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Flow Instabilities and Vortex Dynamics in Intersecting Flows
In the proximity of stagnation points, flow instabilities tend to arise at relatively low Reynolds number (Re). These instabilities often result in vortices that can evolve time-periodic patterns as Re is increased. These types of flows are well studied in cases for which the stagnation point is fixed on an obstacle and the resulting vortices are in the spanwise direction (e.g. the von Kàrmàn vortex street). However, they are less understood at flow intersections, where the stagnation point is not wall-attached and the resulting vortices are stretched by the flow in the streamwise direction. Due to the intermittent nature of these vortices it is a challenge to control and study their dynamics. Here, we induce vortex formation in 4-way intersections, in which the onset of flow instability is highly sensitive to small changes of the experimental parameters (i.e., channel depth:width ratio α, fluid properties and the Reynolds number, Re). Microfluidic cross-slot geometries, with a novel configuration, are fabricated by selective laser-induced etching in fused silica glass, enabling quantitative flow velocimetry measurements at the cross-section of the intersecting region. Our experiments, supported by numerical simulations, show that by tuning α, Re and and fluid elasticity we gain precise control over vortex intensity, core structure, merging-splitting dynamics and periodic fluctuations of the vortical flow field at the intersection.
These experiments capture fundamental processes that govern flow transitions and provide important insights into the mechanisms of symmetry breaking, vortex dynamics and of turbulent drag reduction by polymer additives. Our findings contribute to the improvement of flow control and advancement of applicable technologies in which vortex suppression is required (i.e., stabilization of structures), or when vortex induced motion is desired (i.e., energy harvesting, mixing enhancement) and are transferable to systems with similar flow behavior (i.e., Taylor-Couette apparatus, T-channels, flows around obstacles and at the wake of airplane wings)