Acoustic liners' dissipation analysis in presence of grazing flow using Howe's energy corollary

Acoustic liners are installed in the intake of aircraft engines to absorb noise. In operating conditions, the acoustic liners are subjected to grazing acoustic waves and a turbulent boundary layer that develops over the nacelle. In presence of high sound pressure level (>140 dB) and grazing flow, the liner works in the so-called non-linear regime and the acoustic dissipation is governed by the vortex shedding formed at the mouths of the orifices. In addition, the grazing flow modifies the impedance of the liner. High fidelity simulations are conducted on a spatially evolving turbulent boundary layer interacting with a multi-orifice liner in presence of a grazing acoustic tone. The energy dissipated by vortex shedding and the topology of the dissipation are quantified using Howe's energy corollary, a methodology applied here for the first time to acoustic liners. We demonstrate that, in absence of flow, the dissipation by shedding occurs in the inflow and outflow phases while the effect of the flow is to inhibit the vortex shedding in the outflow. This leads to an overall acoustic dissipation reduction by vortex shedding, with respect to the no-flow case, and a modification of the impedance.