Numerical simulation of the formation of two and three-dimensional structures in shear layers and wakes

The nonlinear formation of two and three-dimensional structures in nominally two-dimensional shear layers and wakes was numerically investigated, assuming spatially periodic flow. The vorticity dynamics was simulated using the Biot-Savart law. The instability of the braid region, the translative instability of the vortex cores, and the helical pairing mechanism were analyzed for the simple shear layer. The wake flow was investigated with regard to possible changes in the topological structure of the vorticity field. Comparisons with experimental observations indicate the importance of subharmonic perturbations in the wake; their origin is, however, unclear. Further investigations are necessary to determine the effects of friction.