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Numerical investigation of flow and turbulence structure through and around a circular array of rigid cylinders

Published online by Cambridge University Press:  06 July 2015

Kyoungsik Chang  and
George Constantinescu
Kyoungsik Chang
Affiliation:
Department of Aeromechanical Engineering, Hanseo University, 356-706, South Korea
George Constantinescu*
Affiliation:
Department of Civil and Environmental Engineering, The University of Iowa, IA 52246, USA
*
†Email address for correspondence: [email protected]
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Abstract

This numerical study investigates flow and turbulence structure through and around a circular array of solid circular cylinders of diameter $d$. The region containing the array of rigid cylinders resembles a porous circular cylinder of diameter $D$. The porous cylinder Reynolds number defined with the steady incoming flow velocity is $\mathit{Re}_{D}=10\,000$. Fully three-dimensional (3D) large eddy simulations (LES) are conducted to study the effects of the volume fraction of solids of the porous cylinder ($0.023<\text{SVF}<0.2$) and $d/D$ on the temporal variation and mean values of the drag/lift forces acting on the solid cylinders and on the porous cylinder. The effects of the bleeding flow through the circular porous cylinder on the wake structure and the influence of the SVF and $d/D$ on the onset of flow three-dimensionality within or downstream of the porous cylinder and transition to turbulence are discussed. Results are compared with experimental data, predictions of theoretical models available in the literature and also with the canonical case of a solid cylinder ($\text{SVF}=1,d/D=1$). Three-dimensional LES predict that large-scale wake billows are shed in the wake of the porous cylinder for $\text{SVF}>0.05$, similar to the von Karman vortex street observed for solid cylinders. As the SVF decreases, the length of the separated shear layers (SSLs) of the porous cylinder and the distance from the back of the porous cylinder at which wake billows form increase. For sufficiently low volume fractions of solids (e.g. $\text{SVF}=0.05$, 0.023), no wake billows are shed and the interactions among the wakes of the solid cylinders are weak. Even for $\text{SVF}=0.023$, SSLs containing large-scale turbulent eddies form on the two sides of the porous cylinder, but their ends cannot interact to generate wake billows. In both regimes, the force acting on some of the solid cylinders within the array is highly unsteady. As opposed to results obtained based on 2D simulations, no intermediate regime in which the force acting on the solid cylinders is close to steady is present. Interestingly, an energetic low frequency corresponding to a Strouhal number defined with the diameter of the porous cylinder of approximately 0.2 is present within the porous cylinder and near-wake regions not only for cases where wake billows are generated but also for cases where no wake billows form. In the latter cases, this frequency is due to an instability acting on the SSLs which induces in-phase large-scale undulatory deformations of the two SSLs. A combined drag parameter for the porous cylinder ${\it\Gamma}_{D}=\overline{C}_{d}\,aD/(1-\text{SVF})$ is introduced, where $aD$ is the non-dimensional frontal area per unit volume of the porous cylinder. This parameter characterizes by how much the velocity of the bleeding flow at the back of the porous cylinder is reduced compared with the incoming flow velocity for a given total drag force acting on the porous cylinder. Results from simulations conducted with different values of the SVF, $d/D$ and mean time-averaged solid cylinder streamwise drag parameter, $\overline{C}_{d}$, show that ${\it\Gamma}_{D}$ increases monotonically with increasing $aD$. Several ways of defining the spatial extent of the wake region in a less ambiguous way are proposed.

JFM classification

Vortex Flows: Vortex shedding Wakes/Jets: Wakes
Type
Papers
Information
Journal of Fluid Mechanics , Volume 776 , 10 August 2015 , pp. 161 - 199
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Copyright
© 2015 Cambridge University Press 

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Chang et al. supplementary movie

Animation showing nondimensional vertical vorticity field in the wake of a porous cylinder. a) SVF=0.2, antisymmetric shedding, von Karman vortex street is present b) SVF=0.05, symmetric shedding, no von Karman vortex street forms

Download Chang et al. supplementary movie(Video)
Video 3.4 MB

Chang et al. supplementary movie

Animations showing the nondimensional vertical vorticity distribution through the array of cylinders. a) SVF=0.2; b) SVF=0.05. The movie visualizes the vortical content of the bleeding flow and the wake-to-cylinder interactions for a case with a high SVF and a case with a low SVF.

Download Chang et al. supplementary movie(Video)
Video 3.6 MB