UFR 2-10 Best Practice Advice: Difference between revisions
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== Numerical Issues == | == Numerical Issues == | ||
The LES presented and discussed were carried out with a finite-volume code that uses a | The LES presented and discussed were carried out with a finite-volume code that uses a | ||
Cartesian grid and an immersed boundary method. 45 and 27 | Cartesian grid and an immersed boundary method. 45 and 27 Million grid points were used for | ||
the ''h/D'' = 2.5 and 5 cases respectively, and an even finer resolution near the ground plate and | the ''h/D'' = 2.5 and 5 cases respectively, and an even finer resolution near the ground plate and | ||
in vertical direction at mid-height would have been desirable. The mesh sizes ∆x and Δy | in vertical direction at mid-height would have been desirable. The mesh sizes ''∆x'' and ''Δy'' | ||
around the cylinder were 0.004 D and 0.008 D for h/D = 2.5 and 5 respectively. For the h/D = | around the cylinder were 0.004''D'' and 0.008 D for h/D = 2.5 and 5 respectively. For the h/D = | ||
2.5 case the mesh sizes Δz in vertical direction vary from 0.0085 D near the ground plate to | 2.5 case the mesh sizes Δz in vertical direction vary from 0.0085 D near the ground plate to | ||
0.0625 D at mid-height and 0.0014 D at the free-end increasing again towards the frictionless | 0.0625 D at mid-height and 0.0014 D at the free-end increasing again towards the frictionless | ||
Revision as of 16:09, 11 January 2011
Best Practice Advice
Key Physics
The flow past finite height circular cylinders mounted on a flat plate is very complex as described in some detail in the Introduction and as illustrated in Fig. 1. The flow is highly three-dimensional with an interaction of various vortex systems. There is generally unsteady vortex shedding behind the cylinder unless the cylinder height is rather small (h/D below 2). The vortex shedding that occurs in the test cases considered (h/D = 2.5 and 5) is strongly influenced by the end effects, and in particular the effects of the free-end at the cylinder top.
Numerical Issues
The LES presented and discussed were carried out with a finite-volume code that uses a Cartesian grid and an immersed boundary method. 45 and 27 Million grid points were used for the h/D = 2.5 and 5 cases respectively, and an even finer resolution near the ground plate and in vertical direction at mid-height would have been desirable. The mesh sizes ∆x and Δy around the cylinder were 0.004D and 0.008 D for h/D = 2.5 and 5 respectively. For the h/D = 2.5 case the mesh sizes Δz in vertical direction vary from 0.0085 D near the ground plate to 0.0625 D at mid-height and 0.0014 D at the free-end increasing again towards the frictionless upper boundary. For the h/D = 5 case, the corresponding values are 0.00175 D, 0.125 D and 0.0028 D. The calculations could of course also be performed with methods using wall- conforming grids, but the mesh sizes should be similar to the ones just given, or in vertical direction even better near the ground and at mid-height of the cylinder. As discussed in some detail in Palau Salvador et al (2010), LES of the h/D = 2.5 case have been obtained with coarser meshes (e.g. 6.4 Mio grid points by Fröhlich and Rodi 2004) and these calculations could also capture most of the main features of the complex flow, including the vortex shedding near the ground and the development of tip vortices, but some of the details like the complex flow behaviour on the top of the cylinder could not be resolved.
Contributed by: Guillermo Palau-Salvador, Wolfgang Rodi, — Universidad Politecnica de Valencia, Karlsruhe Institute of Technology
© copyright ERCOFTAC 2011