Acoustic waves and friction drag in turbulent flows with sound absorbing materials – ACOUDRAG

Reducing the environmental footprint of aircraft, by lowering their fuel consumption and their noise emissions, is a major challenge that involves new scientific and technological advances. With the emergence of high bypass ratio engines and the reduction in jet noise they allow, the acoustic waves emitted by the fan blades have become a major component of the sound emitted and their attenuation is more necessary than ever. Aircraft engines use sound-absorbing materials (called acoustic liners) to reduce their noise emission. In their most widespread architecture, these consist of small resonant cavities placed in the walls of the engine nacelle, which are separated from the flow by a perforated plate. During their propagation in the engine, the acoustic waves emitted by the fan are partly transformed into vortices in the cavities of the absorbent material, which are ultimately dissipated by viscosity. This is the classical mechanism for sound absorption. In aircraft engines, a specificity is that the grazing flow in which the waves travel is high-speed and turbulent, which has several consequences. On the one hand, this flow modifies the acoustic response of the acoustic liner, that is to say its acoustic impedance, the very definition of which poses a problem with flow. On the other hand, the material affects the structure of the turbulent boundary layer, which has a detrimental effect on drag, meaning that sound absorption comes at the expense of consumption. Thus, an interaction between three different actors takes place: the turbulent flow, the acoustic wave, and the acoustic liner. This interaction is complex due to its strongly nonlinear nature and needs to be studied as a whole, which is the subject of the project ACOUDRAG. This project addresses the need to increase the fundamental knowledge in two domains mentioned in Axe B.4 of the Generic Call for Proposals 2023 (Engineering and Process Sciences): fluid mechanics and aeroacoustics. The partners are a public laboratory (Institut PPRIME in Poitiers) and a public institution with industrial ambitions (ONERA Toulouse).
The ACOUDRAG project seeks to elucidate the physics of flow near an acoustically treated surface by considering two related issues: (1) How do liners react to turbulent structures, and how is the acoustic impedance modified? (2) How to quantify the effect of the acoustic wave/wall interaction on the dynamics of the flow? In particular, we will investigate how the acoustic wave can modify the drag, and determine the parameters governing this interaction. To provide an answer to these questions, the positioning of the project consists in using high-fidelity numerical simulations as well as time-resolved measurement techniques. The numerical simulations will be carried out at PPRIME and will investigate fully turbulent channel flows, where the turbulence scales will either be taken fully into account or partially modeled, and the acoustic material will be entirely included in these simulations. The experiments will be carried out at ONERA, which has two high-performance measurement benches combining microphone measurements and laser velocimetry, one of them allowing precise wall shear stress measurement. The partners will define common configurations and share their post-processing methodologies. Their complementary numerical and experimental contributions will provide a reliable and comprehensive database. Its analysis will allow the elucidation and modeling of flow dynamics, and will guide the design of new materials, allowing acoustic liners to maximize their sound absorption properties and minimize their frictional drag. The database will be published in open-access form for the community.