High-fidelity simulations of high speed flows for aerospace problems

The interest in high-speed flows has considerably risen in the last decades, with the growing push of the space economy in the public and private sectors, and with the aim to provide fast long-range methods for air transportation of goods and passengers around the globe. Flight systems operating at high speeds can be quite different: aircrafts, rockets or reentry vehicles, but they are all eventually enveloped by turbulent, hot, highly compressible flow. These effects become dominant in a very thin region in the proximity of solid boundaries, called the boundary layer. This is the region where intense mechanical and thermal loads are transferred to the vehicle, significantly affecting its design and the choice of surface materials. At hypersonic speeds, the enormous heat generated in the boundary layer can also lead to chemical dissociation processes, which significantly change the resulting flow dynamics. The objective of the present thesis is two-fold. On one hand, our aim is to better understand the interplay of controlling flow parameters such as the Reynolds number, the Mach number, the wall temperature, and surface roughness, targeting specific configurations that are not yet well understood by the research community. On the other hand, we work towards improving current engineering models that are used to inform design choices, which are often inspired by a better physical understanding of the underlying process. To this end, Computational Fluid Dynamics (CFD) is leveraged as an invaluable tool to systematically analyze several flow configurations without the need to perform very expensive wind tunnel runs or in flight measurements, often impractical for these applications. Among the plethora of methodologies available in CFD, the present thesis is oriented to high-fidelity simulations, specifically Direct Numerical Simulations (DNS), where all turbulent scales are resolved, and Large Eddy Simulations (LES), where only small scales are modeled, yet maintaining the ability to capture the non-stationary and multiscale nature of turbulence. The first part of this thesis (Papers I and II) investigates flat-plate turbulent boundary layers under varying Reynolds numbers, Mach numbers, and wall temperature conditions to address gaps in high-speed flow DNS reference data. A detailed study of the interplay between Mach number and wall temperature, Paper II, isolates their individual effects, focusing on their relative impact on coupling between kinetic and thermal fields. Paper III conducts one of the first numerical studies of compressible turbulent boundary layers transitioning from smooth to rough surfaces using DNS. It provides a high-fidelity analysis of turbulent scales and a direct comparison with subsonic cases, identifying unique compressible flow features, including compression and expansion waves from 3D surface elements. Papers IV and V focus on modeling high-speed flows. Paper IV presents the URANOS compressible flow solver, ported for running in recent Graphic Processing Units (GPUs) architectures and designed for LES and wall-modeled LES methodologies for calorically-perfect gases. Paper V extends the wall model by Griffin et al. (2023) to incorporate finite-rate chemistry and multicomponent diffusion in chemically reacting hypersonic boundary layers, showing improved a priori predictions for velocity and temperature profiles compared to classical models, which are not visible for the wall-normal distribution of mass fractions. This finding highlights the need for refined modeling of chemical reaction terms as a result of their highly nonlinear nature.