Vortex pairing in a circular jet under controlled excitation. Part 1. General jet response (original) (raw)

1980, Journal of Fluid Mechanics

Hot-wire and flow-visualization studies have been carried out in three air jets subjected to pure-tone acoustic excitation, and the instability, vortex roll-up and transition as well as jet response to the controlled excitation have been investigated. The centreline fluctuation intensity can be enhanced by inducing stable vortex pairing to a level much higher than even that a t the 'preferred mode', but can also be suppressed below the unexcited level under certain conditions of excitation. The conditions most favourable to vortex pairing were determined as a function of the excitation Strouhal number, the Reynolds number (Re,), and the initial shear-layer state, i.e. laminar or turbulent. It is shown that the rolled-up vortex rings undergo pairing under two distinct conditions of excitation: 'the shear layer mode' when the Strouhal number based on the initial shear-layer momentum thickness (Xt,) is about 0.012, and 'the jet column mode' when the Strouhal number based on the jet diameter (Xt,) is about 0.85. The former involves pairing of the near-exit thin vortex rings when the initial boundary layer is laminar, irrespective of the value of St,. The latter involves pairing of the thick vortex rings a t x / D r 1.75, irrespective of St, or whether the initial boundary layer is laminar or turbulent. For laminar exit boundary layer, pairing is found to be stable, i.e., occurring regularly in space and time, for R e , < 5 x lo4, but becomes intermittent with increasing Re, or fluctuation intensity in the initial boundary layer.