Publication
This paper follows a previous publication, where the so-called Advection Boundary Law lining an acoustic waveguide, in absence of mean flow, was studied in terms of its potentials for noise isolation and non-reciprocal propagation. The Advection Boundary Law is a special operator which can be synthesized on the boundary of a waveguide thanks to a programmable Electroacoustic Liner. This special boundary operator proved to achieve enhanced noise isolation with respect to classical local impedance. Moreover, it demonstrated to accomplish non-reciprocal sound propagation along the waveguide, and the non-trivial passivity limits were assessed. Nevertheless, acoustic liners are meant to attenuate noise propagation in waveguides with airflow, such as heating and air-conditioning ventilation systems and aircraft turbofan engines. In particular, the new generation of Ultra-High-By-Pass-Ratio turbofans and the increasingly stringent regulations on aircraft noise pollution, require a significant breakthrough in the acoustic liner technology. This challenge was taken up by the SALUTE H2020 project, during which the experimental campaign reported in this paper was conducted. For the first time, the Advection Boundary Law interfacing an airflow is thoroughly analysed in terms of duct-modes and scattering simulations. The enhancement of isolation performances is confirmed also in presence of mean-flow. Moreover, for the first time, non-reciprocal propagation along the waveguide is achieved against the one naturally induced by the mean-flow. These results, along with the passivity limits, are discussed and confirmed by the experimental campaign, conducted on the CAIMAN test-bench of the Laboratory of Fluid Mechanics and Acoustics of the Ecole Centrale de Lyon. The tools and results provided in this paper should lead the implementation of the Advection Boundary Law for maximizing noise isolation or achieving non-reciprocal sound propagation along waveguides with airflow.