Brent A. Miller

Robust and Efficient Treatment of Temperature Feedback in Fluid–Thermal–Structural Analysis

One of the primary challenges in the development of high-speed systems is accurate and efficient prediction of the aerodynamic heating, particularly for lightweight systems where there is a potential for strong coupling between the aerothermodynamic loads and the structural response. A novel approach is developed that corrects heat flux predictions from a pointwise dependency on surface temperature in order to account for surface temperature gradients. This enables efficient construction of a computational fluid dynamics surrogate for aerodynamic heating without a priori assumptions on the surface temperature profile while also accurately maintaining the critical feedback behavior of the surface temperature profile for a coupled fluid–thermal–structural analysis. The method is compared, in terms of accuracy and efficiency, with a hierarchy of approaches for aerodynamic heating prediction. Comparison cases include simple flat-plate heating, as well as shock impingements in two-dimensional and three-dimensi...

Modeling and Analysis of Shock Impingements on Thermo-Mechanically Compliant Surface Panels

Fluid-Thermal-Structural interactions play an important role in the development of high speed vehicles, impacting various sub-disciplines (i.e., aerodynamic, structural, material, propulsion, and control) at the micro, component and/or vehicle scales. This study focuses on the development of a partitioned fluid-thermal-structural procedure aimed at performing a long time record thermo-structural response prediction of surface panels subject to shock impingements. Specific modeling aspects essential to this are reduction of the computational aerothermodynamics to a tractable model, and partitioned timemarching of the fluid-thermal-structural problem. Additional factors considered are: 1) the movement of the shock impingement due to forced motion of a shock generator, 2) panel backpressure, 3) a 140dB random prescribed pressure load to account for pressure fluctuations associated with turbulent boundary layers, and 4) coupled vs. uncoupled fluid-thermal-structural analysis. Results indicate that quasistatic CFD analysis provides a promising means for generating an aerothermodynamic surrogate model. Differences between quasi-static and unsteady models were under 6% for both panel temperature rise and pressure loads for a forced motion analysis. Several studies using the fluid-thermal-structural model are performed, focusing on the differences between the coupled and uncoupled analyses, as well as the role of backpressure on the panel response. The effect of the backpressure to the direction of panel buckling is investigated, and the backpressure required to buckle the panel into the flow is predicted to be 10% of free stream higher for the uncoupled model than the coupled model. However, generally differences were minor between the coupled and uncoupled analysis. The inclusion of a 140 dB prescribed pressure load, meant to mimic the effect of turbulent boundary layer loadings, results in negligible temperature differences. However, both the shock motion and this load introduce large amplitude oscillations at the start of the response, followed by relatively small oscillations once the buckling amplitude of the panel becomes significant.

Time-Marching Considerations for Response Prediction of Structures in Hypersonic Flows

This study examines time-marching procedures for fluid–thermal–structural analysis using time-accurate thermal and structural solvers combined with quasi-steady aerothermodynamic models. Four coupling schemes are considered, including a conventional scheme with explicit time integration, a basic loose coupling scheme with implicit time integration, a predictor-based implicit scheme, and a subiteration-based implicit scheme with strong coupling. The first three schemes also incorporate structural subcycling, in which the structural time step is smaller than the thermal time step. Convergence studies reveal that the conventional explicit and basic implicit schemes are first-order accurate in time, whereas the predictor implicit and strong implicit schemes retain second-order accuracy. Structural subcycling is found to preserve the order of accuracy for the predictor implicit scheme. Additional analysis of the long-term behavior of the solution of two configurations indicates that the conventional explicit a...

Characterization of structural response to hypersonic boundary-layer transition

The inherent relationship between boundary-layer stability, aerodynamic heating, and surface conditions makes the potential for interaction between the structural response and boundary-layer transition an important and challenging area of study in high-speed flows. This paper phenomenologically explores this interaction using a fundamental two-dimensional aerothermoelastic model under the assumption of an aluminum panel with simple supports. Specifically, an existing model is extended to examine the impact of transition onset location, transition length, and transitional overshoot in heat flux and fluctuating pressure on the structural response of surface panels. Transitional flow conditions are found to yield significantly increased thermal gradients, and they can result in higher maximum panel temperatures compared to turbulent flow. Results indicate that overshoot in heat flux and fluctuating pressure reduces the flutter onset time and increases the strain energy accumulated in the panel. Furthermore, ...

Loosely Coupled Time-Marching of Fluid-Thermal-Structural Interactions

This study focuses on the development of a loosely coupled partitioned multi-physics time marching procedure for fluid-thermal-structural analysis using time-accurate high-fidelity models. The scheme is specifically formulated to maintain global second order temporal accuracy using implicit second order solvers for each discipline, and requires no subiterations between solvers. Furthermore, the scheme is designed to exploit disparities in time scales between the submodels through the use of multicycling, i.e. multiple fluid time steps between each structural step, and multiple structural steps between each thermal step. Preliminary verification of the scheme is carried out by coupling a CFD solver with thermal and structural solvers. A fluid-structural analysis with and without multicycling demonstrates the second order accuracy of structural displacements/velocities, and integrated pressure loads. A fluidthermal analysis is also performed, demonstrating second order accuracy of the temperature rise and integrated heat load in the absence of multicycling. However, currently the fluid-thermal time-marching is less than second order accurate with multicycling.

Computational Modeling for Conjugate Heat Transfer of Shock-Surface Interactions on Compliant Skin Panels

Aerothermal interactions play an important role in the analysis and optimization of high speed vehicles, impacting the design of the aerodynamic, structural, control, and propulsion systems at both the component and multi-disciplinary levels. This study aims to develop a CFD-FEM based partitioned aerothermal solver, for investigating the thermal response of surface panels subject to shock turbulent boundary layer interactions (STBLIs). The focus of the study is assessment of different time marching approaches for the partitioned conjugate heat transfer problem, and also the degree of coupling in STBLIs. An additional consideration is the potential for structural compliance in the system, which leads to a non-stationary shock impingement location. Preliminary results for the initial thermal response of a vibrating skin panel indicate that a one-way coupled and quasi-steady approximation yield similar panel temperatures to a fully unsteady, two-way coupled analysis. Furthermore, a static approximation, where the flow is predicted a priori without consideration of the surface motion, significantly under predicts the peak temperature rise and affected length of the panel.

Surrogate Based Reduced-Order Aerothermodynamic Modeling for Structural Response Prediction at High Mach Numbers

One of the primary challenges for hypersonic aerothermoelastic analysis of air-breathing hypersonic vehicles is accurate and efficient computation of the unsteady aerothermodynamic loads, where currently approaches are limited to either simple engineering level approximations or expensive Computational Fluid Dynamics. This study aims to fill the modeling gap between these two extremes by constructing computationally efficient surrogates for CFD predictions of the unsteady aerodynamic pressure loads. A novel aspect of the work is the use of steady-state CFD data for the surrogate, corrected for unsteady effects using piston-theory aerodynamics. Validation of the steady-state surrogate is achieved by comparing to 500 steady-state full order CFD cases. Results indicate that the surrogate generally has less than 1% RMS error. Comparison of the unsteady generalized aerodynamic forces computed using the corrected surrogate, third-order piston theory, unsteady Euler, and unsteady NavierStokes aerodynamics demonstrates that the developed surrogate method produces predictions superior to unsteady Euler solutions and piston theory, at a computational cost similar to that of piston theory. The surrogate is also implemented into a dynamic aerothermoelastic panel simultation, and compared with previous results using analytical approaches. The comparison reveals differences of over 100% in the onset time to flutter, and significantly altered post-flutter responses. The surrogate framework is found to enable a reasonably accurate, robust, and efficient method for incorporating CFD loads prediction, which would otherwise be impractical, into a long time-record aerothermoelastic analysis of structures for a complete hypersonic trajectory.

Efficient Time-Marching of Fluid-Thermal-Structural Interactions

This study focuses on the development of a subiteration free, loosely coupled partitioned time marching procedure for fluid-thermal-structural analysis using time-accurate thermal and structural solvers combined with surrogate fluid models. The scheme is specifically formulated to maintain global second-order temporal accuracy using implicit solvers. This is achieved by using second-order solvers for the thermal and structural domains combined with second-order accurate extrapolations to estimate the quasi-static heat flux and quasi-steady pressure. Furthermore, the scheme is designed to exploit disparities in time scales between the submodels through the use of structural subcycling, in which multiple structural time steps are taken between each thermal step. Here, second order global accuracy is maintained using a second-order accurate interpolation of temperature. The scheme is evaluated on a simple panel in hypersonic flow, and is found to yield second order accuracy both with and without subcycling of the structural solver. A comparable conventional scheme is found to yield only first order accuracy, to produce spurious structural oscillations for certain time step sizes, and to require significantly smaller time steps for comparable time accuracy. In regards to the latter, the time marching scheme developed in this work is found to reduce the computational expense over the conventional scheme by more than 80 percent.

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

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.

Effects of Strain Hardening on Fluid-Thermal-Structural Interactions

This paper describes an on-going effort towards predicting the role of material plasticity in the structural response of compliant panels in high speed flow. This work is an extension of a previous study, where a fluid-thermal-structural-material interactions framework is used to investigate the response of elastic and elastic-plastic panels subjected to multiple loading cycles. The plastic material response is predicted concurrently using a nonlinear strain hardening law that accounts for cyclic loading. It was found that residual deformation on the order of one panel thickness can produce characteristically different responses for elastic and plastic panels. After several loading cycles, the onset of flutter for the permanently deformed panel increased beyond the time responses considered.