This paper reports on an investigation of coherent structures and the characteristic flow field of trailing edge shed vorticity, which can be found downstream of blade rows as well as behind lobed exhaust mixers. The corresponding, fundamental flow case of a free, skewed mixing layer was studied both experimentally in a low-speed test facility and numerically using Standard k,ε-Model and Large Eddy Simulation (LES).
The investigation gave a new insight into the flow structure. Along the complete development length there is a coexistence of the streamwise vortices generated by cross-shear with the spanwise vorticity of Von-Karman vortex street or two-dimensional Kelvin-Helmholtz instability. This was confirmed by extensive flow-field measurements using five-hole probes and X-wire anemometry as well as by CFD. The measurement of the mixing layer spreading resulted in a growth rate of the skewed mixing layer very similar to that of the two-dimensional flow. The development of energy thickness downstream of the trailing edge, representing the mixing losses, was found to be practically independent of skewing angle. The spacing and the fluctuation of the streamwise vortex cores were not accessible to probe measurement, but were determined by visualization and large eddy simulation. The separation of vorticity-components gave stable distributions in streamwise and spanwise direction until vortex breakdown, which appears to be independent of the initial state of the boundary layer. A criterion for streamwise vortex breakdown was identified by correcting the development length with the equivalent shear layer parameter. The content of turbulent kinetic energy as a measure for turbulence production and mixing efficiency is discussed.
