Numerical Simulation of High-Frequency Flow Instabilities Near an Airblast Atomizer

In the paper it is shown that statistical averaging of transport equations (URANS = Unsteady Reynolds Averaged Navier Stokes) imposes no inherent restriction concerning the ability to predict periodic or other deterministic transient flow processes. This even holds for periodic oscillations at relatively high frequencies lying in the spectral range of the inertial sub-range of flow turbulence. As an application, the unsteady behaviour of an isothermal swirling air flow through and behind an airblast-atomizer of a design typical for modern aeroengine combustors is treated. This flow exhibits self-excited oscillations at a frequency of 2.8 kHz. Computations of this flow behaviour based on the numerical solution of the unsteady statistically averaged Navier-Stokes equations are presented. The turbulence model employed in the computations is a k,ε-model modification for swirling flows. The transport equations are discretized by a Finite-Volume method on a curvilinear grid. Calculated mean velocity profiles as well as the predicted dynamic flow behaviour at the nozzle exit agree very well with appropriate LDV- and microphone-measurements.