Three-dimensional simulation of flows through a rectangular sudden expansion

Abstract

Detailed three-dimensional numerical predictions of the effect of vortex generators on vorticity dynamics and the phenomenon of axis switching are presented for the laminar incompressible flows through a rectangular sudden expansion. As a special case, the axis switching mechanism for flows through a square sudden expansion has also been investigated. The physical domains consist of 2:1 and 1:1 aspect-ratio channels which undergo a sudden expansion such that the backward facing step height is uniform, and equal to 0.75 times the minor width (or width, for the 1:1 aspect-ratio channel) of the inlet channel. The rectangular-shaped protrusions into the flow (“rectangular tabs”) are used as vortex generators, placed on the plane of the expansion. Passive forcing was provided by the system geometry and particularly due to the presence of the tabs. Suitable placement of the tabs could either stop or augment the axis switching mechanism. The rectangular vortex rings formed in the shear layer of the expanding jet undergo nonuniform self-induction and a complex three-dimensional deformation of the jet takes place in its downstream location. Such a deformation of the jet leads to the generation of streamwise vortices. Two mechanisms, the ωθ-dynamics, originated from the difference in induced velocities for different segments of a rolled-up azimuthal vortical structure; and the ωz-dynamics, caused due to the induced velocities of streamwise vortex pairs in the flow, co-exist and are identified to be active in flows through a rectangular sudden expansion. The tendency of axis switching initiated by the ωθ-dynamics are either resisted or enhanced by the ωz-dynamics. While ωθ-dynamics is known to be responsible for axis switching in periodically forced asymmetric jets, the ωz-dynamics mostly govern the effect of tabs. The results of the present simulation reveal the flow features closely consistent with the axis switching mechanism suggested by the laboratory flow visualization.