Flow visualization, basic image processing and statistical treatment are
used to investigate the flow characteristics of an axisymmetric free air jet
having an initially uniform velocity profile. Vortical structures and their
development in the transition zone of the jet are visualized for a range of
Reynolds numbers from 1300 to 1700. It is shown the onset and growth of
these structures which propagate downstream of the nozzle exit at
approximately 0.65 of the velocity of the mean flow. The duration of this
development mechanism, before reaching the zone of turbulence, is estimated
at 7.6 × 10-3. Near-field and intermediate-field region lengths,
separation distance between two neighboring vortical structures, apparition
frequency and jet Strouhal number are measured as functions of the Reynolds
number. In particular, it is shown that the Strouhal number, which is
estimated at 0.64 in our case, is independent of Reynolds numbers.