This paper reviews counterflows, double counterflows, and circulation cells in vortex motion and argues that all these seemingly paradoxical phenomena can be caused by a common swirl-decay mechanism (SDM). It is shown that the SDM explains (a) the counterflow of water and oil in hydrocyclones, (b) the elongated counterflow of hot and cold air in vortex tubes, (c) the double counterflow occurring in vortex combustion chambers, and (d) the back flow in tubular chimneys of vortex units. The SDM also explains the development and disappearance of circulation cells, often referred to as vortex breakdown bubbles, in sealed cylindrical containers where the flow is driven by rotation of one end disk. The SDM also works in two-fluid flows modeling vortex bioreactors. In a few words, the SDM works as follows. Because of the balance of the centrifugal force and the radial gradient of pressure in a fast-swirling flow, the pressure at the rotation axis is smaller than that at the periphery. If the swirl decays downstream, the pressure grows along the axis. This axial gradient of pressure decelerates the near-axis flow and can reverse it, thus developing a local or global counterflow.
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