Reentry vehicle ablation
Maxime Stuck thesis « Ablation d’un matériau de protection thermique en régime turbulent » (« Thermal protection material ablation in turbulent regime ») has been defended on December 12th 2023 (ISAE-Université de Toulouse). The thesis addresses the coupling between ablation and turbulence that triggers the apparition of surface roughnesses on hypersonic vehicles ablative TPS during atmospheric flight.
Keywords : Turbulent boundary layer, Forced linear response, Reynolds Averaged Navier–Stokes, Aerodynamic, Ablation
Support : the thesis framework is a collaboration between by CEA and ONERA
Summary
During their trajectory through the atmosphere, high velocity objects such as re-entry capsules or meteorites are subjected to extreme thermal environments. Heat fluxes can even be so high as to trigger oxidation, sublimation and other chemical reactions between the materials they are made of and the atmosphere they fly in. This leads to the body material thermal erosion, the so-called ablation phenomenon.
For reentry vehicles, this ablation needs to be as small as possible in order to make the reentry mission a success. High temperature materials are used thus for, such as carbon-carbon composite materials which are among the most resistant materials to this kind of environments.
During the ablation process, scallop-like patterns may appear on the surface (Figure 1). When appearing on reentry vehicle body (Figure 2), these patterns, stemming from the coupling between the body wall ablation and the turbulent boundary layer, in turn boost thermal heat fluxes, thereby accelerating the degradation of the thermal protection.
Knowing the physical phenomena behind these observations is of importance for a good sizing of reentry vehicles TPS. The aim of this thesis is therefore to understand the role of turbulence in the growth of scallops during the first instants of their formation.
Similar patterns can also occur in a wide variety of geophysical environments where an erodible wall undergoes fluid flow shearing (Figure 3). In these situations, the flow is incompressible, and there exists an empirical correlation, proposed by Thomas (1979), connecting patterns size to turbulent boundary layer features (Figure 4).
As a first step in the thesis, early times of a flight test simulation was carried out on the basis of available data in the literature (Hochrein & Wright, 1976 – Figure 2). This permited to establish a connection between the patterns encountered during heat shield ablation in the flight test and the Thomas (1979) correlation (Figure 4).
In a second step, the ablation gouges were modeled as a harmonic sinusoidal deformation of the wall. The historical approach (Charru et al., 2013) used to study the growth of such periodic patterns was adopted, and studies of forced linear responses of a turbulent boundary layer inner region developping on a corrugated wall were performed.
The comparison of the results with RANS (Reynolds Averaged Navier–Stokes) simulations, a DNS (Direct Numerical Simulations) database and experimental results from Hanratty and co-workers (Abrams & Hanratty, 1985; Frederick & Hanratty, 1988) highlighted the failure of the Boussinesq hypothesis for a specific range of the wall deformation wavelengths, and its consequences in predicting the emergence of a preferred wavelength during wall regression (Chedevergne et al. 2023).
It was shown that a second-order turbulence model can reproduce the reference results, particularly with regard to shear stress and heat flux at the wall, illustrating the importance of the difference in diagonal Reynolds stresses, poorly represented by the Boussinesq hypothesis.
Drawing on Hanratty’s work, ad-hoc corrections were then proposed and, despite the limitations of this approach, proved their effectiveness in improving the performance of first-order turbulence models (Stuck et al. 2024 and Figure 5).
Finally, to conclude this work, a preliminary study looked at the three-dimensional extension of linear analyses to investigate the influence of wall curvature.
References
| R. Thomas, 1979 | Size of scallops and ripples formed by flowing water. Nature, vol. 277 n◦ 5694 pp. 281–283. |
| G. Hochrein & G. Wright, 1976 | Analysis of the tater nosetip boundary layer transition and ablation experiment. From 14th Aerospace Sciences Meeting, p. 167. |
| F. Charru et al., 2013 | Sand ripples and dunes. Annual Review of Fluid Mechanics, vol. 45 pp. 469–493. |
| J. Abrams & T. Hanratty, 1985 | Relaxation effects observed for turbulent flow over a wavy surface. Journal of Fluid Mechanics, vol. 151 pp. 443–455. |
| K. Frederick & T. Hanratty, 1988 | Velocity measurements for a turbulent non-separated flow over solid waves. Experiments in fluids, vol. 6 n◦ 7 pp. 477–486. |
| M. Stuck thesis, 2023 | Ablation d’un matériau de protection thermique en régime turbulent Thèse de Doctorat, Toulouse, ISAE |
| F. Chedevergne et al., 2023 | About the role of the Hanratty correction in the linear response of a turbulent flow bounded by a wavy wall. Journal of Fluid Mechanics, vol. 967 pp. A39. |
| M. Stuck et al., 2024 | Influence of the turbulent closure for the prediction of the linear response of a flow bounded by a corrugated wall European Journal of Mechanics – B/Fluids, vol. 105, pp 275-284 |
