Nonlinear roll-up of externally excited free shear layers

The effects of strong critical-layer nonlinearity on the spatially growing instabilities of a shear layer between two parallel streams are considered. A composite expansion technique is used to obtain a single formula that accounts for both shear-layer spreading and nonlinear critical-layer effects. Nonlinearity causes the instability to saturate well upstream of the linear neutral stability point. It also produces vorticity roll-up that cannot be predicted by linear theory.

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