Spiral shear layers: Roll-up and incipient instability

The structure and roll-up of thin shear layers generated by the impulsive motion of a plate is investigated through experiments and numerical computations. At large Reynolds numbers, the shear layer rolls up into a self-similar spiral, compatible with a power-law geometry, to which an oscillation is superimposed. This modulation is also manifest in the distributions of the circulation density and the strain rate, which are found to be nonmonotonic along the layer. A sequence of stretching and compression regions is observed, critical for the stability of the sheet. (C) 2005 American Institute of Physics.