Effect of swirl on self-sustained azimuthal aeroacoustic modes in an axisymmetric cavity*

We investigate the effect of a swirling mean flow on azimuthal aeroacoustic modes in an axisymmetric cavity. In absence of imposed swirl, a self-sustained aeroacoustic wave of azimuthal order 1 can emerge from the reflectionally symmetric mean flow, and depending on the initial condition, it spins either in the clockwise or in counterclockwise direction. With the Navier-Stokes equations linearized around the low-Mach turbulent mean flow, we show that an imposed swirl promotes the global mode spinning against the swirl direction, i.e. the co-winding counter-spinning eigenmode. These findings explain our experimental observations based on simultaneous acoustic recordings and time-resolved stereoscopic particle image velocimetry. We show that the fluid mechanics of this intriguing whistle can be leveraged to create an acoustic scatterer that can non-reciprocally transmit acoustic energy without losses. This aeroacoustic realization of loss-immune non-reciprocal scattering is based on the synchronization of the aeroacoustic limit cycle and the incident acoustic wave, and it opens the way for exciting research in acoustic metamaterials.