A three-dimensional simulation of vortex formation at the open end of an acoustic waveguide

For high enough levels of acoustic pressure inside a tube, a nonlinear mechanism is responsible for the formation of annular vortices at the open end of the tube, which results in energy loss. Higher sound pressure levels in the tube lead in turn to larger values of the acoustic velocity at the exit, and thus to higher Reynolds numbers. It has been observed [Buick et al., 2011] that two regimes are possible depending on whether the acoustic velocity is low, in which case vorticity appears in the immediate vicinity of the tube, or high, in which case vortices are formed at the open end of the tube and are advected outwards. We use a Lattice Boltzmann Method (LBM) to simulate the velocity field at the exit of the tube in 3D, for both cases. We plan to compare these numerical results with experimental results obtained through particle image velocimetry (PIV). The effect of varying both the geometry of the tube and the shape of the termination on the magnitude of the nonlinear losses at the exit is also examined.