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Turbulent wake behind a concave curved cylinder

Published online by Cambridge University Press:  18 September 2019

Fengjian Jiang Open the ORCID record for Fengjian Jiang [Opens in a new window] ,
Bjørnar Pettersen  and
Helge I. Andersson
Fengjian Jiang*
Affiliation:
Department of Marine Technology, Norwegian University of Science and Technology (NTNU), 7491 Trondheim, NO-7491, Norway
Bjørnar Pettersen
Affiliation:
Department of Marine Technology, Norwegian University of Science and Technology (NTNU), 7491 Trondheim, NO-7491, Norway
Helge I. Andersson
Affiliation:
Department of Energy and Process Engineering, Norwegian University of Science and Technology (NTNU), 7491 Trondheim, NO-7491, Norway
*
†Email address for correspondence: fengjian.jiang@outlook.com
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Abstract

We present a detailed study of the turbulent wake behind a quarter-ring curved cylinder at Reynolds number $Re=3900$ (based on cylinder diameter and incoming flow velocity), by means of direct numerical simulation. The configuration is referred to as a concave curved cylinder with incoming flow aligned with the plane of curvature and towards the inner face of the cylinder. Wake flows behind this configuration are known to be complex, but have so far only been studied at low $Re$. This is the first direct numerical simulation investigation of the turbulent wake behind the concave configuration, from which we reveal new and interesting wake dynamics, and present in-depth physical interpretations. Similar to the low-$Re$ cases, the turbulent wake behind a concave curved cylinder is a multi-regime and multi-frequency flow. However, in addition to the coexisting flow regimes reported at lower $Re$, we observe a new transitional flow regime at $Re=3900$. The flow field in this transitional regime is dominated not by von Kármán-type vortex shedding, but by periodic asymmetric helical vortices. Such vortex pairs exist also in some other wake flows, but are then non-periodic. Inspections reveal that the periodic motion of the asymmetric helical vortices is induced by vortex shedding in its neighbouring oblique shedding regime. The oblique shedding regime is in turn influenced by the transitional regime, resulting in a unified and remarkably low dominating frequency in both flow regimes. Owing to this synchronized frequency, the new wake dynamics in the transitional regime might easily be overlooked. In the near wake, two distinct peaks are observed in the time-averaged axial velocity distribution along the curved cylinder span, while only one peak was observed at lower $Re$. The presence of the additional peak is ascribed to a strong favourable base pressure gradient along the cylinder span. It is noteworthy that the axially directed base flow exceeded the incoming velocity behind a substantial part of the quarter-ring and even persisted upwards along the straight vertical extension. As a by-product of our study, we find that a straight vertical extension of 16 cylinder diameters is required in order to avoid any adverse effects from the upper boundary of the flow domain.

JFM classification

Wakes/Jets: Wakes Vortex Flows: Vortex shedding Wakes/Jets: Separated flows
Type
JFM Papers
Information
Journal of Fluid Mechanics , Volume 878 , 10 November 2019 , pp. 663 - 699
Copyright
© 2019 Cambridge University Press 

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Jiang et al. supplementary movie 1

A movie of instantaneous cross-flow velocity distribution in the symmetry plane.

Download Jiang et al. supplementary movie 1(Video)
Video 8.7 MB

Jiang et al. supplementary movie 2

Wake dynamics in the z/D = -7.0 plane.

Download Jiang et al. supplementary movie 2(Video)
Video 4 MB

Jiang et al. supplementary movie 3

Wake dynamics in the z/D = -6.0 plane.

Download Jiang et al. supplementary movie 3(Video)
Video 5.8 MB

Jiang et al. supplementary movie 4

Wake dynamics in the x/D = 14.0 plane.

Download Jiang et al. supplementary movie 4(Video)
Video 8.1 MB