Generation of streamwise vortices in square sudden-expansion flows

Abstract

The intent of the present work is to investigate the nature of jet spreading and the process of evolution of the associated embedded streamwise vortices for the steady flow through a two-step square sudden expansion. Simulations are performed to review the flow physics within a square channel which undergoes a first expansion with an uniform step height $0.75h$ ($h$ being the inlet channel width); and at a streamwise distance $8h$ from the plane of first expansion the channel goes through another expansion with the second step height being half of the first step height. Unlike asymmetric jets, the square jet is observed to experience relatively faster nonuniform azimuthal perturbations during its streamwise evolution and at some downstream location the jet expands in such a way that it looks as if it has locally rotated by $45\ifmmode^\circ\else\textdegree\fi{}$. The developed four pairs of outflow type streamwise vortices (with each pair occupying their position at the end of a jet diagonal), which dominated over the first expansion zone, seems to control the azimuthal jet deformation process through their induced outward velocity. Another important aspect of the present investigation is that here we have established a unique pressure analysis which efficiently predicts the presence of all the streamwise vortices in the setup and also their nature of dynamics without any ambiguity. Moreover, the presented pressure analysis suggests that nonuniform lateral flow acceleration within the channel, as induced by the developed transverse pressure gradient skewing, influences the generation of the streamwise vortices. The pressure analysis also successfully predicts every local change in the dynamics of the embedded streamwise vortices, during the downstream evolution of the jet.