How the Nozzle Geometry Impacts Vortex Breakdown in Compressible Swirling-Jet Flows

摘要

The influence of the nozzle geometry, namely, the nozzle-wall thickness d and the nozzle length L, on the vortex breakdown of compressible, swirling nozzle-jet flows is numerically investigated. The Reynolds number is set to Re=5000 and the Mach number is Ma=0.6. The nozzle is either in rotation with the mean-flow direction or kept at rest. In a first set of simulations, the nozzle-wall thickness is varied in the range d={0.1,0.15,0.2,0.25}, whereas the nozzle length is kept constant (L=5). For the rotating nozzle wall, the flowfield changes significantly, mainly due to a variation in the initial swirl number and an enhanced centrifugal instability at the outer side of the nozzle wall. For the setup with a nozzle kept at rest, only minor changes of the flowfield are observed in general for a variation of the nozzle-wall thickness. Changing the nozzle length from L=5 to L=7.5, while keeping the wall thickness constant (d=0.1), leads to a shift of the recirculation region downstream for both setups, and a reduction of its radial extent. The helical instabilities developing in the downstream direction within the boundary layers at the nozzle wall reach higher amplitudes for the long nozzles, leading to the observed differences in the configurations downstream of the nozzle. The results presented reemphasize the important role played by the nozzle configuration upstream of the swirling jet for the development of the vortex-breakdown configuration, and contribute to the clarification of the effects of the nozzle geometry.