Scaled-resolved numerical simulations using the lattice-Boltzmann Very Large Eddy Simulation method are performed to compute the acoustic impedance of a realistic multi-cavity single degree of freedom liner grazed by a turbulent boundary layer. Numerical results are assessed against experimental impedance measurements carried out in grazing flow impedance test facility at the Federal University of Santa Catarina (UFSC), with three different approaches: the in-situ technique, the mode matching method and a Prony-like algorithm. Both experiments and numerical simulations are carried out with and without turbulent grazing flow at Mach number equal to 0.3 and with grazing acoustic tonal plane wave. Acoustic waves with amplitude equal to 130 dB and 145 dB are analyzed. For each amplitude, six frequencies are investigated in the range between 800 Hz and 2300 Hz. For each case, the acoustic wave propagates both in the same direction and opposite to the grazing turbulent flow. Numerical results show very good agreement with experimental data for the no-flow case. In the presence of grazing flow, preliminary numerical results show an overestimation of the resistance with respect to the experimental data. It has been found that using a less dissipative solver for the acoustic simulations and increasing the resolution lead to better agreement. Nevertheless, the numerical database predicts well the different trends between the impedance measurement methods. The presented database, after being recomputed with the less dissipative solver, will be used to understand the physical reasons behind the different impedance measurement results obtained with different eduction methods and clarify the physics of the flow-acoustic interaction.@en

View Video Presentation: https://doi-org.tudelft.idm.oclc.org/10.2514/6.2021-2144.vid

The physics behind acoustic liners attenuation in the presence of flow and high sound pressure level is still a matter of debate. Similarly, discrepancies between experimental results and numerical data have been linked to the boundary conditions used to model the liner and boundary layer effects, and the reasons behind these discrepancies are still not clear. In this sense, to avoid the limitations of the boundary condition approach, fully resolved high fidelity computation models of the liner and its dissipation mechanisms may be an important tool to improve understanding. The present study carries out a numerical analysis using a code based on the Lattice-Boltzmann method, and special focus is given on replicating the results from different experimental techniques used to evaluate the liner impedance: the in-situ method and an impedance eduction method based on the mode-matching technique. The study is conducted with a model including a single degree of freedom liner with multiple cavities based on a real geometry. The model considers high sound pressure level, grazing plane acoustic waves without flow in order to replicate the experimental setup. A mesh convergence analysis is performed, and the liner impedance obtained numerically is compared with experimental results from the in-situ method and the impedance eduction technique. The numerical pressure and velocity fields are also analyzed in detail to better understand the damping mechanisms and physics involved.@en