Experimental investigation of the nonlinear spectral dynamics of planar jet transition

An experimental investigation of the nonlinear spectral dynamics involved in the transition of an initially laminar planar jet shear layer at moderate Reynolds number is presented. Application of a two‐point measurement technique allows the quadratic nonlinear transfer function characterizing the local spectral dynamics to be estimated from digitized time‐series fluctuation data. This approach provides a measure of the efficiency of local nonlinear processes and allows the relevant local quadratic wave interactions characterizing the jet shear layer transition to be unambiguously detected and quantified. The results indicate that the jet shear layer shows a strong preference for difference mode interactions. These function initially to produce low frequency modes near the jet column frequency. The finite amplitude saturation of the fundamental is dominated by sum interactions which enrich the harmonic content of the spectrum. The subharmonic resonant interaction is found to be a weakly nonlinear phenomenon that involves a linear growth rate modification of the subharmonic due to its phase couple with the fundamental. Measurements indicate that the phase couple between the subharmonic and fundamental instabilities develops very near the nozzle lip. This resonant interaction between fundamental and subharmonic is found to dominate the global nonlinearity of the shear layer though it is overshadowed locally by stronger sum and difference wave interactions. Evidence is also presented which demonstrates resonant phase coupling between the disturbance sensitive region near the nozzle lip and vortex interaction events in the downstream flow, thereby indicating that a resonant feedback coupling exists between these two regions of the flow field.