Abstract
Experimental and numerical studies are reported on the vortex breakdown phenomenon in an enclosed cylindrical container in which a top endwall rotates at an angular velocity Ω, and a bottom endwall rotates at an angular velocity Ωb, and a colinder sidewall is stationary. Regime boundaries for the vertex breakdown, the location of the stagnation point of breakdown bubble and the attendant global flow features are in good agreement between the experimental and numerical results. The flow behavior is examined an |Ωb/Ω| was varied for several combinations of the cylinder aspect ratios and the rotational Reynolds numbers based on Ω. For a co-rotation (Ωb/Ω>0), the breakdown bubble is located closer to the bottom endwall. For sufficiently large values of Ωb/Ω, the bubble adheres to the bottom endwall. When Ωb/Ω becames close to 1, the meridional-plane flow tends to become antisymmetric with respect to the midplane of the cylinder. For a counter-rotation (Ωb/Ω<0), the bubble moves toward the top endwall. Finally, as|Ωb/Ω| is further increased, the bubble ceases to exist. The Brown and Lopez criterion based on the production of the negative azimuthal vorticity is considered using the present numerical results.
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