Experimental investigation of transitional unsteady separated vortex flows

Experimental results on transitional separated flows along a rounded backward facing step are reported in the present thesis. Experiments are carried out in a water towing tank using hydrogen bubbles and laser light sheet flow visualizations. Results are given for a Reynolds number range between 8.4×103 and 1.01×105. Velocity-field measurements are obtained with video-based digital particle image velocimetry (DPIV). Flow visualizations and quantitative measurements reveal that the topological structure and dynamics of transitional separated flows are complex and dominated by large-scale flow unsteadiness with travelling vortices, with intense interaction between these flow structures, and with inclined separated shear layers. Vortex shedding appears out and downstream of a mean-flow separation bubble. While laminar flow separation appears stationary and two-dimensional a pronounced three-dimensionality of the flow appears in the region of turbulent flow reattachment. The development of the initially two-dimensional vortex-flow structures into Λ-formed three-dimensional flow structures is visualized. An interpretation of the dynamics of an unsteady vortex flow being the basis of a mean-flow transitional separation bubble is given.