Do large structures control their own growth in a mixing layer? - An assessment

Two different two-dimensional free shear layers, the T-layer developing in time from an initial tangential velocity discontinuity separating the two half-spaces, and the S-layer which develops downstream of the origin where two uniform streams of unequal velocity are brought into tangential contact, are compared. Calculations are performed in order to determine to what extent the perturbations induced upstream by large concentrations of vorticity found downstream hasten or retard the subharmonic instability that leads to the formations of these large structures. The results show that the elliptic influence, or the feedback, in a mixing layer is relatively small for small velocity ratios.