Toward a Predictive Exascale-Class Hypersonic Simulation Capability for Damage Lethality Assessment and Weapons Environment Characterization

Project Overview

This project developed new numerical techniques to significantly improve the fidelity of computational fluid dynamics (CFD) simulations of three-dimensional, unsteady turbulent boundary layer (TBL) flows generated by supersonic and hypersonic vehicles. The work focused on extending the Nonlinear Large-Eddy Simulation (NLES) methodology, largely developed at Lawrence Livermore National Laboratory (LLNL), which has previously demonstrated order-of-magnitude improvements in accuracy over other LES methods for incompressible and variable density flows. We developed these new methods within the framework of LLNL's MARGOT hypersonic LES flow solver, currently used for damage/lethality/environments assessments for NNSA, MDA, and DOD stakeholders.

The project first extended the NLES physical models to fully compressible, supersonic and hypersonic flows. As with previously-treated NLES flow regimes, these new physics models were based on the spatial organization of the smaller scales in compressible flows, using multifractal formalism, a branch of chaos theory. We developed new multifractal models for the density, internal energy, equation of state, pressure and dilatation fields, which for the first time ever used the classic Helmholtz decomposition, which separates vortical and compressible motions in a flow. We also extended the NLES-based backscatter-limiter strategy, used to control simulation energy levels, to these high-speed flows, by developing new limiters treating density advection, internal-energy advection, pressure and pressure-dilatation. We then evaluated the accuracy of these new NLES constructs in static a priori tests, against pre-existing fully-resolved direct-numerical simulation (DNS) databases of supersonic and hypersonic flows. These tests indicated the same order of magnitude accuracy improvements previously seen in NLES methods treating other, slower flow regimes. Finally, these methods were then implemented in a new branch of the MARGOT code, and used to run a series of a posteriori simulations. Tests were first run of the well-known compressible Taylor-Green vortex (TGV) case, which contains unsteady eddy shocklet / local turbulence interactions, similar to those typically seen in supersonic and hypersonic TBL flows. These simulations ran stably and accurately up to Mach 7, the highest Mach number tested. The project concluded with successful simulations of Mach 2 and 5 temporally-evolving TBL flows. Future work will include porting the new methods to the main branch of MARGOT and beginning the production use of these methods in high-speed TBL simulations where unsteady small-scale turbulent features drive significantly-higher drag forces and heat transfer, which are of particular concern for system-level performance assessments in supersonic and hypersonic flow regimes.

Mission Impact

This project has extended the state-of-the-art in high-fidelity, high-resolution CFD modeling and simulation methods for supersonic and hypersonic turbulent boundary layers. Such capabilities are urgently needed to analyze the efficacy of kinetic and non-kinetic damage modes for system-lethality assessment against emerging threats identified by the Department of Defense (DOD) and the Missile Defense Agency (MDA), as well as provide flight vulnerability /survivability /environments analysis to NNSA stakeholders. Thus, these new capabilities are directly applicable to NNSA missions to ensure stockpile effectiveness as well as to enhance nuclear threat deterrence. The project has developed a new paradigm for state-of-the-art, massively-parallel, multiphysics hypersonics CFD solvers, thereby directly enhancing Laboratory Core Competencies in high performance computing, modeling and simulation. The work is also directly applicable to NIF/Photon Sciences Core Competencies in the field of laser/matter interactions and laser ablation.