Main flow oriented vorticity noise experiments.

Entropy inhomogeneities and vorticity spots induce so-called indirect combustion noise when passing through a choked nozzle; referred to as entropy noise and vorticity noise, respectively. We note that vorticity noise depends on the orientation of the vorticity; viz., oriented normal or parallel to the axial main flow. An experimental investigation of parallel component vorticity noise is presented. In the experiment a time-dependent swirling flow was induced by unsteady tangential injection in the pipe upstream of a choked convergent-divergent nozzle. As the resulting swirling flow passes through the nozzle, the axial stretching of the fluid caused an increase in rotation energy. The steady energy conservation in an isentropic flow implies a Mach number higher than unity at the throat and an associated reduction of density. Ergo, the critical mass-flow rate (for fixed reservoir pressure and temperature) decreases quadratically with increasing swirl intensity. The acoustic waves radiated downstream of the nozzle are due to the change in the mass flow through the nozzle. These are a direct measure for this mass-flow modulation. Using a semi-empirical model, this sound production mechanism is demonstrated to be quasi steady. This contradicts the bare assertion found in the literature that sound is produced by the acceleration of vorticity or "vorticity waves.". [ABSTRACT FROM AUTHOR]