This paper begins the process of verifying and validating computational fluid dynamics (CFD) codes for supersonic retropropulsive flows. Four CFD codes (DPLR, FUN3D, OVERFLOW, and US3D) are used to perform various numerical and physical modeling studies toward the goal of comparing predictions with a wind tunnel experiment specifically designed to support CFD validation. Numerical studies run the gamut in rigor from code-to-code comparisons to observed order-of-accuracy tests. Results indicate that this complex flowfield, involving time-dependent shocks and vortex shedding, design order of accuracy is not clearly evident. Also explored is the extent of physical modeling necessary to predict the salient flowfield features found in high-speed Schlieren images and surface pressure measurements taken during the validation experiment. Physical modeling studies include geometric items such as wind tunnel wall and sting mount interference, as well as turbulence modeling that ranges from a RANS (Reynolds-Averaged Navier-Stokes) 2-equation model to DES (Detached Eddy Simulation) models. These studies indicate that tunnel wall interference is minimal for the cases investigated; model mounting hardware effects are confined to the aft end of the model; and sparse grid resolution and turbulence modeling can damp or entirely dissipate the unsteadiness of this self-excited flow.
A high-fidelity three-dimensional approach is developed to simulate emission signatures from the shock layer around the Stardust sample return capsule at several points on its best-estimated trajectory. Calculations are performed with various gas chemistry, thermodynamic, and radiation models. Results are compared against calibrated imaging data acquired by a slitless echelle spectrometer. The present analysis is based on flowfields computed without the inclusion of ablation products, and the comparison is focused on radiation from atomic oxygen and nitrogen lines. The purpose is to apply and improve, if necessary, the current models used in nonequilibrium atmospheric entry simulations.
The Stardust Sample Return Capsule (SRC) was launched in February 1999 on a mission to retrieve samples of interstellar dust from the tail of comet WILD-2. Stardust returned to Earth in January 2006 entering the atmosphere with a velocity of 12.6 km/s, the fastest Earth reentry and highest energy reentry of any artificial vehicle to date. Several optical instruments captured the reentry of Stardust through an observation campaign aboard the NASA DC-8 airborne observatory. Flow environments obtained from Computational Fluid Dynamics (CFD) solutions are loosely coupled with material response modeling to predict the surface temperature of Stardust throughout the reentry. The calculated surface temperatures are compared with the data from two spectral instruments onboard the airborne observatory, the Echelle camera and SLIT telescope. The gray body curves corresponding to the average and area-averaged surface temperatures predicted by the material response simulation have excellent agreement with the recorded Echelle data at lower altitudes. At these altitudes the CFD/material response coupling can predict the surface temperature to within 50 K. The CFD calculations alone overestimates surface temperatures because it does not take into account ablation, as the material response modeling does. At higher altitudes, the presence of paint on the heatshield could have contribution to the lower observed surface temperatures and explain the over-prediction by the simulated data, which does not account for the paint. The over-prediction of the simulated surface temperature coincides in time with the high emission intensity lines corresponding to paint products. The average surface temperatures resulting from the SLIT telescope analysis agree to within 5% with the average surface temperatures predicted by the material response. Surface temperature is one of the critical parameters used in the design of thermal protection systems since it is an indicator of material performance. The combined CFD/material response approach employed in the present analysis can thus be reliably used for future missions such as CEV Orion.
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