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INVESTIGATION OF A TRANSONIC AXISYMMETRIC BACKWARD-FACING STEP FLOW BY MEANS OF HIGH RESOLUTION PIV

Sven Scharnowski
Institute of Fluidmechanics and Aerodynamics Bundeswehr University Munich 85577 Neubiberg, Germany

Christian J. Kahler
Institute for Fluid Mechanics and Aerodynamics Universitat der Bundeswehr Munchen (UniBw) Werner-Heisenberg-Weg 39, 85577 Neubiberg, Germany

Résumé

This work analyzes the axisymmetric backward-facing step flow at a Mach number of 0.7 and a Reynolds number of 106, based on the forebody's diameter, in order to achieve fundamental understanding of the separating and reattaching flow and to generate a data basis for the validation of numerical flow simulations. Due to the strong progress of optical flow measurements in the last years it was possible for the first time to resolve all flow scales down to 180 µm (~1% of the step height) with high precision. A large ensemble, consisting of 21500 statistically independent PIV image pairs, allows for the reliable and accurate estimation of the mean velocity distribution as well as of the Reynolds shear stress distribution on the plane of symmetry. It was found that the ensemble-averaged flow reattaches 1.06 times the fore body's diameter downstream of the point of separation on the models rear sting. In the corner of the primary recirculation region a secondary vortex with opposite mean circulation direction is generated on average. The Reynolds shear stress distribution shows high intensity within the developing shear layer as well as at the centre of the primary recirculation region and develops a broad maximum at an axial location around the reattachment location. The shear stress in the separated region was found to be strongly influenced by small scale turbulent structures.