Vortex breakdown in the shear-driven flow in a rectangular cavity

The vortex dynamics of laminar flow past a rectangular cavity is investigated using numerical simulations and microfluidic experiments. The flow is inherently three-dimensional and is characterized by a large, dominant vortex structure that fills most of the cavity at moderate Reynolds numbers with a weak, yet significant, flow in the axial direction along the vortex core. Classical bubble-type vortex breakdown is observed within the cavity above a certain critical Reynolds number, which is a function of the channel width. The critical Reynolds number for the onset of breakdown is determined as a function of the channel width, and the evolution and dynamical transitions of the breakdown regions are investigated as functions of the channel width and Reynolds number. At large cavity widths, two vortex breakdown bubbles emerge near the sidewalls symmetric about the center plane, which grow and eventually merge as the Reynolds number increases. For large-enough widths, the vortex breakdown regions remain well separated, and their structures become independent of the cavity width. The stability and bifurcations of the stagnation points and their transitions to stable and unstable limit cycles are analyzed. At the intermediate width regime, a single vortex breakdown bubble appears above the critical Reynolds number. In the narrow width regime, the flow exhibits more complicated modes. An additional vortex breakdown mode with reversed flow patterns is observed in this width regime, along with multiple shifts in the stability of stagnation points. The experimental and numerical results also demonstrate the sensitivity of the flow to the inlet conditions, such that relatively small asymmetries upstream can result in significant changes to the vortex breakdown behavior in the cavity.