Free shear flows - Organized structures and effects of excitation

Recent studies of free shear flows are reviewed. Included are experimental studies of: excited and unexcited circular and elliptic jets, plane wake and mixing layers, and effects of excitation on augmentation and reduction of jet turbulence and jet noise. It is shown that proper excitation can produce both large increases and large decreases in turbulence and Reynolds stress level, mixing and noise generation, suggesting promising technological applications of self-excited jets such as whistler jets. It is also argued that 'ribs' or longitudinal vortices play an important role in production and mixing in free, and perhaps all, turbulent shear flows. The 'cut-and-connect' interaction of vortical structures is proposed as a key mechanism for coherent structure breakdown, mixing, turbulence production, and aerodynamic noise generation. A simple analysis of the viscous vorticity equation using symmetry of configurations of vortical structures before and after the interaction gives realistic predictions.