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 Shirin Ghaffari
B.S. Sharif University of Technology

Prediction of the heat load on the surface of vehicles (re-)entering a planetary atmosphere is important for heat shield design. Turbulent flow induces a much higher heating in comparison to laminar flow. Therefore, prediction of the location of laminar-turbulent transition is a key factor in designing the dimensions and materials used for the thermal protection system (TPS). Fundamental physical processes related to the laminar-turbulent transition in high-speed boundary layers are not well understood. This is true even in the first stage of the process, that is, amplification of small linear perturbations. Due to small amplification rates, this first stage can take place for a long downstream distance ultimately leading to nonlinear interaction and transition. Therefore, understanding the factors that affect the laminar to turbulent transition is crucial. My project focuses on the effect of ablation on high-speed boundary layer stability in particular. Ablation diverts a significant fraction of the convective and radiative energy flux away from the surface via the creating of volatile reaction products – thus “blowing” from the surface is one facet of ablating heat shields (figure 1). Early transition triggered by surface ablation in the boundary layer may create larger heat transfer and, in turn, more an undesirably high ablation rate. This project will attempt to characterize the phenomena initially through linear stability analysis and ultimately combine these results with large-scale numerical simulation. In the latter stages of the project we will couple the modeling of complex chemical reactions at the surface with the fluid mechanics in the high-speed boundary layer to determine whether ablation has a significant effect on transition at high Mach number.

                                   Figure 1: Example of an ablated heat shield

Preliminary plots of linear disturbance amplification rates (upper left corner) and wall-normal amplitude functions for the 2-D disturbance show the destabilizing effect of blowing on a high-speed boundary layer.

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