Bobby Philip | Los Alamos National Laboratory (original) (raw)

Papers by Bobby Philip

2. The build system has been upgraded to use CMake/CTest/CDash, from KitWare Inc., to simplify th... more 2. The build system has been upgraded to use CMake/CTest/CDash, from KitWare Inc., to simplify the process of building and testing AMP. (Section 3) 3. The Materials component has been refactored to allow for a vector based interface, better organization, and to make it easier for users to add their own materials. (Section 4) 4. The Mechanics component has been refactored to support both small deformation analysis and large deformation analysis using the Updated Lagrangian approach. (Section 5) 5. The TrilinosMLSolver class has been refactored to allow the use of the ML solver without having to create Epetra matrices. (Section 6) 6. The Mesh component is currently being redesigned to allow for multiple mesh packages using a common interface. (Section 7) 7. The AMP Nuclear Fuel Performance code is being split into two codes: a base package that will provide general purpose routines and design and can leverage other work, and a nuclear fuel performance code that contains all of the fuel specific and export controlled code. (Section 8) 8. An AMP-MPI class was added to manage all parallel communication and allow for serial build. (Section 9) 2 Vision, stakeholders, and requirements During the prototyping effort of Fiscal Year 2010 (FY10), the concept and requirements for the software were discussed and the software was developed with an agile approach that allowed the team to adjust the plans and direction of the software as the team developed cohesiveness. However, the vision, stakeholders, and requirements were never explicitly defined and documented, which created misunderstanding within, and beyond, the team.

Journal of Power Sources, Jun 1, 2014

h i g h l i g h t s Generalized 3D computational model of an electric double layer supercapacitor... more h i g h l i g h t s Generalized 3D computational model of an electric double layer supercapacitor. 3D microstructural aspects do not have a significant impact on the performance. Specific capacitance, ionic conductivity, and tortuosity are critical. State-of-the-art numerical methods provide accurate and robust solutions.

Numerical Mathematics-theory Methods and Applications, Nov 1, 2014

ABSTRACT This paper presents a method for solving the linear semi-implicit immersed boundary equa... more ABSTRACT This paper presents a method for solving the linear semi-implicit immersed boundary equations which avoids the severe time step restriction presented by explicit-time methods. The Lagrangian variables are eliminated via a Schur complement to form a purely Eulerian saddle point system, which is preconditioned by a projection operator and then solved by a Krylov subspace method. From the viewpoint of projection methods, we derive an ideal preconditioner for the saddle point problem and compare the efficiency of a number of simpler preconditioners that approximate this perfect one. For low Reynolds number and high stiffness, one particular projection preconditioner yields an efficiency improvement of the explicit IB method by a factor around thirty. Substantial speed-ups over explicit-time method are achieved for Reynolds number below 100. This speedup increases as the Eulerian grid size and/or the Reynolds number are further reduced.

Advances in Computational Mathematics, Oct 12, 2014

The immersed boundary (IB) method is an approach to fluidstructure interaction that uses Lagrangi... more The immersed boundary (IB) method is an approach to fluidstructure interaction that uses Lagrangian variables to describe the deformations and resulting forces of the structure and Eulerian variables to describe the motion and forces of the fluid. Explicit time stepping schemes for the IB method require solvers only for Eulerian equations, for which fast Cartesian grid solution methods are available. Such methods are relatively straightforward to develop and are widely used in practice but often require very small time steps to maintain stability. Implicit-time IB methods permit the stable use of large time steps, but efficient implementations of such methods require significantly more complex solvers that effectively treat both Lagrangian and Eulerian variables simultaneously. Several different approaches to solving the coupled Lagrangian-Eulerian equations have been proposed, but a complete understanding of this problem is still emerging. This paper presents a geometric multigrid method for an implicit-time discretization of the IB equations. This multigrid scheme uses a generalization of box relaxation that is shown to

Journal of Computational Physics, Oct 1, 2008

An implicit structured-adaptive-mesh-refinement (SAMR) solver for 2D reduced magnetohydrodynamics... more An implicit structured-adaptive-mesh-refinement (SAMR) solver for 2D reduced magnetohydrodynamics (MHD) is described. The time-implicit discretization is able to step over fast normal modes, while the spatial adaptivity resolves thin, dynamically evolving features. A Jacobian-free Newton-Krylov method is used for the nonlinear solver engine. For preconditioning, we have extended the optimal "physics-based" approach developed in [11] (which employed multigrid solver technology in the preconditioner for scalability) to SAMR grids using the well-known Fast Adaptive Composite grid (FAC) method [35]. A grid convergence study demonstrates that the solver performance is independent of the number of grid levels and only depends of the finest resolution considered, and that it scales well with grid refinement. The study of error generation and propagation in our SAMR implementation demonstrates that high-order (cubic) interpolation during regridding, combined with a robustly damping second-order temporal scheme such as BDF2, is required to minimize impact of grid errors at coarse fine interfaces on the overall error of the computation for this MHD application. We also demonstrate that our implementation features the desired property that the overall numerical error level is dependent only on the finest resolution level considered, and not on the base-grid resolution or on the number of refinement levels present during the simulation. We demonstrate the effectiveness of the tool on several challenging problems.

Journal of Computational and Applied Mathematics, Mar 1, 2012

Springer eBooks, Jul 29, 2007

Computational study of the macroscopic stability of plasmas is a challenging multi-scale problem.... more Computational study of the macroscopic stability of plasmas is a challenging multi-scale problem. Implicit time integration can be used to relieve stability constraints due to fast Alfvén waves, and adaptive mesh refinement (AMR) can be used to resolve highly localized solution features. The strong nonlinearities and numerical stiffness of magnetohydrodynamics (MHD) models present further challenges that must be solved to make implicit AMR practical. We present initial results on the application of implicit AMR to a reduced resistive MHD model.

SIAM Journal on Scientific Computing, 2003

Bulletin of the American Physical Society, Oct 24, 2005

Submitted for the DPP05 Meeting of The American Physical Society Fully implicit adaptive mesh ref... more Submitted for the DPP05 Meeting of The American Physical Society Fully implicit adaptive mesh refinement MHD algorithm BOBBY PHILIP, MICHAEL PERNICE, LUIS CHACON, LANL-In the macroscopic simulation of plasmas, the numerical modeler is faced with the challenge of dealing with multiple time and length scales. The former results in stiffness due to the presence of very fast waves. The latter requires one to resolve the localized features that the system develops. Traditional approaches based on explicit time integration techniques and fixed meshes are not suitable for this challenge, as such approaches prevent the modeler from using realistic plasma parameters to keep the computation feasible. We propose here a novel approach, based on implicit methods and structured adaptive mesh refinement (SAMR). Our emphasis is on both accuracy and scalability with the number of degrees of freedom. To our knowledge, a scalable, fully implicit AMR algorithm has not been accomplished before for MHD. As a proof-of-principle, we focus on the reduced resistive MHD model as a basic MHD model paradigm, which is truly multiscale. The approach taken here is to adapt mature physics-based technology 1 to AMR grids, and employ AMR-aware multilevel techniques (such as fast adaptive composite-FAC-algorithms) for scalability. We will demonstrate that the concept is indeed feasible, featuring optimal scalability under grid refinement. Results of fully-implicit, dynamically-adaptive AMR simulations will be presented on a variety of problems.

Bulletin of the American Physical Society, Nov 19, 2008

Lecture Notes in Computer Science, 1999

Journal of Computational Physics, Apr 1, 2014

The time dependent non-equilibrium radiation diffusion equations are important for solving the tr... more The time dependent non-equilibrium radiation diffusion equations are important for solving the transport of energy through radiation in optically thick regimes and find applications in several fields including astrophysics and inertial confinement fusion. The associated initial boundary value problems that are encountered often exhibit a wide range of scales in space and time and are extremely challenging to solve. To efficiently and accurately simulate these systems we describe our research on combining techniques that will also find use more broadly for long term time integration of nonlinear multiphysics systems: implicit time integration for efficient long term time integration of stiff multiphysics systems, local control theory based step size control to minimize the required global number of time steps while controlling accuracy, dynamic 3D adaptive mesh refinement (AMR) to minimize memory and computational costs, Jacobian Free Newton-Krylov methods on AMR grids for efficient nonlinear solution, and optimal multilevel preconditioner components that provide level independent solver convergence.

Journal of Computational Physics, Apr 1, 2015

This paper describes an efficient and nonlinearly consistent parallel solution methodology for so... more This paper describes an efficient and nonlinearly consistent parallel solution methodology for solving coupled nonlinear thermal transport problems that occur in nuclear reactor applications over hundreds of individual 3D physical subdomains. Efficiency is obtained by leveraging knowledge of the physical domains, the physics on individual domains, and the couplings between them for preconditioning within a Jacobian Free Newton Krylov method. Details of the computational infrastructure that enabled this work, namely the open source Advanced Multi-Physics (AMP) package developed by the authors is described. Details of verification and validation experiments, and parallel performance analysis in weak and strong scaling studies demonstrating the achieved efficiency of the algorithm are presented. Furthermore, numerical experiments demonstrate that the preconditioner developed is independent of the number of fuel subdomains in a fuel rod, which is particularly important when simulating different types of fuel rods.

ASPC, 2013

We describe methods for accurate and efficient long term time integra- tion of non-equilibrium ra... more We describe methods for accurate and efficient long term time integra- tion of non-equilibrium radiation diffusion systems: implicit time integration for effi- cient long term time integration of stiff multiphysics systems, local control theory based step size control to minimize the required global number of time steps while control- ling accuracy, dynamic 3D adaptive mesh refinement (AMR) to minimize memory and computational costs, Jacobian Free Newton-Krylov methods on AMR grids for efficient nonlinear solution, and optimal multilevel preconditioner components that provide level independent solver convergence.

Bulletin of the American Physical Society, Oct 31, 2006

Submitted for the DPP06 Meeting of The American Physical Society Fully implicit adaptive mesh ref... more Submitted for the DPP06 Meeting of The American Physical Society Fully implicit adaptive mesh refinement algorithm for reduced MHD BOBBY PHILIP, MICHAEL PERNICE, LUIS CHACON, LANL-In the macroscopic simulation of plasmas, the numerical modeler is faced with the challenge of dealing with multiple time and length scales. Traditional approaches based on explicit time integration techniques and fixed meshes are not suitable for this challenge, as such approaches prevent the modeler from using realistic plasma parameters to keep the computation feasible. We propose here a novel approach, based on implicit methods and structured adaptive mesh refinement (SAMR). Our emphasis is on both accuracy and scalability with the number of degrees of freedom. As a proofof-principle, we focus on the reduced resistive MHD model as a basic MHD model paradigm, which is truly multiscale. The approach taken here is to adapt mature physics-based technology 1 to AMR grids, and employ AMR-aware multilevel techniques (such as fast adaptive composite grid-FAC-algorithms) for scalability. We demonstrate that the concept is indeed feasible, featuring near-optimal scalability under grid refinement. 2 Results of fully-implicit, dynamically-adaptive AMR simulations in challenging dissipation regimes will be presented on a variety of problems that benefit from this capability, including tearing modes, the island coalescence instability, and the tilt mode instability.

Nuclear Science and Engineering, Jul 1, 2014

Abstract The Integrated Fuel Assessment IFA-432 experiments from the International Fuel Performan... more Abstract The Integrated Fuel Assessment IFA-432 experiments from the International Fuel Performance Experiments database were designed to study the effects of gap size, fuel density, and fuel densification on fuel centerline temperature in light water reactor fuel. An evaluation of nuclear fuel pin heat transfer in the FRAPCON-3.4 and Exnihilo codes for uranium dioxide (UO2) fuel systems was performed, with a focus on the densification stage (2.2 GWd/tonne UO2). In addition, sensitivity studies were performed to evaluate the effect of the radial power shape and approximations to the geometry to account for the thermocouple hole. The analysis demonstrated excellent agreement for rods 1, 2, 3, and 5 (varying gap thicknesses and density with traditional fuel), demonstrating the accuracy of the codes and their underlying material models for traditional fuel. For rod 6, which contained unstable fuel that densified an order of magnitude more than traditional, stable fuel, the magnitude of densification was overpredicted, and the temperatures were outside the experimental uncertainty. The radial power shape within the fuel was shown to have a significant impact on the predicted centerline temperatures, whereas the effect of modeling the fuel at the thermocouple location as either annular or solid was relatively negligible. This has provided an initial benchmarking of the pin heat transfer capability of Exnihilo for UO2 fuel with respect to a well-validated nuclear fuel performance code.