Investigation of Time-Resolved Nozzle Interference Effects

Automotive wind tunnels still see an increasing significance in the vehicle development process. Nevertheless, the flow topology in the test sections of open jet wind tunnels is not yet understood completely. A large source for transient structures is the shear layer developing between the high velocity nozzle flow and the calmly air in the plenum.

For a better understanding, the shear layer is investigated in the symmetry plane with a multi-hole probe and PIV measurements in this paper. The analysis of the recorded data shows fluctuations with a Strouhal number of 0.46 based on the nozzle hydraulic diameter. This value matches other researches of jet flow. However, the concept of vortices separating at the nozzle edge seems to be incomplete. The PIV measurements reveal a wave structure in the shear layer at the nozzle exit. These waves release larger vortices moving along the outer boundary of the shear layer. Mode decomposition (POD) shows changing intensities of these vortices for varying wind tunnel velocities. Different flow structures are detected when delta wings are mounted to the nozzle edges.