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Hufnagel, Lorenz
Publications (2 of 2) Show all publications
Schlatter, P., Hufnagel, L., Canton, J., Merzari, E., Marin, O. & Örlü, R. (2017). Swirl Switching in bent pipes studied by numerical simulation. In: 10th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2017: . Paper presented at 10th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2017, Swissotel ChicagoChicago, United States, 6 July 2017 through 9 July 2017. International Symposium on Turbulence and Shear Flow Phenomena, TSFP10
Open this publication in new window or tab >>Swirl Switching in bent pipes studied by numerical simulation
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2017 (English)In: 10th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2017, International Symposium on Turbulence and Shear Flow Phenomena, TSFP10 , 2017Conference paper (Refereed)
Abstract [en]

Turbulent flow through pipe bends has been extensively studied, but several phenomena still miss an exhaustive explanation. Due to centrifugal forces, the fluid flowing through a curved pipe forms two symmetric, counter-rotating Dean vortices. It has been observed, experimentally and numerically, that these vortices change their size, intensity and location in a quasi-periodic, oscillatory fashion, a phenomenon known as swirl-switching. These oscillations are responsible for failure due to fatigue in pipes, and their origin has been attributed to a recirculation bubble, disturbances coming from the upstream straight section and others. The present study tackles the problem by direct numerical simulations (DNS) of turbulent pipe flow at moderate Reynolds number, analysed, for the first time, with three-dimensional proper orthogonal decomposition (POD) in an effort to distinguish between the spatial and temporal contributions to the oscillations. The simulations are performed at a friction Reynolds number of about 360 with a divergence-free synthetic turbulence inflow, which is crucial to avoid the interference of low-frequency oscillations generated by a standard recycling method. Two different bends are considered, with curvature 0.1 and 0.3, preceded and followed by straight pipe segments. Our results indicate that a single low-frequency, three-dimensional POD mode is responsible for the swirl-switching. This mode represents a travelling wave, and was previously mistaken by 2D POD for two different modes. Low-order reconstruction clearly shows that the upstream turbulent flow does not play a role for the swirl-switching.

Place, publisher, year, edition, pages
International Symposium on Turbulence and Shear Flow Phenomena, TSFP10, 2017
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-217853 (URN)2-s2.0-85033214965 (Scopus ID)9780000000002 (ISBN)
Conference
10th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2017, Swissotel ChicagoChicago, United States, 6 July 2017 through 9 July 2017
Note

QC 20171117

Available from: 2017-11-17 Created: 2017-11-17 Last updated: 2017-11-17Bibliographically approved
Hufnagel, L., Canton, J., Örlü, R., Marin, O., Merzari, E. & Schlatter, P. (2017). The three-dimensional structure of swirl-switching in bent pipe flow. Journal of Fluid Mechanics, 835, 86-101
Open this publication in new window or tab >>The three-dimensional structure of swirl-switching in bent pipe flow
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2017 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 835, p. 86-101Article in journal (Refereed) Published
Abstract [en]

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 degrees 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-tau approximate to 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.

Place, publisher, year, edition, pages
Cambridge University Press, 2017
Keywords
pipe flow boundary layer, turbulence simulation, turbulent flows
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-220487 (URN)10.1017/jfm.2017.749 (DOI)000416940800003 ()2-s2.0-85038601971 (Scopus ID)
Funder
Swedish Research CouncilSwedish e‐Science Research Center
Note

QC 20180103

Available from: 2018-01-03 Created: 2018-01-03 Last updated: 2018-05-21Bibliographically approved
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