Influence of aspect ratio on vortex formation in X-junctions: direct numerical simulations and eigenmode decomposition

CORREA, P. G.; GOMBA J. M.; MAC INTYRE J. R.; UBAL S.; PERAZZO, CARLOS ALBERTO; HULIN J. P.; AURADOU H.

PHYSICS OF FLUIDS

AMER INST PHYSICS

Lugar: New York; Año: 2020

1070-6631

We study numerically the appearance and number of axial vortices in the outletsof X-shaped junctions of two perpendicular channels of rectangular sections withfacing inlets. We explore the effect of the aspect ratio of the cross section,AR,on the number of vortices created at the center of the junction. Direct numericalsimulations (DNS) performed for different values of the Reynolds numberReandARdemonstrate that vortices with their axis parallel to the outlets, referred to asaxial vortices, appear above critical Reynolds numbersRec. AsARincreases from1 to 11, the number of vortices observed increases from 1 to 4, independently ofRe.ForAR= 1, the single axial vortex induces an interpenetration of the inlet fluidsin the whole section; instead, for largerAR?s for which more vortices appear, thetwo inlet fluids remain largely segregated in bands, except close to the vortices. Thelinear stability analysis demonstrates that only one leading eigenmode is unstable fora given set of values ofARandRe. This mode provides a simplified model of the flowfield, reproducing its key features such as the number of vortices and their distance.Its determination with this method requires a much smaller computational load thanthe DNS. This approach is shown to allow one to determine quickly and precisely thecritical Reynolds numberRecand the sensitivity functionSwhich characterizes theinfluence of variations of the base flow on the unstable one.