Generation (or Degeneration) of a Separation Bubble in a Liquid-Filled Cylinder Through Spin-Up (or Spin-Down) Process

Abstract

The present study deals with the spin-up and spin-down of viscous incompressible fluid confined in a cylindrical enclosure. The spin-up (or spin-down) is developed by increasing (or decreasing) abruptly the angular velocity of the upper endwall where initial steady flow is either due to uniform rotation of the lower endwall or due to rotation of both end walls in the same direction with small rotation ratio of the top-bottom endwalls. We have seen numerically that through spin-up, an on-axis separatiorbubble can be generated (vortex breakdown) in a flow when the Reynolds number is much lower than the critical value of the Reynolds number (Re)c for the onset of vortex breakdown. At the ultimate steady state the separation bubble appears and a waviness in the stream surfaces in the vicinity of the lower endwall is found. The spin-down process acts in the opposite way. We found that for the flow with Reynolds number above the critical value (Re)c , the vortex breakdown can be prevented through spin-down and the ultimate steady-state flow is also free from it.