RESPONSE OF THE AXISYMMETRIC MIXING LAYER TO SUBHARMONIC FORCING AT DIFFERENT AMPLITUDES.

An initially laminar axisymmetric mixing layer of a large air jet (of 27 cm diameter) has been subjected to high-amplitude forcing at the subharmonic of the natural instability frequency, f//N. The excitation is induced acoustically through an axisymmetric slit at the lip, without inducing bulk excitation of the jet exit flow. Subharmonic (f//N/2) forcing can significantly increase the shear layer width in the initial region but reduces the spread rate in the self-preserving region. On the other hand, forcing at a lower subharmonic frequency (i. e. , f//N/10) is effective in producing a significantly higher spread rate in the self-preserving region. Both the amplification rate of the velocity perturbation at the forcing frequency and its saturation amplitude are found to be distinct functions of the forcing amplitude.