Title: The three-dimensional structure of swirl-switching in bent pipe flow

Abstract

Swirl-switching is a low-frequency oscillatory phenomenon which affects the Dean vortices in bent pipes and may cause fatigue in piping systems. Despite thirty years worth of research, the mechanism that causes these oscillations and the frequencies that characterise them remain unclear. In this paper, we show that a three-dimensional wave-like structure is responsible for the low-frequency switching of the dominant Dean vortex. The present study, performed via direct numerical simulation, focuses on the turbulent flow through a $$90^{\circ }$$pipe bend preceded and followed by straight pipe segments. A pipe with curvature 0.3 (defined as ratio between pipe radius and bend radius) is studied for a bulk Reynolds number $$Re=11\,700$$, corresponding to a friction Reynolds number $$Re_{\unicode[STIX]{x1D70F}}\approx 360$$. Synthetic turbulence is generated at the inflow section and used instead of the classical recycling method in order to avoid the interference between recycling and swirl-switching frequencies. The flow field is analysed by three-dimensional proper orthogonal decomposition (POD) which for the first time allows the identification of the source of swirl-switching: a wave-like structure that originates in the pipe bend. Contrary to some previous studies, the flow in the upstream pipe does not show any direct influence on the swirl-switching modes. Finally, our analysis further shows that a three-dimensional characterisation of the modes is crucial to understand the mechanism, and that reconstructions based on two-dimensional POD modes are incomplete.

Authors:

Hufnagel, Lorenz ^[1]; Canton, Jacopo  ^[1];

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Örlü, Ramis  ^[1];

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Marin, Oana ^[2]; Merzari, Elia ^[2]; Schlatter, Philipp  ^[1]

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+ Show Author Affiliations

1. KTH Royal Inst. of Technology, Stockholm (Sweden). Linne FLOW Centre. Swedish e-Science Research Centre (SeRC). KTH Mechanics
2. Argonne National Lab. (ANL), Argonne, IL (United States). Mathematics and Computer Science Division

Publication Date:

2017-11-27

Research Org.:

Argonne National Laboratory (ANL), Argonne, IL (United States); KTH Royal Inst. of Technology, Stockholm (Sweden)

Sponsoring Org.:

USDOE Office of Science (SC); Swedish Research Council (VR)

OSTI Identifier:

1427509

Grant/Contract Number:  

AC02-06CH11357

Resource Type:

Accepted Manuscript

Journal Name:

Journal of Fluid Mechanics

Additional Journal Information:

Journal Volume: 835; Journal ID: ISSN 0022-1120

Publisher:

Cambridge University Press

Country of Publication:

United States

Language:

English

Subject:

42 ENGINEERING; 71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS; turbulence simulation; pipe flow boundary layer