UFR 2-10 Best Practice Advice: Difference between revisions
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== Key Physics == | == Key Physics == | ||
The flow past finite height circular cylinders mounted on a flat plate is very complex as | 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 | described in some detail in the Introduction and as illustrated in | ||
[[UFR_2-10_Description#fig1|Fig. 1]]. The flow is highly | |||
three-dimensional with an interaction of various vortex systems. There is generally unsteady | 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). | 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 | 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. | influenced by the end effects, and in particular the effects of the free-end at the cylinder top. | ||
== 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 | in vertical direction at mid-height would have been desirable. The mesh sizes ∆''x'' and Δ''y'' | ||
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'' | ||
2.5 case the mesh sizes | for ''h/D'' = 2.5 and 5 respectively. For the | ||
0.0625 D at mid-height and 0.0014 D at the free-end increasing again towards the frictionless | ''h/D'' = 2.5 case the mesh sizes Δ''z'' in vertical direction | ||
upper boundary. For the h/D = 5 case, the corresponding values are 0.00175 D, 0.125 D and | vary from 0.0085''D'' near the ground plate to | ||
0.0028 D. The calculations could of course also be performed with methods using wall- | 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 | 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 | 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 | detail in | ||
coarser meshes (e.g. 6.4 | [[UFR_2-10_References#palau|Palau-Salvador ''et al'' (2010)]], LES of the ''h/D'' = 2.5 case have been obtained with | ||
coarser meshes (e.g. 6.4 Million grid points by | |||
[[UFR_2-10_References#frohlich|Fröhlich and Rodi 2004]]) and these calculations | |||
could also capture most of the main features of the complex flow, including the vortex | 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 | 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. | complex flow behaviour on the top of the cylinder could not be resolved. | ||
== Computational Domain and Boundary Conditions == | |||
A computational domain similar to the one sketched in | |||
[[UFR_2-10_Test_Case#fig3|Fig. 3]] should be used. The extent of | |||
the domain upstream of the cylinder (1.6''h'') is the minimum | |||
for the ''h/D'' = 2.5 case and should | |||
actually be chosen larger if possible. For wall-resolving LES, no-slip conditions should be | |||
used at the cylinder and ground plate walls while the top wall and side walls can be treated as | |||
frictionless rigid lid. At the outflow, the convective boundary condition should be used. At the | |||
inflow boundary the mean velocity profile taken from the measurements should be specified. | |||
In the LES presented, fluctuations at the inflow were set to zero and developed then in the | |||
boundary layer approaching the cylinder. By prescribing fluctuations corresponding to the | |||
turbulent boundary layer, a more realistic boundary layer approaching the cylinder could | |||
probably be achieved. | |||
== Physical Modelling == | |||
As was discussed in | |||
[[UFR_2-10_References#palau|Palau-Salvador ''et al'' (2010)]], | |||
steady RANS methods can get some of the | |||
mean-flow features like separation on the sides and on the top, horseshoe vortex and tip | |||
vortices, but none of the many details discussed above and of course no features that are due | |||
to the unsteady nature of the flow which includes the mean pressure in the separation region. | |||
Hence, an eddy-resolving method is necessary to do justice to this complex flow. The wall- | |||
resolving LES presented have shown good results using the dynamic Smagorinsky model | |||
approach. However, with the fine grids as used in the LES presented, the subgrid-scale model | |||
is not very influential and probably a standard Smagorinsky model with near-wall damping | |||
could also be used. | |||
== Application Uncertainties == | |||
One of the main uncertainties is the oncoming boundary layer developing on the ground plate | |||
when the flow approaches the cylinder. Also, in the experiments the free-stream turbulence | |||
above the boundary layer was 2% which was not accounted for at all in the calculations. | |||
Further, the top and side walls were treated as frictionless rigid lids in the LES presented, but | |||
the boundary layers at these walls may have had some influence. | |||
== Recommendations for Future Work == | |||
LES with an even finer grid, particularly with better resolution near the ground plate and in | |||
vertical direction at mid-height of the cylinder would be of interest. Also a study on the | |||
influence of specifying more realistic boundary-layer characteristics at the inflow would be of | |||
interest. Further it would be intriguing to see how hybrid RANS/LES methods, e.g. the DES | |||
method, would perform for this flow. | |||
<br> | <br> | ||
<br> | <br> | ||
Latest revision as of 11:40, 12 February 2017
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 0.008D for h/D = 2.5 and 5 respectively. For the h/D = 2.5 case the mesh sizes Δz in vertical direction vary from 0.0085D near the ground plate to 0.0625D at mid-height and 0.0014D at the free-end increasing again towards the frictionless upper boundary. For the h/D = 5 case, the corresponding values are 0.00175D, 0.125D and 0.0028D. 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 Million 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.
Computational Domain and Boundary Conditions
A computational domain similar to the one sketched in Fig. 3 should be used. The extent of the domain upstream of the cylinder (1.6h) is the minimum for the h/D = 2.5 case and should actually be chosen larger if possible. For wall-resolving LES, no-slip conditions should be used at the cylinder and ground plate walls while the top wall and side walls can be treated as frictionless rigid lid. At the outflow, the convective boundary condition should be used. At the inflow boundary the mean velocity profile taken from the measurements should be specified. In the LES presented, fluctuations at the inflow were set to zero and developed then in the boundary layer approaching the cylinder. By prescribing fluctuations corresponding to the turbulent boundary layer, a more realistic boundary layer approaching the cylinder could probably be achieved.
Physical Modelling
As was discussed in Palau-Salvador et al (2010), steady RANS methods can get some of the mean-flow features like separation on the sides and on the top, horseshoe vortex and tip vortices, but none of the many details discussed above and of course no features that are due to the unsteady nature of the flow which includes the mean pressure in the separation region. Hence, an eddy-resolving method is necessary to do justice to this complex flow. The wall- resolving LES presented have shown good results using the dynamic Smagorinsky model approach. However, with the fine grids as used in the LES presented, the subgrid-scale model is not very influential and probably a standard Smagorinsky model with near-wall damping could also be used.
Application Uncertainties
One of the main uncertainties is the oncoming boundary layer developing on the ground plate when the flow approaches the cylinder. Also, in the experiments the free-stream turbulence above the boundary layer was 2% which was not accounted for at all in the calculations. Further, the top and side walls were treated as frictionless rigid lids in the LES presented, but the boundary layers at these walls may have had some influence.
Recommendations for Future Work
LES with an even finer grid, particularly with better resolution near the ground plate and in
vertical direction at mid-height of the cylinder would be of interest. Also a study on the
influence of specifying more realistic boundary-layer characteristics at the inflow would be of
interest. Further it would be intriguing to see how hybrid RANS/LES methods, e.g. the DES
method, would perform for this flow.
Contributed by: Guillermo Palau-Salvador, Wolfgang Rodi, — Universidad Politecnica de Valencia, Karlsruhe Institute of Technology
© copyright ERCOFTAC 2011