Trapped modes in a non-axisymmetric cylindrical waveguide

Lyapina, A. A.; Pilipchuk, A. S.; Sadreev, A. F. JOURNAL OF SOUND AND VIBRATION, 421 48-60; 10.1016/j.jsv.2018.01.056 MAY 12 2018

We consider acoustic wave transmission in a non-axisymmetric waveguide which consists of a cylindrical resonator and two cylindrical waveguides whose axes are shifted relatively to each other by an azimuthal angle $Δ ϕ$ . Under variation of the resonator's length $L$ and fixed $Δ ϕ$ we find bound states in the continuum (trapped modes) due to full destructive interference of resonant modes leaking into the waveguides. Rotation of the waveguide adds complex phases to the coupling strengths of the resonator eigenmodes with the propagating modes of the waveguides tuning Fano resonances to give rise to a wave faucet. Under variation of $Δ ϕ$ with fixed resonator's length we find symmetry protected trapped modes. For $Δ ϕ \neq 0$ these trapped modes contribute to the scattering function supporting high vortical acoustic intensity spinning inside the resonator. The waveguide rotation brings an important feature to the scattering and provides an instrument for control of acoustic transmittance and wave trapping.