Transient Vortex Modelling of Flow Induced Acoustic Resonances Near Cavities or Obstructions in Ducts

When flow in a duct passes a cavity or an obstruction and generates a shear layer, an acoustic resonance may be excited which interacts with that shear layer. The result is a sustained resonance and changed flow pattern with generally two distinct vortices forming during one cycle of the acoustic resonant frequency. This effect occurs over discrete ranges of flow velocity. To explain the fluid mechanics of the process Howe’s theory of aerodynamic sound is used to calculate the energy exchanged, during one acoustic cycle, between the sound field and the flow. The flow velocities corresponding to sustained resonance correlate well with velocities where the net flow of energy from the flow to the sound field is calculated to be greatest.