Attachment saddle point topology and transformation of 3-D separation in juncture flow

An experimental investigation of laminar boundary layer 3-D steady separation vortex flow upstream of junctures had been conducted to verify the existence of attachment saddle point topology and to study the transformation of the flow topology. The laser light sheet of 2-D particle image velocimetry(PIV) system was set on the symmetry plane of junctures formed by different shape models mounted on a fixed length flat plate. A micro lens was used to take the local flow topology around the singular point. The results confirmed the existence of attachment saddle point structure under certain flow parameters in different shape of junctures. There is transformation between attachment saddle point topology and classical separation saddle point topology as Reynolds number increase or model bluntness changes. An analysis of relation between attachment saddle point topology and Lighthill-s classical steady 3-D separation mode is presented to show that Lighthill-s classical separation mode is applicable for attachment saddle point topology.