Controlled perturbation of circular jets

The near flow fields of periodically perturbed circular air jets have been experimentally investigated for the Strouhal number range 0 ≤ StD ≤ 4.0 and the Reynolds number range 11,000–113,000. The largest total fluctuation intensity growth on the centerline occurs at StD ≌ 0.85 while the largest fundamental amplitude growth occurs at StD ≌ 0.30. At StD ≌ 0.85, most of the fluctuations is at the subharmonic frequency associated with vortex pairing in the ‘jet mode’, which is independent of the ‘shear layer mode’ of vortex pairing occurring at Ste ≌ 0.011. Smoke visualization at lower speeds provided further details of pairing in either mode. In the jet mode of vortex pairing, the Reynolds stress undergoes large excursions even though its time average values are not altered appreciably by the excitation.