One-Way Acoustic Guiding under Transverse Fluid Flow

In a moving acoustic medium, sound waves travel differently with and against the fluid flow. This well-established acoustic effect is backed by the intuition that the fluid velocity bias imparts momentum on the propagating acoustic waves, thus violating reciprocity. Based on this conception, fluid flow that is transverse to the wave direction of propagation will not break reciprocity. In this paper we contrast this common wisdom and theoretically show that the interplay between transverse mean flow and transverse structural gliding asymmetry can yield strong nonreciprocity and even, surprisingly, one-way acoustic waveguiding. To demonstrate that, we analyze a waveguide that comprises a few adjacent acoustic subdiffraction chains, i.e., linear chains of acoustic scatterers with monopolar or dipolar response. The structure is embedded within a medium with mean flow velocity that is transverse to the waveguide axis. We find the symmetry breaking conditions under which nonreciprocity is obtained, and we show how, under transverse mean flow, with Mach numbers as low as 0.02, one-way propagation of the acoustic wave is obtained on a sub-wavelength-thick acoustic waveguide. To demonstrate the phenomenon, we introduce two models. One in which the scatterers are assumed to be pointlike and the flow is homogeneous, and a second, in which we accommodate the issue of possible wakes created by the presence of scatterers inside the flow. The latter setup exhibits no direct interaction of the scatterers with the flow, by placing the waveguide in a quiescent medium adjacent to a flowing medium. Our results push forward the understanding of acoustic nonreciprocity, and may open another venue for the design of nonreciprocal acoustic wave devices for various applications.