An experimental investigation into the role of simultaneous amplitude and phase modulation in the transition of a planar jet

An experimental investigation into the role of simultaneous amplitude and phase modulation of velocity fluctuations characterizing the near-field transition of an initially laminar planar jet at moderate Reynolds number is presented. Very low level periodic acoustic excitation at selected frequencies is applied upstream of the nozzle exit in order to introduce an organized carrier wave into the flow. Instantaneous amplitude and phase modulates are computed directly from hot-wire velocity fluctuation data which allows their respective role in the jet transition process to be quantified. It is found that amplitude modulation serves to provide energy to amplifying sideband fluctuations which multitone phase modulation effectively redistributes to neighboring spectral modes in the nonlinear stages of transition. The phase modulation is found to result from an experimentally verified amplitude dependence in the dispersion relation. The phase angle between amplitude and phase modulation is found to be an important parameter in governing the spectral development of the shear layer by its effect on sideband asymmetry. While the essential physics associated with the modulation phenomenon is found to be quite similar to that previously reported in plane wakes, the presence of the subharmonic resonant interaction in the planar jet shear layer adds an additional dimension to the modulation phenomenon.