The article deals with the design of a metadevice able to trap acoustic waves in a duct. The aim is to limit the outgoing acoustic power and confine the perturbation inside the duct exploiting the unconventional reflection of the optimized metasurface. The target engineering application is related to the improvement of the efficiency of standard acoustic liners in the ultra-high-bypass-ration engines, where the ratio between diameter and length of the nacelle inlet increases significantly. The metabehaviour is modeled by means of the generalized Snell's law for reflection from acoustically rigid surfaces. The realization of the device relies on a modular concept, which building set is made of eight elementary cells, able to induce a reflected field suitably phase-delayed with respect to the incident wave. The acoustic perturbation is produced by a source placed inside the duct. The set spans the whole 0-2$pi$ phase delay range, and the anomalous reflection is obtained by the tailored design of the phase delay gradient profile on the metasurface. The cells are designed through numerical optimisation in order to extend the effective frequency range of the device, keeping the overall thickness of the metadevice smaller than a quarter of the nominal wavelength. The duct and the source are considered co--moving within the fluid at rest. The numerical analysis is performed in the frequency domain in a frame of reference rigidly connected to the duct, and considering several values for the Mach number. Preliminary numerical results show thatr the phase-delay profile can be effectively tailored to achieve the required reflection steering.
