GALCIT Colloquium

The advancement of hypersonic capabilities presents a formidable challenge, requiring transformative progress across various research domains. Paramount among these is the development of models that can accurately and efficiently simulate hypersonic environments characterized by strong non-equilibrium flows. This becomes particularly critical in wake flow environments, an aspect of modeling that has been misunderstood for decades yet is critical to operating hypersonic vehicles for Defense and Space Exploration applications.

This talk outlines a new paradigm for constructing predictive modeling and simulation tools from a fundamental physics perspective, rejecting the empiricism that has prevented progress in modeling hypersonic flows for decades.

The most physically consistent description of non-equilibrium hypersonic flows relies on the solution of the Boltzmann equations for particles and photons. However, for problems of interest, the exponentially large number of degrees of freedom and the wide range of spatial and temporal scales involved make these equations unsolvable. Inspired by model reduction strategies developed in statistical physics, this work addresses the challenges of the combinatorial explosion of the possible configurations of the system, obtaining new hydrodynamic governing equations by projecting the high-dimensional Boltzmann equations onto a few lower dimensional subspaces. The distribution function within each subspace is then reconstructed using the Maximum Entropy Principle, thus ensuring compliance with the Detailed Balance.

The model's application encompasses two key areas:

a) Non-equilibrium Chemical Kinetics: Analysis of thermochemical relaxation in simplified flow environments with validation against Master Equation results and experimental data.

b) Rarefied Gas-dynamics: Analysis of shock structure in a wide range of Mach numbers validated through experimental data and Direct Simulation Monte Carlo (DSMC) calculations.

The concluding segment of the talk will highlight the application of this model to the study of radiative heating in spacecraft's back-shell region during atmospheric entry. By analyzing the recombination of gas molecules in the wake region, this research clarifies the previously misunderstood heating phenomenon observed in the back shell under high-speed entry conditions, resolving a longstanding puzzle in hypersonic aerothermodynamics.