Investigation of attachment saddle point structure of 3-D steady separation in laminar juncture flow using PIV

An experimental study of laminar horseshoe vortex upstream of a cylinder/flat plate juncture has been conducted using PIV in symmetric plane to verify the existence of attachment saddle point topology. The experimental results confirm the existence of attachment saddle point structure, which is different from conventional separation saddle point flow topology. In one, two and three primary vortices condition, upstream streamlines near surface do not rise up or separate from the plate surface but attach to plate surface. The outmost vortex initiates from the spatial singular point and not from surface singular point. Especially, the skin-friction-lines convergence corresponds to spatial attached flow but not separated flow as in case of conventional separation saddle point topology, alternatively, the spatial attached flow correspond to convergent but not divergent skin-friction-lines, as happens in conventional attachment flows. The results indicate that attachment saddle point structure is a kind of separation in which vorticity separates from spatial layer of boundary layer but not from the plate surface. The study also reveals three types of attachment saddle point topologies, these three topologies all satisfy the topological, singular point index rule.