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Direct numerical simulation of boundary layer transition control via acoustic metasurfaces – [ED MEGEP]

The wall-bounded boundary layer flow can transition from a laminar to a turbulent regime, leading to a significant increase in friction and heat transfer at the surface. Consequently, high-speed vehicles may experience increased drag and excessive wall heating. This transition phenomenon, from an ordered flow to chaotic motion, is initiated when relatively small atmospheric perturbations penetrate the boundary layer flow, triggering the flow’s natural instabilities. These instabilities then undergo linear amplification until critical amplitudes. Beyond these amplitudes, nonlinear interactions and three-dimensional effects arise, progressively filling the wall-bounded flow until it becomes fully turbulent. This thesis focuses on the linear growth of instabilities in both subsonic and hypersonic twodimensional boundary layers and explores the use of acoustic metasurfaces passive control systems to delay or prevent the onset of turbulence. The first year was dedicated to adapting and implementing boundary conditions in a high-fidelity simulation code to extend its capabilities for incorporating the acoustic response effects of complex metasurfaces and accurately simulating the interactions between instabilities and control devices. Leveraging these tools, direct numerical simulations (DNS) of canonical configurations were subsequently performed, with the acoustic response of reactive acoustic surfaces modeled by a time-domain impedance boundary condition (TDIBC). These unsteady simulations, covering a wide range of spatiotemporal scales, aimed to provide a detailed description of the dynamics of Tollmien-Schlichting wave development in subsonic flows and the second Mack mode in hypersonic flows, as well as their interaction with an acoustically reactive surface. These numerical investigations were complemented by linear stability analyses. A first case involving the boundary layer on a flat plate at Mach 0.12 was simulated with an experimentally measured acoustic impedance as the boundary condition, aiming to mimic the passive effect of the acoustic response of a laminar flow control (LFC) system. In a similar fashion, the Mach 7.4 flow around a blunt cone was numerically simulated, accounting for the acoustic response of a ceramic matrix composite (CMC) material used in thermal protection systems (TPS) for hypersonic vehicles. An optimization process for the macroscopic geometric parameters of a CMC material was also carried out in order to enhance its acoustic absorption performance. Another aspect of the thesis concerned the analysis of the dynamics of the second mode of Mack, both on a cold wall and on an adiabatic wall, in the context of an academic case of a flat plate at Mach 6. These dynamics are described using an original approach in terms of the interaction of coherent structures associated with the fluid-thermodynamic, vortical, acoustic, and thermal components of the perturbation field. 

Jury

Mme Estelle PIOT	ONERA	Directrice de thèse
M. Guillaume DAVILLER	CERFACS	Co-directeur de thèse
M. Guillaume LEHNASCH	ISAE ENSMA	Rapporteur
Mme Taraneh SAYADI	CNAM	Rapporteure
M. Christophe AIRIAU	Université Toulouse III – Paul Sabatier	Examinateur
M. José CARDESA	ONERA	Examinateur
M. Xavier GLOERFELT	ENSAM Paris	Examinateur
M. Alexander WAGNER	Deutsches Zentrum für Luft-und Raumfahrt (DLR)	Examinateur