Untersuchung der turbulenten Strukturen in koaxialen, begrenzten Strahlen mit Drall

This is an experimental investigation of the flow structure and turbulence characteristics of a turbulent axisymmetric shear layer between two primary streams under the influence of three-dimensional stationary boundary conditions. These conditions result from skewing the primary streams relative to the trailing edge. Two configurations were investigated: both primary streams were parallel but skewed relative to the trailing edge (i.e. slanted trailing edge) and the primary streams were non-planar (i.e. had cross-shear). Although these flows are simplified models, their understanding should provide more insight into the mechanisms of real life three-dimensional shear flows than studies on classical two-dimensional shear flows have done to date. The investigated shear layers were generated at the interface between two coaxial jets with different axial velocities. Additionally, azimuthal velocity components were imposed by means of a swirl generator on the shear layer either in the same or the opposite direction. Results from various experimental measurements including 4-sensor hot-wire probes, hot-wire rakes and visualizations indicate an increase of the total turbulent kinetic energy, and of the growth rates compared to cases with no skewing. Further, the vortex rings resulting from the the Kelvin-Helmholtz instability were deformed by coherent structures having axis not parallel to the trailing edge. It is assumed that these structures are helical vortices competing with the vortex rings in a complex structural manner which lead to the observed increase of the turbulent quantities