Thermal Protection Systems
The Thermal Protection System (TPS) of a re-entry vehicle is one of the key components of its design. The materials used for TPS usually have ablative properties. Such materials use energy for controlled chemical reactions instead of heating, thus keeping the vehicle at a reasonably “cold” temperature. In order to properly model the heat flux at the surface of the vehicle, flow field, ablation and material response must be taken into account.
Kentucky Aerothermodynamics and Thermal-response System
Kentucky Aerothermodynamics and Thermal-response System (KATS), has been developed over the past few years. Using a C++ foundation, KATS uses an implicit first-order backward Euler time integration to solve for modified conservation equations accounting for chemical behavior within the gas and material. These equations are used to characterize the behavior of the flow in and around the body at hand with high fidelity. KATS has implemented the following features:
- Reads 3D Unstructured grid in CGNS format
- ParMETIS for domain decomposition
- MPI for inter-processors communications
- PETSC Krylov subspace method as linear solver for iteration
- Schemes include Steger-Warming flux-vector splitting, AUSM+up, Roe, etc.
KATS-MR
The Material Response (MR) module in KATS is being developed since January, 2012. Currently it features three-components solid decomposition model, chemical equilibrium pyrolysis gas model, time-based gas transport model, and anisotropic material properties model. The gas and solid materials are programmed in a multi-species way, which makes it easy to implement higher fidelity models in the future, like comprehensive pyrolysis models, carbon oxidization models.
KATS-CFD
The flow solver, KATS-CFD, is a three-dimensional, finite-volume Navier-Stokes code for the simulation of weakly ionized hypersonic flows in thermo-chemical non-equilibrium developed at the University of Kentucky. Together with pre-conditioner and switch convective flux scheme to AUSM+-up, it however can still work with rather low speed viscous flow for Mach below 1.e-4. Its coupling with KATS-thermal response solver is being extended to account for the interaction between the flow field and the thermal protection system.
The code enjoys the features:
- 3D Structured/unstructured grids
- Thermal equilibrium/nonequilibrium
- Standard finite-rate chemistry kinetics
Spallation
One cause of surface ablation is due to spallation of fibers. As the phenolic resin sublimates, the pyrolysis gas builds in pressure within the material until it is high enough to blow out. The blowing of this pyrolysis gases passes by weaken oxi- dized fibers and can be strong enough to break and carry away fragmented charred particles. Code has been generated that computes the projected path of these spalled particles at various parameters.
KATS-Coupling
Coupling of FD and MR solver is accomplished by adding a Surface Module.
Each module solves its governing equation on a given mesh, i.e. the FD solver solves the equations on the fluid mesh, and the MR solver on another. The only requirement is that the meshes are aligned at their interfaces so that the area of the faces is identical on both grids. At each time step, a surface module solves the flux balance equations to obtain the primitive values at the interface.Simulation based on flow-tube oxidation experiment carried out in the NASA Ames flow-tube facility.
KATS-Micro-scale Approach
The development of a material response code is not possible without understanding the physical phenomena that occurs within the ablation material as it reacts to it’s environment. Historically, rough and overly simplified estimations of the of the material were used to obtain material properties, such as: tortuosity, permeability, porosity, etc. However, when compared to Micro-CT images many inaccuracies arise within the geometry.
Due to these inaccuracies, attention has been put into reconstruction of the
material geometry to gain higher precession in computational property estimations. This is done by making uses of the the Lawrence Berkeley National Laboratory’s Advance Light Source Facility to take tomographs of the ablation material. These images are then imported into code using marching cube algorithms to regenerate the geometry while also numerically estimating material properties and oxidation recession rates.
KATS-Validation
As KATS begins to incorporate information from all of the above, a sense of validation still must be completed to ensure the accurate of the mathematical models. Although arc-jets offer a convent and an ”affordable” means to test TPS materials, the experiments still are not capable to fully replicate re-entry environment. In August 2012, a herculean achievement was mades as NASA successfully delivered a nearly 2000 lb payload to the martian surface. The Mars Science Laboratory (MSL) project is of vital importance to the Gas Surface Interactions Lab and other groups alike as it returned pressure and temperature data from the heat shield. This data provides a valuable opportunity to compare the obtained data from MSL to that of the computed values though KATS.
