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Wake behind a discontinuous cylinder: unveiling the role of the large scales in wake growth and entrainment

Published online by Cambridge University Press:  07 February 2022

V.S.R. Mandava Open the ORCID record for V.S.R. Mandava [Opens in a new window] ,
Joan Herrero ,
Gregory A. Kopp  and
Francesc Giralt Open the ORCID record for Francesc Giralt [Opens in a new window]
V.S.R. Mandava
Affiliation:
Departament d'Enginyeria Química, Universitat Rovira i Virgili, Tarragona, Catalunya43007, Spain
Joan Herrero
Affiliation:
Departament d'Enginyeria Química, Universitat Rovira i Virgili, Tarragona, Catalunya43007, Spain
Gregory A. Kopp
Affiliation:
Boundary Layer Wind Tunnel Laboratory, Faculty of Engineering, University of Western Ontario, London, ONN6A 5B9, Canada
Francesc Giralt*
Affiliation:
Departament d'Enginyeria Química, Universitat Rovira i Virgili, Tarragona, Catalunya43007, Spain
*
†Email address for correspondence: fgiralt@urv.cat
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Abstract

The turbulent flow in the wake of a discontinuous cylinder (DC) was investigated. The DC geometry consisted of cylinder segments $5D$ long (with $D$ being the diameter of the cylinder) separated by gaps of width $2.5D$. Particle image velocimetry and hot-wire anemometry were used to analyse the flow at two Reynolds numbers, $Re = 4000$ and $10\,000$, for $x/D \le 180$. Large eddy simulations for both the DC and the infinite continuous cylinder (CC) wakes were also carried out at $Re = 10\,000$. The DC configuration was devised to trigger the shedding of horseshoe vortices (HSV) in the very-near-wake region with the intent of illustrating the role that these three-dimensional HSVs, previously identified in the far-wake region of CC, play in the entrainment process in turbulent wakes. The DC geometry produced HSVs by the interaction between the high momentum flow through the gaps and the main spanwise vortex shed behind each cylinder segment, while in the CC wake they evolve from near-wake instabilities straddled with hairpin vortices that detach spanwise vorticity from the shed Kármán vortices. The DC wake was found to grow and spread in the transverse direction with a much faster rate than for the CC wake, up until approximately $x/D \approx 50$. Prior to this location, the enhanced growth rate caused by the shear-aligned HSV led to a wake width of approximately 3 times that of the CC wake, with a maximum mean velocity deficit that was approximately half.

JFM classification

Vortex Flows: Vortex shedding Wakes/Jets: Wakes Turbulent Flows: Turbulent convection
Type
JFM Papers
Information
Journal of Fluid Mechanics , Volume 936 , 10 April 2022 , A4
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Copyright
© The Author(s), 2022. Published by Cambridge University Press

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References

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