Browsing by Subject "Computational Physics"

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    Microscale Modeling of Porous Thermal Protection System Materials
    (2015-05) Stern, Eric
    Ablative thermal protection system (TPS) materials play a vital role in the design of entry vehicles. Most simulation tools for ablative TPS in use today take a macroscopic approach to modeling, which involves heavy empiricism. Recent work has suggested improving the fidelity of the simulations by taking a multi-scale approach to the physics of ablation. In this work, a new approach for modeling ablative TPS at the microscale is proposed, and its feasibility and utility is assessed. This approach uses the Direct Simulation Monte Carlo (DSMC) method to simulate the gas flow through the microstructure, as well as the gas-surface interaction. Application of the DSMC method to this problem allows the gas phase dynamics -- which are often rarefied -- to be modeled to a high degree of fidelity. Furthermore this method allows for sophisticated gas-surface interaction models to be implemented. In order to test this approach for realistic materials, a method for generating artificial microstructures which emulate those found in spacecraft TPS is developed. Additionally, a novel approach for allowing the surface to move under the influence of chemical reactions at the surface is developed. This approach is shown to be efficient and robust for performing coupled simulation of the oxidation of carbon fibers. The microscale modeling approach is first applied to simulating the steady flow of gas through the porous medium. Predictions of Darcy permeability for an idealized microstructure agree with empirical correlations from the literature, as well as with predictions from computational fluid dynamics (CFD) when the continuum assumption is valid. Expected departures are observed for conditions at which the continuum assumption no longer holds. Comparisons of simulations using a fabricated microstructure to experimental data for a real spacecraft TPS material show good agreement when similar microstructural parameters are used to build the geometry. The approach is then applied to investigating the ablation of porous materials through oxidation. A simple gas surface interaction model is described, and an approach for coupling the surface reconstruction algorithm to the DSMC method is outlined. Simulations of single carbon fibers at representative conditions suggest this approach to be feasible for simulating the ablation of porous TPS materials at scale. Additionally, the effect of various simulation parameters on in-depth morphology is investigated for random fibrous microstructures.
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    Safety in Numbers: Creating and Contesting the Los Alamos Approach to Supercomputing, 1943 to 1980
    (2019-08) Lewis, Nicholas
    Since its origins during World War II as the primary R&D site of the Manhattan Project, the Laboratory at Los Alamos has pushed the limits of computational technologies and systems. In response to its heavy computing demands, and the infancy of the computing industry during the early Cold War, the Theoretical Physics (T) Division at Los Alamos, which provided computing resources for all divisions at the Lab, played an active role in researching and developing the computing technologies that its scientists required. T Division's hardware and software R&D effort formed the basis of the distinctive Los Alamos approach to computing, which placed computing on an equal footing with other scientific research, and encouraged the formation of local, world-class expertise in computing technologies essential to the Lab's mission. As particular computing technologies matured, and vendors were able to provide commercial products adequate for the Lab's needs, T Division shifted its R&D focus to other, less mature lines of computing research. This approach provided Los Alamos computer users with technologies that were otherwise unavailable or unsuitable from outside sources, and ensured that T Division could rapidly adapt the Lab's computing operation to unpredictable changes in the technical and strategic demands of the Cold War. The Los Alamos approach to computing remained largely uncontested until its transfer in 1968 with T-Division personnel to the Lab's new Computing (C) Division, where the inherited approach became a point of contention for a growing number of weapons-program computer users who opposed the Lab's basic computer-science research and C Division's operational independence. Radical changes to the Lab's administrative structure over the 1970s upended the power dynamics between the supporters of the traditional Los Alamos approach to computing and those who sought to bring the Lab's computing resources under the direct control of the weapons program. A focal point for critics of C Division's R&D policies, the local development of a cutting-edge operating system for the iconic Cray-1 supercomputer became the final battle ground over the decades-old Los Alamos approach to computing.