An experimental investigation of the natural transition of an untuned planar jet

Abstract

The result of an experimental investigation into the natural near-field transition of an untuned planar jet at moderate Reynolds number is presented. Here the term ‘‘untuned’’ refers to the case where the ratio of the thin shear layer instability frequency to the jet column frequency is not given by an integer power of 2, so that a sequence of shear layer vortex pairing or amalgamation events is incapable of yielding the jet column frequency near the end of the potential core. This case is of interest because the jet shear layer instability must undergo more dramatic frequency and phase adjustments in order to satisfy the downstream jet column constraint. This provides a unique opportunity to investigate how instabilities that scale with the jet column interact with those that scale with the nascent shear layer instability to configure the initial evolution of the natural planar jet. Auto-bicoherence spectra are used in conjunction with conventional power spectra in order to provide quantitative measurements of the nonlinear phase coupling between wave triads that characterizes the near-field transition of the natural planar jet. These measurements are complemented by two-point correlation, coherence, and phase spectra that document the streamwise evolution and cross-stream symmetry of structural patterns in the flow throughout the initial, interaction, and early self-preserving regions. These measurements indicate that the wave interactions that characterize the planar jet near-field transition are quite different from the sequence of subharmonic instabilities that typically characterize the planar mixing layer. In particular, suppression of the subharmonic instability and the formation of modulating sidebands are observed. The modulation occurs at the jet column frequency and the measurements suggest that this has an origin that is due to a kinematic effect associated with the lateral oscillation of the nascent shear layers near the nozzle lip. The origin of this oscillation appears fully consistent with Biot–Savart induction from the downstream region of the flow associated with the loss of symmetry of the large-scale vorticity field.