Vortex Breakdown

FERGUL - Fluids Engineering Research Group at the University of Liverpool.

The video shows the development of vortex breakdown in a swirling pipe flow.  The experiment consists of two smoothly joined pipe sections that can be rotated independently about the same axis.  In this clip, the pipe on the left hand side of the video is rotating whilst the pipe on the right hand side remains stationary.  The flowrate through the pipe remains fixed (giving a Reynolds number of 25 based on the average velocity and pipe diameter) whilst the rotational speed of the left pipe is slowly increased.  Once the swirl imparted to the flow goes above a critical value then flow reversal - so-called "vortex breakdown" occurs.

D. J. C. Dennis, C. Seraudie and R. J. Poole "Controlling vortex breakdown in swirling pipe flows: Experiments and simulations". Phys. Fluids 26, 053602 (2014)
ABSTRACT
A laminar, incompressible, viscous pipe flow with a controllable swirl induced by wall rotation has been studied both numerically and experimentally up to an axial Reynolds number (Re) of 30. The pipe consists of two smoothly joined sections that can be rotated independently about the same axis. The circumstances of flow entering a stationary pipe from a rotating pipe (so-called decaying swirl) and flow entering a rotating pipe from a stationary pipe (growing swirl) have been investigated. Flow visualisations show that at a certain swirl ratio the flow undergoes a reversal and vortex breakdown occurs. The variation of this critical swirl ratio with Reynolds number is explored and good agreement is found between the experimental and numerical methods. At high Re the critical swirl ratio tends to a constant value, whereas at low Re the product of the Reynolds number and the square of the swirl ratio tends to a constant value in good agreement with an existing analytical solution. For decaying swirl the vortex breakdown manifests itself on the pipe axis, whereas for growing swirl a toroidal zone of recirculation occurs near the pipe wall. The recirculating flow zones formed at critical conditions are found to increase radially and axially in extent with increasing Reynolds number and swirl ratio.

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