It is unreliable or infeasible that normal PIV is used to test the weak secondary flow field whose velocity is less than 20% of the main flow velocity. In this study
a new method was proposed to obtain reliable measuring results for instantaneous vector field of weak secondary flow in high velocity flow. The second light sheet was modulated to the plane of 1.0-1.5 mm downstream from the first light sheet to trace the motion of particles. In order to measure quantitatively the evolvement of particle motion and vortex structure for time-dependent secondary flow
the sequence in time and space of four laser light sheets was modulated so that the motion of the particles in four transient states could be continuously traced by two sets of double-exposure CCD. The results show that three instantaneous vector fields with high time resolution can be obtained.
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references
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