On the non-linear influence of the edge geometry on vortex shedding in Helmholtz resonators

This study investigates the effect of edge profile of a Helmholtz resonator neck in non-linear regime by means of experiments and large eddy simulations. The dissipation mechanisms in a Helmholtz resonator differ significantly depending on the sound pressure level. At low levels, i. e., in the linear regime, thermo-viscous effects are responsible for the dissipation of the acoustic energy since the oscillating flow follows the neck geometry. However, increasing the sound pressure level results in flow separation at the edges. At these points, vortices form which convert acoustic perturbation energy to the hydrodynamic mode. This is a strong non-linear effect increasing the dissipation considerably. To observe this effect, experiments and numerical simulations are carried out for combinations of various backing volumes, sound pressure levels, and neck profiles. The neck profiles are selected as 45°-chamfers due to manufacturing concerns. Hereby, a strong dependence on the edge shape is observed in both experiments and numerical simulations. The presence of the chamfer reduces the vortex shedding in comparison to the sharp edge significantly, which leads to a lower acoustic resistance.