On the Relationship Between the Formation Number and Passive Scalar Pinch-Off in Starting Jets

A series of experiments were conducted in a water tank to investigate the behavior of starting vortex ring in starting jets at Reynolds numbers up to 2 × 10⁴. The starting vortex separates from the rest of jet in the near-field. The velocity vectors were measured via the digital particle image velocimetry while the passive scalar concentrations were observed by the laser sheet technique. The measured Formation number was in the range of 4.5 to 5.5, consistent with previous low Reynolds number results. The Formation number appears to be a robust parameter for characterizing the pinch-off of vorticity in starting jets.