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Final Report for "Verification and Validation of Radiation Hydrodynamics for Astrophysical Applications"

Description: The motivation for this work is to gain experience in the methodology of verification and validation (V&V) of astrophysical radiation hydrodynamics codes. In the first period of this work, we focused on building the infrastructure to test a single astrophysical application code, Castro, developed in collaboration between Lawrence Livermore National Laboratory (LLNL) and Lawrence Berkeley Laboratory (LBL). We delivered several hydrodynamic test problems, in the form of coded initial conditions and documentation for verification, routines to perform data analysis, and a generalized regression test suite to allow for continued automated testing. Astrophysical simulation codes aim to model phenomena that elude direct experimentation. Our only direct information about these systems comes from what we observe, and may be transient. Simulation can help further our understanding by allowing virtual experimentation of these systems. However, to have confidence in our simulations requires us to have confidence in the tools we use. Verification and Validation is a process by which we work to build confidence that a simulation code is accurately representing reality. V&V is a multistep process, and is never really complete. Once a single test problem is working as desired (i.e. that problem is verified), one wants to ensure that subsequent code changes do not break that test. At the same time, one must also search for new verification problems that test the code in a new way. It can be rather tedious to manually retest each of the problems, so before going too far with V&V, it is desirable to have an automated test suite. Our project aims to provide these basic tools for astrophysical radiation hydrodynamics codes.
Date: March 17, 2010
Creator: Zingale, M & Howell, L H
Partner: UNT Libraries Government Documents Department

A Discrete Ordinates Algorithm for Radiation Transport Using Block-Structured Adaptive Mesh Refinement

Description: The discrete ordinates method is well-suited to implementation with block-structured adaptive mesh refinement (AMR). AMR meshes group points into logically-rectangular patches, and provide the benefits of adaptivity without sacrificing the efficiency and geometric regularity of regular grids. In particular, these meshes preserve the directional ordering of points required for explicit ordinate transport sweeps. A number of algorithmic issues must be addressed to make such a method practical. These include sequencing of ordinates and grids for parallel execution, simultaneous solution of the transport equation on multiple levels of the grid hierarchy, implicit coupling to the fluid energy, and conservation of energy in a time-dependent context where different grid levels advance with different timesteps. The author discusses these and other issues and present example calculations in two and three spatial dimensions.
Date: February 3, 2003
Creator: Howell, L H
Partner: UNT Libraries Government Documents Department

The modeling of a laboratory natural gas-fired furnace with a higher-order projection method for unsteady combustion

Description: A higher-order, embedded boundary projection method for axisymmetric, unsteady, low-Mach number combustion is used to model a natural gas flame from a 300 kW IFRF burner in the Burner Engineering Research Laboratory (BERL) at Sandia National Laboratory under hot wall conditions. The numerical predictions presented are the late simulated-time results of a computation of unsteady flow in the furnace. The predictions are compared both with measurements completed in the BERL as part of the GRI SCALING 400 Project and with results from a steady-state axisymmetric reacting flow code in order to evaluate the combustion model and the numerical method. The results compare favorably with the experimental data.
Date: February 1, 1996
Creator: Pember, R.B.; Colella, P.; Howell, L.H. & Almgren, A.S.
Partner: UNT Libraries Government Documents Department

An adaptive mesh refinement algorithm for the discrete ordinates method

Description: The discrete ordinates form of the radiative transport equation (RTE) is spatially discretized and solved using an adaptive mesh refinement (AMR) algorithm. This technique permits the local grid refinement to minimize spatial discretization error of the RTE. An error estimator is applied to define regions for local grid refinement; overlapping refined grids are recursively placed in these regions; and the RTE is then solved over the entire domain. The procedure continues until the spatial discretization error has been reduced to a sufficient level. The following aspects of the algorithm are discussed: error estimation, grid generation, communication between refined levels, and solution sequencing. This initial formulation employs the step scheme, and is valid for absorbing and isotopically scattering media in two-dimensional enclosures. The utility of the algorithm is tested by comparing the convergence characteristics and accuracy to those of the standard single-grid algorithm for several benchmark cases. The AMR algorithm provides a reduction in memory requirements and maintains the convergence characteristics of the standard single-grid algorithm; however, the cases illustrate that efficiency gains of the AMR algorithm will not be fully realized until three-dimensional geometries are considered.
Date: March 1, 1996
Creator: Jessee, J.P.; Fiveland, W.A.; Howell, L.H.; Colella, P. & Pember, R.B.
Partner: UNT Libraries Government Documents Department

An embedded boundary method for the modeling of unsteady combustion in an industrial gas-fired furnace

Description: A new methodology for the modeling of unsteady, nonpremixed, axisymmetric reacting flow in industrial furnaces is presented. The method is an extension of previous work by the authors to complex geometries, multistep kinetics mechanisms, and realistic properties, especially thermochemical data. The walls of the furnace are represented as an embedded boundary in a uniform, rectangular grid. The grid then consists of uniform rectangular cells except at the furnace wall where irregular (mixed) cells may be present. We use finite volume differencing techniques for the convective, viscous, and radiative heat transport terms in the mixed cells, while a finite element-based technique is used to solve the elliptic equation arising from the low-Mach number formulation. Results from the simulation of an experimental natural gas-fired furnace are shown.
Date: October 18, 1995
Creator: Pember, R.B.; Almgren, A.S.; Crutchfield, W.Y.; Howell, L.H.; Bell, J.B.; Colella, P. et al.
Partner: UNT Libraries Government Documents Department

An adaptive projection method for the modeling of unsteady, low-Mach number combustion

Description: In this paper the authors present an adaptive projection method for modeling unsteady, low-Mach reacting flow in an unconfined region. The equations they solve are based on a model for low-Mach number combustion that consists of the evolution equations for density, species concentrations, enthalpy, and momentum coupled with a constraint on the divergence of the flow. The algorithm is based on a projection methodology in which they first advance the evolution equations and then solve an elliptic equation to enforce the divergence constraint. The adaptive mesh refinement (AMR) scheme uses a time-varying, hierarchical grid structure composed of uniform rectangular grids of varying resolution. The integration scheme on the grid hierarchy is a recursive procedure in which a coarse grid is advanced, fine grids are advanced multiple steps to reach the same time as the coarse grid, and the coarse and the fine grids are synchronized. The method is valid for multiple grids on each level and multiple levels of refinement. The method is currently implemented for laminar, axisymmetric flames with a reduced kinetics mechanism and a Lewis number of unity. Two methane-air flames, one steady and the other flickering, are presented as numerical examples.
Date: October 1, 1997
Creator: Pember, R.B.; Howell, L.H.; Bell, J.B.; Colella, P.; Crutchfield, W.Y.; Fiveland, W.A. et al.
Partner: UNT Libraries Government Documents Department