Experiments on subharmonic resonance in a shear layer

The subharmonic resonance phenomenon is studied using hot-wire measurements and flow visualization in an initially laminar shear layer forced with two-frequencies for various choices of the fundamental frequency f and its subharmonic f/2 with controlled initial phase difference φ_in between them. We explore the effects of the controlling parameters, namely: (i) forcing frequencies and their initial amplitudes, (ii) initial phase difference φ_in, and (iii) detuning (i.e. when the second forcing frequency is slightly different from f/2). While several of our experimental observations support predictions based on weakly nonlinear theory, others do not. We explain our data in terms of vortex dynamics concepts.