Tunability of superradiant scattering

A low-Mach airflow passing over a cavity can cause aeroacoustic instabilities, resulting in whistling sounds. These instabilities take place when the hydrodynamic mode of the fluid and the acoustic mode of the cavity form constructive feedback exceeding the dissipative losses. The resulting limit cycle has been observed to enable tunable compensation of transmission losses, which is desirable in a broad range of phenomena from lossless non-reciprocal transmission to acoustic cloaking. The general method works by scattering harmonic incident waves with sufficiently large amplitude by a cavity limit cycle oscillating in the weakly non-linear regime, forcing the limit cycle to synchronize with the incident waves. At suitable conditions, the resulting non-linear wave-mode coupling leads to superradiant amplification of the outgoing waves. This work is focused on the tunability of superradiant scattering with regard to the incident wave amplitude and the bifurcation parameter. To investigate the tunability, experiments were performed on a superradiant aeroacoustic meta-atom, which has previously been demonstrated to amplify harmonic acoustic waves. These experimental validations show excellent agreement with corresponding theoretical predictions. As a key result, it is demonstrated that superradiant amplification can be achieved at small incident wave amplitudes, given that the bifurcation parameter is also small enough.