Description du sujet de thèse

At CNRS, on the Campus of the University of Poitiers, the Institut Pprime is recruiting a PhD student to perform numerical simulations of turbulent wall-bounded flows in the presence of wall cavities acting as sound absorbers.

The French National Research Agency (ANR) ACOUDRAG project founded the thesis, associating Institut Pprime, ONERA Toulouse, and Cerfacs. The latter will be involved in the supervision of the thesis.

Contexte de travail

A significant part of the noise emitted by airplanes is generated by their engines, and more particularly by the fan. To reduce this noise, sound absorbing materials (acoustic liners) are used. These are made up of a series of Helmholtz resonators placed within the wall of the nacelle and are separated from the grazing flow by a perforated plate. The objective of the thesis is to better understand these materials from an aerodynamic and acoustic point of view, through numerical simulations. These simulations will be carried out in academic configurations.

In a duct with flow, the turbulent grazing flow eventually interacts with an acoustic absorber. This has two consequences. First, the acoustic properties of the liner are affected: the acoustic impedance, which constitutes the main characteristic of the liner, is modified in a non-trivial way compared to its value without flow. Second, acoustic liners modify the mean flow and increase friction (induced drag), which is a more recent area of concern. How this happens is still poorly understood and only a few numerical [1,2] or experimental [3,4] studies have addressed this question.

The general objective of this thesis is to understand the complex interaction between the flow, the acoustic waves, and the absorbing material, by means of numerical simulations. Simulations of wall turbulent flows (plane channel or boundary layer) will be performed in the presence of an incident acoustic wave to be attenuated and of an absorbent material placed on a portion of the walls. Until now, simulations of this kind have been carried out at Institut Pprime in which materials have been modeled by wall impedances [5,6]. This made it possible to study the modification of turbulence by these materials, in particular due to instabilities. However, such a framework implies that the feedback of the flow on the behavior of the liner (its impedance) is ignored, and in this thesis the aim is to go beyond this simplifying modeling by taking into account the full liner geometry in the simulations. That is, wall resonant cavities will have to be meshed. The simulations will be carried out with the AVBP simulation code developed by Cerfacs (https://www.cerfacs.fr/avbp7x/), which has been used to study the thermal behavior of materials similar to the acoustic absorbing materials considered here [7]. This code solves the compressible Navier-Stokes equations via direct numerical simulations (DNS) or large scale simulations (LES). The simulations performed during this thesis will make it possible to study absorbent materials from two different perspectives:
1. that of acoustic behavior. The dependence of the impedance on the flow will be studied.
2. that of the drag induced by the liner. We will particularly wonder to what extent the latter is linked to the excitation by the incident acoustic wave.

-Subject available on the internet website of the lab:
https://pprime.fr/emploi/phd-thesis-2024-numerical-simulations-of-acoustic-liners-in-turbulent-wall-bounded-flows/

*Bibliography :
[1] Q. Zhang and D. J. Bodony, Numerical Investigation of a Honeycomb Liner Grazed by Laminar and Turbulent Boundary Layers, Journal of Fluid Mechanics, vol. 792, pp. 936-980, 2016.
[2] L. M. Pereira, L. A. Bonomo, A. R. da Silva, J. A. Cordioli and F. Avallone, Lattice-Boltzmann Numerical Investigation of a Realistic Multi-Cavity Acoustic Liner with Grazing Flow, in 28th AIAA/CEAS Aeroacoustics 2022 Conference, Southampton, 2022.
[3] C. Jasinski and T. Corke, Mechanism for Increased Viscous Drag over Porous Sheet Acoustic Liners, AIAA J., vol. 58, 2020.
[4] M. Zheng, C. Chen and X. Li, Experimental investigation of factors influencing acoustic liner drag using direct measurement, Aerospace Science and Technology, vol. 130, 2022.
[5] Sebastian R, Marx D, Fortuné V, Numerical simulation of a turbulent channel flow with an acoustic liner, J. Sound Vib. 456 (2019) 306-330.
[6] Marx D, Sebastian R, Fortuné V, Simulation of instability and sound production in a turbulent
channel flow with an acoustic liner, J. Sound Vib. 573 (2024) 118223.
[7] Esnault S, Duchaine F, Gicquel L. Y. M., Moreau S., Analysis of upstream turbulence impact on wall heat transfer in an acoustic liner with Large-Eddy Simulations, Appl. Sci. 13 (2023), 3145.

The candidate should hold a Master 2 and/or an engineering degree, with knowledge of fluid mechanics, acoustics and numerical simulations.

Le poste se situe dans un secteur relevant de la protection du potentiel scientifique et technique (PPST), et nécessite donc, conformément à la réglementation, que votre arrivée soit autorisée par l'autorité compétente du MESR.

Contraintes et risques

The PhD student will have to travel a few times in France and abroad.