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LES of the aero-acoustic coupling in acoustic liners containing multiple cavities

  • Bauerheim, Michaël
  • Joly, Laurent
Publication Date
Jun 15, 2020
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Acoustic liners are specific devices dedicated to reduce noise pollution of aircraft. They contained a large number of cavities which are submitted to a turbulent grazing boundary flow. In some cases, the shear layer can become unstable, and couples with the acoustic waves propagating in the cavity, leading to a strong vortex-noise interaction. Whereas this vortex-noise coupling is well known in single-cavity configurations where the boundary layer can be described by its upstream characteristics, the interaction between a turbulent grazing flow and multiple cavities is less understood, mainly because the boundary layer may evolve along the liner. This study investigates numerically the coupling between acoustic waves and hydrodynamic fluctuations in an academic liner configuration containing multiple deep cavities. First a single 3D cavity is simulated using LES, revealing a strong vortex-noise coupling at the frequency of the quarter-wave mode at 750Hz, due to a shear layer instability with a wavelength shorter than the cavity width. Then, both a 2D and 3D configuration containing 101 cavities are computed and analyzed. When the acoustic feedback loop is cut off, no vortex-noise coupling occurs, however large flow structures can be observed in the boundary layer. When switching on the acoustic feedback loop, the vortex-noise coupling at 750Hz is again present, yet a more complex noise spectrum is obtained. In particular, lower frequencies are observed, associated to intermittency due to the desynchronization between cavities. Moreover, it is shown that the boundary layer evolves along the liner. One key phenomenon is the massive instability of the turbulent boundary layer, generating large coherent structures with a wavelength larger than the cavity width in the downstream part of the liner. A scenario is then proposed to explain this competition between these two phenomena: in the upstream part of the liner, the flow is dominated by the vortex-noise coupling at 750Hz. Further downstream, however, the shear layer becomes thicker, and is therefore more robust to transverse acoustic excitation, leaving the place for the large turbulent flow structures in the boundary layer to dominate the flowfield.

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