Aerothermodynamics and hypersonic flows involve complex multi-disciplinary physics, including finite-rate gas-phase kinetics, finite-rate internal energy relaxation, gas-surface interactions with finite-rate oxidation and sublimation, transition to turbulence, large-scale unsteadiness, shock-boundary layer interactions, fluid-structure interactions, and thermal protection system ablation and thermal response. Many of the flows have a large range of length and time scales, requiring large computational grids, implicit time integration, and large solution run times. The University of Minnesota NASA US3D code was designed for the simulation of these complex, highly-coupled flows. It has many of the features of the well-established DPLR code, but uses unstructured grids and has many advanced numerical capabilities and physical models for multi-physics problems. The main capabilities of the code are described, the physical modeling approaches are discussed, the different types of numerical flux functions and time integration approaches are outlined, and the parallelization strategy is overviewed. Comparisons between US3D and the NASA DPLR code are presented, and several advanced simulations are presented to illustrate some of novel features of the code.

/pdf/development-of-the-us3d-code-for-advanced-compressible-and-4s9elxcocl.pdf

Development of the US3D Code for Advanced Compressible and Reacting Flow Simulations

This paper describes the development and implementation of a partitioned-based, multiphysics, multi-fidelity simulation framework for flexible hypersonic vehicles. This simulation framework will enable the study of the dominant physics needed for the generation of control-oriented models and will provide a reference model for flight control evaluation. A partitioned-based solution is employed to approach the problem of modeling a flexible hypersonic vehicle by dividing the vehicle into discrete regions within which unique combinations of physical processes are relevant. The dominant physics of each region are then modeled locally and information is exchanged across region interfaces at predetermined time intervals as the vehicle simulation is marched forward in time. The highly coupled physical processes within each region are modeled using an array of variable fidelity reduced order models. The partition solution implementation is compared to a monolithic solution through trim and time simulation of a sample hypersonic vehicle geometry. Trim results of rigid and flexible vehicle models show good matching between the partitioned and monolithic solutions for Mach 6, 26 km altitude, steady level flight. Time simulations at the same flight conditions also show good qualitative matching between the solutions, but a mismatch in effective mass distribution allows the monolithic vehicle model to have a slightly faster control response to a commanded elevon deflection. The partitioned solution code architecture is then extended to the characterization of flutter for a hypersonic lifting surface, showing a significant reduction of flutter Mach number as a function of flight time.

/pdf/aerothermoelastic-simulation-of-air-breathing-hypersonic-4lixcz6lh0.pdf

Aerothermoelastic Simulation of Air-Breathing Hypersonic Vehicles

A set of reduced-order models are considered to determine the variation of the material thermal capacity and thermal conductivity with respect to temperature for a representative hypersonic vehicle...

Nonlinear Thermal Reduced-Order Modeling for Hypersonic Vehicles

This study focuses on the development of a loosely coupled partitioned multi-physics time marching procedure for fluid-thermal-structural analysis using time-accurate solvers. The scheme is specifically formulated to maintain global second order temporal accuracy using implicit solvers for each discipline, and requires no subiterations between solvers. This is achieved by using second-order time integrators for each solver combined with second-order extrapolations of the pressure and heat flux to predict the thermo-structural loadings at the proper time step. Furthermore, the scheme is designed to exploit disparities in time scales between the solvers through the use of fluid and structural subcycling, in which multiple fluid steps are taken between each structural step, and multiple structural steps between each thermal step. Here, second order global accuracy is maintained using second order interpolations of the displacement, velocity, and temperature over the structural time step, and temperature over the thermal time step. The accuracy of the scheme is evaluated on a simple panel in high supersonic flow, and is found to yield second-order accurate solutions both with and without subcycling. The impact of neglecting the extrapolation and interpolation mechanisms on accuracy is also investigated. In general disabling any of these mechanisms degrades the global accuracy to first order. However the heat flux extrapolation is shown to have a weaker impact than the others, retaining near-second order solutions for displacement, pressure, and heat flux.

Loosely Coupled Time-Marching of Fluid-Thermal-Structural Interactions with Time-Accurate CFD

This study focuses on the development of time-marching procedures for efficient and accurate fluid-thermal-structural analysis with time-accurate computational fluid dynamics. The developed procedu...

Efficient Fluid-Thermal-Structural Time Marching with Computational Fluid Dynamics

The Preliminary Aerothermal Structural Simulation (PASS) is a tool suite for MultiDisciplinary Analysis and Optimization of high speed vehicles at the conceptual or preliminary design level. The PASS process includes geometry definition, preliminary packaging, aerodynamic estimation, trajectory modeling, structural sizing, and thermal analysis. PASS currently includes Missile Datcom (MDC) to provide aerodynamic data, Mass Estimator for Monocoque Aero Structures (MEMAS) to estimate the structural mass, Aerothermal Targets Analysis Program (ATAP) to provide thermal analysis, and MATLAB to perform the integration. MDC and ATAP are standard AFRL tools. MEMAS is a new in-house code developed in MATLAB for sizing high speed attack missile structures, including articulated fins. The Packaging algorithm is a new in-house code developed for placement of representative sub-systems and calculation of mass properties/moments of inertia.

Preliminary Aero-thermal Structural Simulation

Fluid-thermal-structural interactions can create significant uncertainty in the performance of high speed vehicles. A new toolset presented in this paper enables assessment of flexible effects at the preliminary design level which complements and augments the Preliminary Aerothermal Structural Simulation (PASS) tool suite for Multi-Disciplinary Analysis and Optimization of high-speed vehicles. The toolset allows calculation of structural deflections due to loads using Abaqus finite element software, mode shapes for use in modal superposition analysis and generation of aerodynamic databases which include flexible effects. Missile DATCOM (MDC) provides aerodynamic data for rigid and axisymmetric vehicles flexible in bending and Cart3D provides aerodynamic data for general vehicles in all flexible modes. PASS provides a structural design and temperature maps from the Mass Estimator for Monocoque Aero Structures (MEMAS) and Aerothermal Targets Analysis Program (ATAP) modules.

Fluid-Thermal-Structural Interaction Effects in Preliminary Design of High Speed Vehicles

Dynamic Fluid-Thermal-Structural Interaction Effects in Preliminary Design of High Speed Vehicles

This thesis describes potential configurations and responses of panels for experimental tests investigating fluid-thermal-structural interactions. The United States Department of Defense (DoD) and the National Aeronautics and Space Administration (NASA) are both interested in hypersonic vehicles due to their incredible capabilities for long range strike, surveillance and responsive access to space. These vehicles experience combined extreme aerodynamic heating and pressure loads. In addition, the structural loads are greatly affected by temperature-dependent material properties. Prediction of the response of these structures requires accurate, coupled fluid-thermal-structural analysis over long time records. High fidelity modeling over these long trajectories is prohibitively expensive computationally. In addition, validation of approximate models is limited due to the lack of experimental data capturing coupled fluid-thermal-structural interactions. The aim of this thesis is to explore the potential for experimental studies of fluid-thermal-structural interactions for validation and basic discovery purposes. Panel flutter is examined in this study, due to a simpler configuration for modeling. In order to accomplish the goal of this work, potential experimental faciliii ties are explored to determine the conditions under which panels could be tested. An examination of common nondimensional parameters related to panel flutter is conducted to determine the utility of these parameters. A study of different panel configurations is conducted to determine the effect on panel response of varying flow conditions, boundary conditions and panel materials. Finally, the potential panel configurations that could be tested in two different facilities are discussed. It is found that there is a range of potential panel configurations that could be examined experimentally. For smaller facilities, the difference between the thinnest and thickest panels which exhibit a buckled response followed by the onset of instability is small and could lead to difficulties when manufacturing test articles. For larger facilities, this range is larger leading enabling more relaxed tolerances. In addition, it is found that while the structural boundary conditions have a significant effect on panel response, changing the thermal support conditions has a less significant effect on overall panel response. Varying thickness and panel material can result in larger thermal moments in the panel which can affect the utility of existing nondimensional parameters. It is also found that existing nondimensional parameters can provide some use in the sizing of panels for experimental tests. The overall conclusion is that limited experimental testing of fluid-thermalstructural interactions is feasible in existing facilities.

Zachary Witeof

Papers

Preliminary Aero-thermal Structural Simulation

Fluid-Thermal-Structural Interaction Effects in Preliminary Design of High Speed Vehicles

Dynamic Fluid-Thermal-Structural Interaction Effects in Preliminary Design of High Speed Vehicles

Exploratory Study on the Design of Combined Aero-Thermo-Structural Experiments in High Speed Flows