Coherent structures in turbulent pipe flow

Abstract

The presence of organized motions in turbulent wall-bounded flows has been explored in progressively greater detail for more than half a century. It is only within recent years that the largest of these structures - the so-called very large-scale motions - has been recognized for its important energetic and shear stress content.

As we gain more understanding of these motions and their contribution to the production of turbulence and their linkage to the mean flow and turbulence intensities, their further exploration become even more intriguing.

The work presented in this thesis investigates the two largest of the known coherent structures - the large-scale motions (LSM) and the very large-scale motions (VLSM) - and their relationship and contribution to the known statistical view of turbulence. To this end, I report an analysis of experimentally acquired data sets in turbulent pipe flow. The turbulent flow is broken down into a set of energetic modes using proper orthogonal decomposition (POD), where each mode can be argued to represent a coherent structure, or at least one phase of its evolution. The results support the existing understanding that these structures are energetically important with a large shear stress contribution. The work also provides a clear link between the large-scale and very large-scale motions, suggesting that the latter is composed of a streamwise pseudo-alignment of the shorter large-scale motions. This result is the principal conclusion of the thesis.

The POD analysis is also expanded to include flow structures induced by pipe curvature. These structures are shown to completely overwhelm the underlying turbulent structures and are shown to exhibit an unsteady behavior, governed by a single cell vortex structure of alternating rotational direction, referred to as "swirl switching"

This motion is suggested to be a governed by an alternating suppression of one of the cells of the Dean motion; a steady, dual cell solution with each cell located on either side of the bend symmetry plane.