  Twisted tapes are used to induce swirling flow and improve mixing. The flow
induced by a 180 degree twisted tape with length (pitch) 60 mm and diameter
25.4 mm in a circular pipe was investigated using Laser Doppler Velocimetry
(LDV) measurements. Tangential velocity profiles downstream of the twisted tape
swirler were measured at multiple locations along the pipe axis, across the
horizontal diameter of the pipe. The profiles showed an unexpected transition
along the pipe axis from regular swirling flow to an apparent counter-rotation
near the pipe axis, and then reverting back to regular swirling flow. Injecting
fine air bubbles into the flow showed the existence of two co-rotating helical
vortices superimposed over the main swirling flow. The close proximity of the
two co-rotating vortices creates the local reversing flow at the pipe
centerline. The secondary vortices are analyzed with high speed camera videos
and numerical simulations.
