Eric C. Stern

Estimation of dynamic stability coefficients for aerodynamic decelerators using CFD

A method for performing dynamic simulations of entry vehicles is developed. This capability uses existing infrastructure within the US3D flow solver, developed for doing fluid structure interaction (FSI) simulations, to allow for up to six degree of freedom (6DoF) simulations. Inviscid, free-to-oscillate simulations of the Mars Science Laboratory (MSL) capsule at Mach2.5and3.5are used to evaluate di↵erent data reduction methods. It is found that many simulations may be required to get reliable data. The computed pitch damping coecients show comparable trends to ballistic range data, though they di↵er in magnitude, particularly at high angles of attack. Preliminary viscous simulation results are presented, and show improved agreement with experimental data compared to the inviscid analysis.

Microscale simulations of porous TPS materials: Application to permeability

An approach for performing microscale simulations of porous spacecraft thermal protection systems (TPS) is presented. This approach uses the Direct Simulation Monte Carlo (DSMC) method to accurately model the flow of gas through the porous media, which in many cases is rarefied. Here we present this method applied to two cases of porous media flow. The first is flow through idealized two-dimensional cylinders. Here, we found very good agreement between both CFD and DSMC calculations and an empirical relation from the literature. For the second case, we simulate flow through an approximation of a realistic three-dimensional microstructure. Here we also present a method for creating complex random fibrous microstructures. In our analysis, we find reasonably good agreement with experiment. However, discrepancies in the experimental data prevent definitive validation using this dataset.

Microscale Simulations of Porous TPS Materials: Ablating Microstructures and Micro-tomography

A previous work outlined a new approach for perform microscale simulations of spacecraft thermal protection system (TPS) materials. This approach uses the Direct Simulation Monte Carlo (DSMC) method to accurately model the flow of gas through the porous media, which in many cases is rarefied. In this paper, we outline an efficient and robust new method for allowing the fibers in a microstructure to ablate due to chemical reactions at the surface. This method is applied to doing high fidelity coupled gas and material simulations, and the results of these qualitatively reproduce the expected morphologies in the diffusion limited and reaction limited regimes. The approach is also applied to simulating the flow through microstructures obtained from X-ray computed micro-tomography. The results show that these simulations are feasible at relevant scales and conditions.

Progress in Payload Separation Risk Mitigation for a Deployable Venus Heat Shield

A deployable decelerator known as the Adaptive Deployable Entry and Placement Technology (ADEPT) offers substantial science and mass savings for the Venus In Situ Explorer (VISE) mission. The lander and science payload must be separated from ADEPT during atmospheric entry. This paper presents a trade study of the separation system concept of operations and provides a conceptual design of the baseline: aft-separation with a subsonic parachute. Viability of the separation system depends on the vehicles dynamic stability characteristics during deceleration from supersonic to subsonic speeds. A trajectory sensitivity study presented shows that pitch damping and Venusian winds drive stability prior to parachute deployment, while entry spin rate is not a driver of stability below Mach 5. Additionally, progress in free-flight CFD techniques capable of computing aerodynamic damping parameters is presented. Exploratory simulations of ADEPT at a constant speed of Mach number of 0.8 suggest the vehicle may have an oscillation limit cycle near 5 angle-of-attack. The proposed separation system conceptual design is thought to be viable.

Nonequilibrium flow through porous thermal protection materials, Part I: Numerical methods

Abstract Numerical methods are developed to simulate high temperature gas flow and coupled surface reactions, relevant to porous thermal protection systems used by hypersonic vehicles. Due to the non-continuum nature of these flows, the direct simulation Monte Carlo (DSMC) method is used, and the computational complexity of the simulations presents a number of unique challenges. Strategies for parallel partitioning, interprocessor communication, complex microstructure geometry representation, cutcell procedures, and parallel file input/output are presented and tested. Algorithms and data structures are developed for a microstructure generation tool called FiberGen that enables realistic microstructures to be constructed based on targeted fiber radius, orientation, and overall porosity, with user defined variations about these values. The data structures and algorithms associated with FiberGen are robust and efficient enough to enable DSMC simulations where the microstructure geometry changes to directly simulate ablation problems. Subsonic boundary conditions are described and validated, and a number of example solutions are presented. The example problems demonstrate the difference between surface ablation and in-depth volume ablation regimes for porous TPS materials.

Investigation of transonic wake dynamics for mechanically deployable entry systems

A numerical investigation of transonic flow around a mechanically deployable entry system being considered for a robotic mission to Venus has been performed, and preliminary results are reported. The flow around a conceptual representation of the vehicle geometry was simulated at discrete points along a ballistic trajectory using Detached Eddy Simulation (DES). The trajectory points selected span the low supersonic to transonic regimes with freestream Mach numbers from 1.5 to 0.8, and freestream Reynolds numbers (based on diameter) between 2.09 × 106 and 2.93 × 106. Additionally, the Mach 0.8 case was simulated at angles of attack between 0° and 5°. Static aerodynamic coefficients obtained from the data show qualitative agreement with data from 70° sphere-cone wind tunnel tests performed for the Viking program. Finally, the effect of choices of models and numerical algorithms is addressed by comparing the DES results to those using a Reynolds Averaged Navier-Stokes (RANS) model, as well as to results using a more dissipative numerical scheme.

Nonequilibrium flow through porous thermal protection materials, Part II: Oxidation and pyrolysis

Abstract Micro scale simulations are performed of flow through porous (pyrolyzing) thermal protection system (TPS) materials using the direct simulation Monte Carlo (DSMC) method. DSMC results for permeability are validated with computational fluid dynamics (CFD) calculations and theory, for simple porous geometries under continuum flow conditions. An artificial fiber-microstructure generation code FiberGen is used to create triangulated surface geometry representative of FiberForm® (FiberForm) material. DSMC results for permeability of FiberForm are validated for a range of pressures (transitional flow conditions) and agree with experimental measurements. Numerical uncertainty is determined to be within 2% if sufficiently large portions of the microstructure are included in the computation. However, small variations in fiber size and angle bias can combine to give + 30 % uncertainty when comparing with experimental permeability data. X-ray microtomography scans of FiberForm are used to create microstructure geometry for incorporation within DSMC simulations of coupled oxygen diffusion and gas-surface chemistry in the presence of a blowing pyrolysis gas. In-depth penetration of atomic oxygen is limited to 0.2 – 0.4 mm for the range of Knudsen number and pyrolysis gas conditions studied.

Implementation of a Newton Method for the Rapid Propagation of Thermochemical Model Uncertainty in Hypersonic Flows

An ew method for reducing the cost of propagating uncertainty in hypersonic fl ow simulations due to physical modeling parameters is proposed. This method uses a sensitivity gradient formulation, similar to a Newton method, to quickly compute new solutions to the Navier-Stokes equations given a small perturbation to the nominal input parameters. This makes the method well suited to doing probabilistic uncertainty propagation, where many solutions must be computed. There are numerical stability issues associated with using this method, and they are explored, and possible remedies suggested. The applicability of this method to uncertainty propagation is demonstrated with a simple test case.