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.@en

Commercial aircraft electrification and novel urban air mobility vehicles under development have brought renewed interest in propeller noise to the research community. The complex noise generation mechanisms and broad range of possible configurations lead to the development of a large number of propeller noise test rigs. However, the effects of test rig configuration, installation, and instrumentation on the experimental results are still unclear. In this work, the comparison of the results obtained by two different institutes, the Federal University of Santa Catarina (Brazil) and TU Delft (The Netherlands), is presented.A pair of identical propellers, used for acoustic benchmark studies, were tested under the same conditions to compare the effects of the test rigs on the noise results. A good agreement is observed for thrust and torque. Some divergences on the acoustic data were observed. However, good agreement was noted at the first harmonic and overall sound pressure level, with a maximum difference of 1.8 and 2.8 dB, respectively.@en

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.

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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