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TURBULENT PIPE FLOW THROUGH A 90° BEND

Stereoscopic PIV is used to investigate the structure of fully developed turbulent pipe-flow through a 90◦ bend. The results show that Dean motions downstream of the bend are very weak in the mean, and time resolved stereoscopic PIV demonstrates that the Dean motions co-exist with large scale swirling motions that switch sign and do not contribute to the mean. Snapshot POD analysis of the flow field reveals that the structures found upstream of the bend persist after the bend and survive the strong secondary motions induced by the pipe curvature. In particular, the Very Large Scale Motions (VLSM) are no longer the most energetic mode in the flow, but they are still the most energetic structure in terms of the streamwise fluctuations. INTRODUCTION Turbulent flow in pipe bends is of great practical interest due to the associated pressure losses and the distortion of the velocity profile. Bends tend to introduce secondary flows that lead to scour, and non-uniform heat transfer (Berger et al. 1983). Much research has been done in the area of secondary flows, as it is a phenomena related to many fluid problems, including flow through heat exchangers, industrial plant piping systems, scouring and river meandering. Early studies showed that secondary vortices in the streamwise direction downstream of the bend (Harlock 1955), the so-called Dean motions (Dean 1928). The strength of the secondary motions is often described using the Dean number, which is the ratio of the square root of the product of the inertia and centrifugal forces to the viscous force. That is,