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The three-dimensional structure of swirl-switching in bent pipe flow
Published online by Cambridge University Press: 27 November 2017
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. Here 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. 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.
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- © 2017 Cambridge University Press
References
Hufnagel et al. supplementary movie 1
Instantaneous flow field in the bent pipe depicted by pseudocolors of velocity magnitude.
Hufnagel et al. supplementary movie 2
Reconstruction of the flow field based on modes 0-2 extracted by three-dimensional POD.
Hufnagel et al. supplementary movie 3
Reconstruction of the flow field based on modes 0-4 extracted by three-dimensional POD. The large view clearly shows the travelling wave responsible for the swirl-switching, which is visible in the two cross-flow sections. See figure 5 for a static view of the modes.
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