Spatially periodic reversing core in a twisted-fin generated swirling pipe flow

Abstract

Experimental results for swirling turbulent flow in a pipe, generated by a 180° twisted fin inside the pipe, are presented. The results show that the core region undergoes a spatially periodic change in direction of rotation from counter-rotating to co-rotating and back to counter-rotating flow relative to the main swirling flow. Up to four transitions in direction of rotation have been recorded with a two-component laser-Doppler velocimeter downstream of the swirl generating fin. The Reynolds number is varied from 25 000 to 85 000 and the average swirl number varies from 0.5 to 0.25 downstream of the fin. The underlying cause of the periodicity in the direction of rotation of flow in the core region is conjectured to be based on the secondary flows generated by a pair of co-rotating helical vortices forming upstream at the spiral-shaped swirl generating fin.