Vortex dynamics and mass entrainment in a rectangular channel with a suddenly expanded and contracted part

Abstract

We report results of three-dimensional simulation of the spatially developing incompressible flow through a rectangular channel with a suddenly expanded and contracted part. The dynamics and evolution of the streamwise vortices through the expansion and contraction regions, and their influence on mass entrainment has been presented to extend our knowledge base to such flows. In addition to the observed axis-switching mechanism of the jet, profound differences as found in the flow structure on two symmetric planes, and the relatively unknown and interesting three-dimensional free-slip separation patterns that formed away from the symmetry planes, particularly in presence of tabs, are included in the presentation. Rectangular tabs, placed on the plane of expansion, generated streamwise vortices which controlled the lateral spreading of the jet and the mass entrainment through their streamwise dynamics. A complete and extensive analysis of the pressure distribution over the channel has been presented, which helps to identify the major source of streamwise vortices, and clearly explains the role played by the pressure variation over the channel in the generation of the streamwise vortices. A relation concerning relative positions of the developed local high and low pressure regions inside the channel to the streamwise dynamics of the vortices in the setup has been established which can predict the presence of all the streamwise vortices in the flow field, and characterize their nature of dynamics without any ambiguity. The results of the present simulation reveal flow features closely consistent with the vortex dynamics and the axis switching mechanism suggested by previous experimental observations. In an interesting contrast with the two-dimensional flow in a symmetric channel with a suddenly expanded and contracted part which exhibited the symmetry breaking flow bifurcations, the corresponding three-dimensional flow investigated here remained symmetric and surprisingly without much sign of recirculation on the minor axis plane.