Controlling sound propagation and scattering via spinning media*

The propagation of waves in time-varying and/or moving media is expected to lead to enhanced control of optical or acoustical waves and a plethora of intriguing applications. In this realm, we will discuss in this talk our recent advances on physics of sound propagation in spinning fluids (air or water). For instance, a spinning cylindrical column of fluid in the same medium will be shown to possess intrinsic spin angular momentum. We will study the torque and force it experiences in evanescent acoustic fields, and we will show that the resulting discontinuity can scatter sound in unusual ways, e.g., a negative radiation force, although it has no imaginary part in its parameters [M. Farhat, et al. PRB(L) 104, L060104 (2021)].

Similarly, scattering cancellation technique (SCT) has proved to be an effective way to conceal static objects from acoustic waves. Yet, spinning objects carry peculiar scattering features that cannot be canceled by regular cloaks. Here, we will propose a generalized SCT theory to cloak spinning objects, and hide them from static observers, based on rotating shells with different angular velocity. Our work extends the realm of SCT and brings it one step closer to its practical realization that involves moving objects [M. Farhat, et al. PRB 101, 174111(2020)].