Characterisation methods of sound-absorbing materials are affected by a lack of description of the interactions between acoustic waves and the turbulent boundary layer over the material surface. Funded by the European Research Council, the LINING project will investigate unexplained experimental findings discovered when a sound wave encounters turbulent air currents over surfaces treated with acoustic liners. Researchers will measure acoustic and hydrodynamic velocities near an acoustically treated surface with new experiments and high-fidelity numerical simulations. Describing how flow and acoustics interact over aircraft engine noise reduction technologies could pave the way for the development of future sound-absorbing surfaces with greater noise reduction and lower drag.
The lack of fundamental knowledge of the interaction between an acoustic wave and a turbulent boundary layer grazing an acoustically treated surface, such as an acoustic liner, is the cause of unexpected and unphysical results found when performing the acoustic characterization of the sound absorbing surface with inverse eduction methods. This is because, in this field, acoustic and aerodynamic have never been fully coupled.
To fill this knowledge gap, the acoustic and hydrodynamic velocities near an acoustically treated surface must be measured. Since it cannot be done only with state-of-the-art experiments, because of hardware and field-of-view limitations, I propose to complement experiments with scale-resolved high-fidelity numerical simulations based on the lattice-Boltzmann very-large-eddy simulation method.
Numerical results will be used to explain the physics of the acoustic-flow interaction. Advanced data analysis methodologies will be developed and applied to separate the acoustic-induced velocity near the wall from the hydrodynamic one. At the same time, the numerical database will be used to compare inverse methods, employed to acoustically characterize the sound absorbing surfaces, in order to explain the physical reasons behind the unexpected results, and propose physics-based corrections. Furthermore, by describing the flow-acoustic interaction, it will be possible to model and predict the drag increase caused by the coupling between the acoustic-induced velocity and the free-stream one.
My description of the flow-acoustic interaction will solve the scientific debate about the unexpected results and pave the way towards future broadband low-noise low-drag acoustic meta-surfaces to increase propulsion efficiency and reduce noise of future, more sustainable, aircraft engines.