible and incompressible, steady and unsteady flows, as well as coupled fluid–structure problems.

  Abstract
  A simple embedded domain method for node-based unstructured grid solvers is presented. The key modification of the original, edge-based solver is to remove all geometry-parameters [...]

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• Computation of Compressible Flows using a Two-equation Turbulence Model on Unstructured Grids

  H. Luo, J. Baum, R. Lohner

  International journal of Computational Fluid Dynamics (2003). Vol. 17 (1), pp. 87-93

  
  

  Abstract

  This paper presents a numerical method for solving compressible turbulent flows using a k - l turbulence model on unstructured meshes. The flow equations and turbulence equations are solved in a loosely coupled manner. The flow equations are advanced in time using a multi-stage Runge-Kutta time stepping scheme, while the turbulence equations are advanced using a multi-stage point-implicit scheme. The positivity of turbulence variables is achieved using a simple change of dependent variables. The developed method is used to compute a variety of turbulent flow problems. The results obtained are in good agreement with theoretical and experimental data, indicating that the present method provides a viable and robust algorithm for computing turbulent flows on unstructured meshes.

  Abstract
  This paper presents a numerical method for solving compressible turbulent flows using a k - l turbulence model on unstructured meshes. The flow equations and turbulence equations [...]

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• A Linelet preconditioner for incompressible flows

  O. Soto, R. Lohner, F. Camelli

  Int. J. Numer. Meth. Heat and Fluid Flows (2003). Vol. 13 (1), pp. 133-147

  
  

  Abstract

  A parallel linelet preconditioner has been implemented to accelerate finite element (FE) solvers for incompressible flows when highly anisotropic meshes are used. The convergence of the standard preconditioned conjugate gradient (PCG) solver that is commonly used to solve the discrete pressure equations, greatly deteriorates due to the presence of highly distorted elements, which are of mandatory use for high Reynolds-number flows. The linelet preconditioner notably accelerates the convergence rate of the PCG solver in such situations, saving an important amount of CPU time. Unlike other more sophisticated preconditioners, parallelization of the linelet preconditioner is almost straighforward. Numerical examples and some comparisons with other preconditioners are presented to demonstrate the performance of the proposed preconditioner.

  Abstract
  A parallel linelet preconditioner has been implemented to accelerate finite element (FE) solvers for incompressible flows when highly anisotropic meshes are used. The convergence [...]

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• Applications of patient-specific CFD in medicine and life science

  R. Lohner, J. Cebral, O. Soto, P. Yim, J. Burgess

  Int. J. Numer. Meth. Fluids (2003). Vol. 43 (6-7), pp. 637-650

  
  

  Abstract

  Recent advances in medical image segmentation, grid generation, flow solvers, realistic boundary conditions, fluid–structure interaction, data reduction and visualization arc reviewed with special emphasis on patient‐specific flow prediction. At the same time, present shortcomings in each one of these areas are identified. Several examples are given that show that this methodology is maturing rapidly, and may soon find widespread use in medicine.

  Abstract
  Recent advances in medical image segmentation, grid generation, flow solvers, realistic boundary conditions, fluid–structure interaction, data reduction and visualization [...]

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• Blood-flow models of the circle of Willis from magnetic resonance data

  J. Cebral, M. Castro, O. Soto, R. Lohner, N. Alperin

  Journal of Engineering Mathematics (2003). Vol. 47, pp. 369-386

  
  

  Abstract

  Detailed knowledge of the cerebral hemodynamics is important for a variety of clinical applications. Cerebral perfusion depends not only on the status of the diseased vessels but also on the patency of collateral pathways provided by the circle of Willis. Due to the large anatomical and physiologic variability among individuals, realistic patient-specific models can provide new insights into the cerebral hemodynamics. This paper presents an image-based methodology for constructing patient-specific models of the cerebral circulation. This methodology combines anatomical and physiologic imaging techniques with computer simulation technology. The methodology is illustrated with a finite element model constructed from magnetic resonance image data of a normal volunteer. Several of the remaining challenging problems are identified. This work represents a starting point in the development of realistic models that can be applied to the study of cerebrovascular diseases and their treatment.

  Abstract
  Detailed knowledge of the cerebral hemodynamics is important for a variety of clinical applications. Cerebral perfusion depends not only on the status of the diseased vessels [...]

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• Convergence study for the discrete particle method

  D. Pelessone, J. Baum, R. Lohner, C. Charman, J. Baylot

  (2003). Proceedings Second MIT Conference on Compurational Fluid and Solid Mechanics June 17–20, pp. 2093-2096

  
  

  Abstract

  The Discrete Particle Method (DPM) is a numerical technique in the class of the discrete element methods for the modeling of cementitious material in the pre- and post-failure regimes. Because the DPM is not based on continuum mechanics, the conventional convergence properties of Galerkin based methods, such as the finite element method, are not expected to apply. This chapter presents the results of a study to assess the convergence properties of the DPM for elastic problems. The DPM belongs to the general class of methods denoted as discrete element methods. The benchmark problem is based on the vibrations of a concrete beam free in space. Axial oscillations are used to adjust DPM model parameters, bending oscillations are used to test convergence characteristics. Results show that the DPM solution converges to the equivalent converged finite element solution.

  Abstract
  The Discrete Particle Method (DPM) is a numerical technique in the class of the discrete element methods for the modeling of cementitious material in the pre- and post-failure [...]

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• Coupling of discrete particle model with embedded mesh flow solver

  C. Charman, D. Pelessone, R. Lohner, J. Baum

  (2003). Proceedings Second MIT Conference on Compurational Fluid and Solid Mechanics June 17–20, pp. 1282-1284

  
  

  Abstract

  This chapter describes a unique coupling between a discrete particle model and an embedded mesh flow solver, with the focus on solving blast loadings on reinforced concrete structures. The development of a discrete particle method (DPM) model is more complex and time-consuming than the mesh generation of a typical finite element method (FEM) analysis. The DPM process is a bootstrapping process, where the discrete particle solver is used in the development of the model. Two methods are commonly used for creating particle models: “fill and expand” (F+E), and the "depositional" methods. There have been numerous successes in coupling the finite element-based structural models to the finite-element fluid-flow solver (FEFLO). Two methods for solving the coupled fluid/structure interaction problem are: the body-conforming and the embedded grid methods. In the body-conforming method, the external faces of the structure are used to define the fluid domain. In the embedded method, the structure is placed inside a larger flow region with special treatment of the fluid elements close to the structural surfaces.

  Abstract
  This chapter describes a unique coupling between a discrete particle model and an embedded mesh flow solver, with the focus on solving blast loadings on reinforced concrete [...]

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