DNS of swirling turbulent pipe flow

A new flow problem is designed with the view to explore the effects of swirl on turbulent pipe flow. A tailor-made azimuthal mean velocity is produced by means of a prescribed circumferential mean pressure gradient. The resulting flow field resembles that of an axially rotating pipe, but offers a greater flexibility with respect to swirl topology. Results from direct numerical simulations of fully developed turbulent pipe flow with swirl are presented. Three different strengths of the swirl have been considered and the results are compared with the data from the conventional pipe flow. The induced azimuthal mean velocity induces a skewing of the coherent near-wall structures. The flow resistance is increased in the low-swirl case, whereas a drag reduction is observed with stronger swirl. This interesting flow phenomenon is interpreted by means of the Reynolds stress components, joint pdfs of the fluctuating velocity components, as well as the helicity. The low-swirl case exhibits a qualitatively different helicity distribution in the near-wall region as compared with the cases with stronger swirl. The skewing of the coherent near-wall structures alone is not sufficient to reduce the wall-friction. Copyright (C) 2009 John Wiley & Sons, Ltd.