Data-parallel Line Relaxation Method Research Articles

Validation of Ablation and Thermal Response Model for Three-Dimensional Multifunctional Ablator

A semi-empirical physics-based ablation and thermal response model was developed for the 3-Dimensional Multifunctional Ablative Thermal Protection System (3D-MAT) material. Model validation was achieved through comparison between computation and available data obtained in the arc-jet test series, which were conducted at NASA Ames Research Center from 2014 to 2020. The charring ablator simulations for arc-jet test models presented in this work were computed by the Two-Dimensional Implicit Thermal Response and Ablation (TITAN) code, and the Data-Parallel Line Relaxation Method (DPLR) code was used for arc-jet flow simulations to estimate the arc-jet stream total enthalpy and define the aerothermal boundary conditions over the test model surface for TITAN simulations. Because of low surface catalytic efficiency of 3D-MAT char, the exact surface heating could not be determined. Thus, three different types of boundary conditions, including 1) fully catalytic surface heating, 2) noncatalytic surface heating, and 3) surface temperature and recession, were used in the TITAN simulation for model validation. The predicted surface and in-depth temperature history for arc-jet test models were compared with pyrometer and thermocouple data, and the predicted test model surface recession and char depth were compared against the posttest measurements.

Evaluation of Sublimation Kinetics for Phenolic Impregnated Carbon Ablator

Recession analyses of stagnation tests conducted in NASA Ames Research Center arcjet facilities with surface heat fluxes high enough for carbon char to sublimate are performed and compared with data. The axisymmetric recession computations are based on both the finite-rate gas–surface interaction model and the standard chemical equilibrium surface ablation model. The finite-rate gas–surface interaction model is studied using three different sets of sublimation coefficients, which are obtained through direct measurements. The standard chemical equilibrium ablation model is established through the tables. The ablating material used in these arcjet tests is phenolic impregnated carbon ablator. The prediction of axisymmetric surface recession is achieved by implicitly coupling a material thermal response code and a flow code. The material thermal response code used in this study is the Two-Dimensional Implicit Thermal-Response and Ablation program, which predicts charring material thermal response and shape change. The flow code solves the reacting Navier–Stokes equations using the data parallel line relaxation method. The recession predictions for three selected arcjet conditions with various base radii are presented. The effect of sticking coefficient for carbon sublimation on recession prediction is investigated, and the prediction discrepancy between finite-rate and chemical equilibrium models is examined and discussed in detail.

Evaluation of Finite-Rate Gas/Surface Interaction Models for Carbon-Based Ablator

Two sets of finite-rate gas/surface interaction model between air and the carbon surface are studied. The first set is an engineering model with one-way chemical reactions, and the second set is a more detailed model with two-way chemical reactions. Each of these two proposed models intends to cover the carbon surface ablation conditions including the low-temperature rate-controlled oxidation, the midtemperature diffusion-controlled oxidation, and the high-temperature sublimation. The prediction of carbon surface recession is achieved by coupling a material thermal response code and a Navier–Stokes flow code. The material thermal response code used in this study is the Two-Dimensional Implicit Thermal-Response and Ablation Program, which predicts charring material thermal response and shape change on hypersonic space vehicles. The flow code solves the reacting full Navier–Stokes equations using the data parallel line relaxation method. Recession analyses of stagnation tests conducted in NASA Ames Research Center arc-jet facilities with heat fluxes ranging from 45 to are performed and compared with data for model validation. The ablating material used in these arc-jet tests is phenolic impregnated carbon ablator. Computational predictions of surface recession and shape change are in good agreement with measurement for arc-jet conditions.

Implicit Coupling Approach for Simulation of Charring Carbon Ablators

This study demonstrates that coupling of a material thermal response code and a flow solver with nonequilibrium gas–surface interaction for simulation of charring carbon ablators can be performed using an implicit approach. The material thermal response code used in this study is the three-dimensional version of fully implicit ablation and thermal response program, which predicts charring material thermal response and shape change on hypersonic space vehicles. The flow code solves the reacting Navier–Stokes equations using data-parallel line relaxation method. Coupling between the material response and flow codes is performed by solving the surface mass balance in the flow solver and the surface energy balance in the material response code. Thus, the material surface recession is predicted in the flow code, and the surface temperature and pyrolysis gas injection rate are computed in the material response code. It is demonstrated that the time-lagged explicit approach is sufficient for simulations at low surface heating conditions, in which the surface ablation rate is not a strong function of the surface temperature. At elevated surface heating conditions, the implicit approach has to be taken because the carbon ablation rate becomes a stiff function of the surface temperature, and thus the explicit approach appears to be inappropriate, resulting in severe numerical oscillations of predicted surface temperature. Implicit coupling for simulation of arc-jet models is performed, and the predictions are compared with measured data. Implicit coupling for trajectory-based simulation of Stardust forebody heat shield is also conducted. The predicted stagnation point total recession is compared with that predicted using the chemical equilibrium surface assumption.

Data-Parallel Line Relaxation Method for the Navier-Stokes Equations

The Gauss‐Seidel line relaxation method is modie ed for the simulation of viscous e ows on massively parallel computers. The resulting data-parallel line relaxation method is shown to have good convergence properties for a seriesoftestcases.Thenewmethodrequiressignie cantlymorememorythanthepreviouslydevelopeddata-parallel relaxation methods, but it reaches a steady-state solution in much less time for all cases tested to date. In addition, the data-parallel line relaxation method shows good convergence properties even on the high-cell-aspect-ratio grids required to simulate high-Reynolds-number e ows. The new method is implemented using message passing on the Cray T3E, and the parallel performance of the method on this machine is discussed. The data-parallel line relaxation method combines the fast convergence of the Gauss ‐Seidel line relaxation method with a high parallel efe ciency and thus shows promise for large-scale simulation of viscous e ows.

Data-parallel line relaxation method for the Navier-Stokes equations

Data-parallel line relaxation method for the Navier-Stokes equations