Motivation

For very high speed flight conditions, reusable (ceramic) materials cannot withstand the heat flux and ‘ablative’ thermal protection systems (TPS), which are designed to erode during flight, are employed.

Typically these heat shields are composed of carbon-fibers embedded within a carbon-matrix or phenolic resin. Especially for vehicles where the ablative surfaces must remain aerodynamic, understanding the gas-surface reaction process is important for understanding and controlling the surface shape change and aerodynamics during flight.

Results and Publications

Carbon Oxidation:

    Poovathingal, S., Schwartzentruber, T.E., Srinivasan, S.G., and van Duin, A.C.T., “Large Scale

    Computational Chemistry Modeling of the Oxidation of Highly Oriented Pyrolytic Graphite”, J.

    Phys. Chem. A (2013), 117, pp. 2692-2703.

    Poovathingal, S. and Schwartzentruber, T.E, “Effect of Microstructure on Carbon-based Surface    

    Ablators using DSMC”, AIAA Paper 2014-1210, Jan. 2014, presented at the 52nd Aerospace

    Sciences Meeting, AIAA SciTech, National Harbor, Maryland.

Surface and in-depth ablation is a very multi-scale problem. Understanding involves individual reactions on surface defects at the atomistic scale, the complex microstructure of the TPS material including fibers and matrix components, and finally macroscopic shape change and aerodynamics. Our approach is to use molecular dynamics to investigate the precise chemical reactions, steric factors, and activation energies occurring at the atomistic level. Feed these results into DSMC simulations of the fiber microstructure (in some cases in-depth gas-surface simulations of porous material). Finally, the DSMC results are used to formulate continuum level models that can be used as CFD boundary conditions. Some images depicting this general multiscale strategy are shown below.

The above images are from the Von Karman Institute (VKI) for Fluid Dynamics (Belgium). From left to right the images depict a TPS sample (fiber pre-form) during a high-enthalpy inductively coupled plasma (ICP) facility test, a fiber sample after the test, and a close-up image of a single fiber that has undergone significant oxidation.

The left image shows a DSMC simulation of flow through a porous fiber microstructure. The microstructure geometry is obtained through collaboration with Dr. Nagi Mansour’s group at NASA Ames Research Center. The right image shows a MD simulation of etch pit growth due to atomic oxygen bombardment.