Abstract

Free shear flows, such as mixing layers, jets and wakes, are inviscidly unstable due to their inflectional velocity profiles. Instability modes, which are usually excited by external perturbations, amplify on the shear flow, leading to vortex roll-up and randomization in the nonlinear stage. Interestingly, in turbulent state free shear flows exhibit a high degree of order, characterised by the prevalent presence of so-called coherent structures, the most striking of which are ‘Brown-Roshko rollers’.

Instability waves and coherent structures are known to be dynamically significant for entrainment and mixing, noise generation as well as for turbulence modelling. In this talk, I will present a nonlinear critical-layer theory to describe first the development of instability modes on laminar free shear layers. The theory predicts vortex roll-up and randomisation through a generalized side-band instability mechanism. The theory is then modified to describe formation and evolution of ‘Brown-Roshko rollers’ on turbulent mixing layers. The mechanism of acoustic radiation by coherent structures will be highlighted.