LAGRANGE IDENTIFICATION OF ABSOLUTELY UNSTABLE REGIMES IN WAKES

The development of a pulsewise perturbation in a two-dimensional incom pressible wake is simulated numerically using a vortex dynamics method . The main attraction of using vortex methods lies in their Lagrangian nature, which in inviscid flows preserves the identity of rotational fluid elements and thereby allows for the tracking of individual vorte x lines or tubes. Because of the non-linearity of this novel approach to identify regions of local absolute and local convective instability , the analysis is not restricted to small amplitudes. Moreover the bas ic state does not have to be locally parallel, and the method is easil y applicable to unsteady flows. A comparison of the resulting stabilit y features of the symmetric wake profiles as determined by the numeric al simulation with the prediction of the Orr-Sommerfeld analysis confi rms the validity of the linear, local stability theory. An extension o f the vortex dynamics method to rotational flow fields in which the ef fects of viscosity become important is enabled by the particle strengt h exchange scheme.