Analysis of the secondary vortices in rotating ow induced by a twisted tape inside a pipe

Flow through a twisted tape (swirler) creates a complicated vortex structure downstream in the pipe. Detailed velocity measurements with Laser Doppler Velocimetry (LDV) along horizontal and vertical axes perpendicular to the axial ow direction have shown a strange ow pattern at the center of the rotating ow{a counter-rotating vortex seems to be present at the center with periodically varying magnitude in the axial direction. In highly detailed measurements, it is shown that this behavior results from a pair of co-rotating secondary vortices that are superimposed on the primary rotating ow in a helical formation. The source of these secondary vortices has remained unclear. This study presents numerical simulations of the ow through 180 twisted tape in a pipe, complementing the previous experimental results. The simulations reproduce the characteristics of the helical co-rotating vortices observed in experiments and provide details of the ow eld. The results provide an insight into the formation of the secondary vortices inside the twisted tape, thus explaining the experimental observations. The mechanism of the vortex formation is described, showing that the secondary co-rotating helical vortices are produced by a pair of single co-rotating vortex formed on each side of the twisted tape.