An experimental investigation of the transition of a planar jet: Subharmonic suppression and upstream feedback

An experimental investigation into the transition of an initially laminar planar jet at moderate Reynolds number is presented. Very low level acoustic excitation is applied upstream of the nozzle exit in order to organize the initial instability wave and thereby facilitate investigation of its downstream evolution. Particular attention is focused upon quantifying the quadratic nonlinear wave interactions that characterize the near-field transition as evidenced by measurements of phase locking between resonant wave triads. The spectral evolution of fluctuations in the planar jet shear layer indicate that the subharmonic is significantly suppressed. This subharmonic suppression was found to result from an interesting combination of the dynamics of subharmonic resonance and the upstream feedback of perturbations from the interaction region beyond the jet potential core to the disturbance sensitive region near the nozzle lip. The upstream feedback was found to be associated with the large-scale restructuring of the flow that characterizes the interaction region of the planar jet and results in the loss of symmetry of the coherent vorticity field with respect to the centerline. The imposed near-exit perturbations interact nonlinearly with the shear layer instability wave to produce the observed suppression. The interplay between the initial and interaction regions of the flow provides a unique glimpse of the rich behavior that even seemingly simple flows can exhibit during transition to turbulence.