Role of streamwise dynamics in spreading and mixing of flows through a rectangular sudden expansion

Abstract

Direct numerical simulations are performed to examine roles of streamwise dynamics in the spreading and mixing of flows in a two-step rectangular sudden expansion channel. The configuration is observed to facilitate higher entrainment by virtue of developed inflow type streamwise dynamics, and the system-generated passive forcing provided the necessary impetus for sustainable growth/evolution of the vortices. In addition, with the introduction and proper placement of two tiny rectangular "tabs" over the inlet walls, the downstream growth/dynamics of the vortices could be effectively modified. Through their inflow/outflow type dynamics, the streamwise vortices are found to decisively dictate the transverse jet spreading. The physical process thereby either led to a quick axis switching of the jet, or stopped axis switching altogether. However, the stretching of the azimuthal vortices is found to remain directly linked to the streamwise continuation of jet's azimuthal curvature variation. A pressure analysis as presented herein, and the simulated nature of dynamics of two azimuthal components of vortices further reveal that the transverse pressure gradient skewing is a dominant source of streamwise vorticity in such flows. With the knowledge of simulated transverse pressure distribution, we formulate here a mechanism which efficiently predicts the inception/dynamics of all the streamwise vortices. Moreover, a novel mathematical foundation in support of the pressure analysis has been outlined here, which ensures further broader universal scope of applicability of our proposed pressure analysis. (C) 2011 American Institute of Physics. [doi:10.1063/1.3623422]