Acoustic liners, passive devices to mitigate engine noise, operate under high-speed grazing flow and grazing acoustic waves. To investigate the complex physics governing this interaction, high-fidelity numerical simulations of a spatially evolving turbulent boundary layer grazing a multi-orifice acoustic liner at a bulk Mach number of 0.32 are performed. The simulations replicate conditions from a reference experiment. Grazing tonal plane acoustic waves with amplitudes of 130 dB and 145 dB and propagating in the same direction and the direction opposite to the mean flow are analyzed. The results show that the boundary layer displacement thickness doubles in the presence of the liner and its growth rate is affected by the amplitude and propagation direction of the acoustic wave. The acoustic liner also promotes the formation of an outer hump in both the logarithmic region of the streamwise and wall-normal velocity variance, with these effects becoming more pronounced under acoustic forcing. Furthermore, impedance estimation, using Dean’s method, reveals that near-wall flow modifications, quantified through the displacement thickness, influence the local value of the computed impedance.

This computational study investigates the impact of manufacturing inaccuracies of face sheet orifice geometries on acoustic liners’ impedance and flow dynamics. Normal Impedance Tube (NIT) lattice-Boltzmann very-large eddy simulations at 130 and 145 dB and 800, 1400, and 2000 Hz reveal that sharp-edged geometries present increased acoustic resistance and absorption than geometries with smoother edges. Rounded and double-chamfered edge shapes, mimicking real-world imperfections, reduce the resistance component of impedance by up to 28%, thus reducing the absorption coefficient. The inspection of the velocity field shows the flow features that cause these differences. Results demonstrate that minor edge imperfections, potentially due to manufacturing, may alter liner performance. This underscores the need to account for geometric imperfections in industrial design and quality control.

Eduction methods are adopted to characterize acoustic liners. In this paper, several impedance eduction techniques are compared using a numerical database obtained with scale resolved lattice-Boltzmann simulations of a reference acoustic liner in the presence or not of a grazing turbulent flow. Three impedance eduction techniques are compared: an inverse approach based on the Mode-Matching (MM) method, the straightforward method based on the Prony-like Kumaresan-Tufts (KT) algorithm, and one approach based on a minimization problem between reference measurements and the solution of the Pierce’s equation. Furthermore, the educed impedance is compared with the one obtained using local impedance measurements with the Dean’s method with virtual probes located on the entire face-sheet. Results show that impedance values obtained with the Deans’ method are highly dependent on the sampling location and that they vary largely over each cavity. Results from the eduction methods are similar amongst them with few discrepancies found for the method based on the Pierce’s equation. In particular, the highest value of resistance obtained using the Deans’ method is similar to the one obtained using the KT and MM eduction methods.

This study investigates the aerodynamic and acoustic response of a multi-orifice acoustic liner grazed by a planar acoustic wave and turbulent flow, at centerline Mach number equal to 0.32. High-fidelity flow simulations are carried out using a Lattice-Boltzmann Very-LargeEddy-Simulation solver and the in-situ technique is used to calculate impedance. The triple decomposition technique is adopted to separate the mean-flow effects from those due to grazing tonal acoustic waves with different frequencies and amplitudes. This study highlights the sensitivity of in-situ measurements on the position of the face-sheet probe used to sample the unsteady pressure fluctuations. It is found that the resistance changes up to a factor of three along each cavity. The acoustic-induced velocity field reveals the intricate interaction between the acoustic waves and the turbulent flow. It is shown that the wake shed by the upstream cavity impacts the downstream one, affecting the spatial distribution and the amplitude of the acoustic-induced velocity within the orifice. Furthermore, a vortex within the hole is observed; it is found that its impact on resistance depends on the acoustic wave propagation with respect to the mean flow.