the second laser light sheet is modulated to trace the motion of particles and main flow
the secondary flow instantaneous vector field is measured in aft-loaded annular cascade of turbine
and strong unsteady evolution of vortex is found and shown quantitatively. The results indicate that one Hopf bifurcation and unstable limit cycle appear in the vortex core of local channel diffusion area
and the streamline structure of channel vortex belongs to the unstable focus in other areas. The analysis results of vorticity aerodynamics and physical nature of the channel vortex indicate that the streamline structure is determined by the contest between pressure gradient and positive or negative dissipation. Moreover
even though different kinds of method to restrain secondary flow have been taken
the flow loss can be further reduced if the streamline structure of channel vortex is kept the unsteady focus.
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