Flow Around a Surface-Mounted Finite Cylinder: A Challenging Case for LES
Results of large eddy simulation of a flow around a cylinder with a finite aspect ratio (length/diameter) of 6 placed on a ground plane, is presented. The Reynolds number based on the inlet velocity and the cylinder diameter is 2×104. The boundary layer t
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Abstract Results of large eddy simulation of a flow around a cylinder with a finite aspect ratio (length/diameter) of 6 placed on a ground plane, is presented. The Reynolds number 4 based on the inlet velocity and the cylinder diameter is 2 × 10 . The boundary layer thickness at the position of the cylinder is approximately 7 % of the cylinder’s diameter. In addition to the comparison of the time-averaged results with existing experimental data, the instantaneous flow was investigated. The fine mesh LES predicted the vortex shedding frequency in agreement with the experimental observations. Several important flow mechanisms are predicted and explained such as: the downwash of coherent structures from the region above the free end into the near wake or the formation of the alternate vortices in the far wake. Instantaneous flow structures around the cylinder such as horseshoe vortex, Kelvin Helmholtz and hairpin vortices are identified and their influence on the cylinder and the resulting aerodynamic forces is explained.
1 Introduction Most studies of flows around cylinders are of infinite cylinders. However, many applications are using finite cylinders with one end being mounted on a surface and other and being free. Examples of such applications are tall buildings, factory chimneys, concrete TV towers or space rockets on launching pads. Such a configuration complicates the cylinder flow dramatically. The intersection of the cylinder with the mounting plane results in a horseshoe vortex which is often not steady in terms of its location and size. Around the other, free, end of the finite cylinder, a complex three-dimensional flow is formed. As the flow goes over the top a down-wash is produced that enters the wake and interacts with the flow shed along the cylinder. Resulting flow is very different from an infinite-cylinder flow or even from a flow around a cylinder with both ends fixed at walls. The present paper studies the flow around a finite cylinder with an aspect ratio of L/D=6 (length/width). The objective of this work is twofold; to test the capability of large eddy simulation to predict this flow and to investigate this flow in detail and produce a knowledge base. Such a knowledge base can be used for validation of hybrid LES-RANS and DES simulations and for deeper understanding of this kind of flow by researchers. S.-H. Peng and W. Haase (Eds.): Adv. in Hybrid RANS-LES Modelling, NNFM 97, pp. 305–315, 2008. © Springer-Verlag Berlin Heidelberg 2008 springerlink.com
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S. Krajnović
2 Description of the Set-Up The finite cylinder studied in this paper is that from the experiments by Park and Lee (2002). The cylinder was mounted vertically on a flat plate. Free stream inlet velocity U 0 = 10 m/s and diameter D equal with 0.03 m give a Reynolds number of approximately 2 × 10 4 . A test section of 24D × 20D × 28D (width × height × length) was used in our simulation (Fig. 1a). The inlet and the outlet in our numerical wind tunnel are placed 8D and 19D from the cylinder, respectively.
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Fig. 1. a) Com
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