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Daniel October 26, 2008 06:59

How to use DES well?
Dear all,

Several questions again.

After reading some thesises, I feel deeply impressed by DES. My major is bridge engg., and when wind blows on the bridge girder sections, the girder will vibrate with buffeting, vortex-excited-oscillation and even fluttering, it's obvious that my main concern is Fluid-Structure Interaction (FSI), as I have been convinced now, DES works very well for large Re and flow with strong separation, that's exactly what I need, so the question remains:

1. Is DES able to apply to FSI (I think of it as a NP-hard problem), I mean, Would DES be capable to predict flutter phenomenon (in future)? Any ideas? Is there any thing special that I need to pay attention? I remember someone told me that even URANS is able to predict the drag-reduction, I am not saying that flutter is related with drag-reduction, but It would be impossible for DES to predict flutter phenomenon if it failed to predict drag-reduction...

2. For a large-span bridge girder section, the Re would reach to 10e6~10e7, it's quite high, but what's more, the flow will separate immediately at the head of girder wind fairing, Adverse Pressure Grdient is therefore strong since the separation is fierce, so, I need be careful about the APG as I use DES, Please correct me if I was wrong.

3. Are there any DES benchmarks that I can follow? I feel the first tough thing is how should I mesh well!!?? I am reading some articles about DES such as "Young-Person's Guide to Detached-Eddy Simulation Grid" by Philippe R. Spalart, but I feel hard to grasp Spalart's writing style, his writing style is just different from others. It's obvious the unstructured grid is inevitable for my bridge case, which software do you guys use to mesh? Could you give my some advice, gmsh? To my knowledge now, I found meshing in DES is not too special especially some concern for the wall-parallel meshing......Comparing to Full-LES, in DES I should use a large spacing in the streamwise and spanwise direction. And the spacing "must" be of the order of BL-thickness/C_DES, please correct me if I am wrong? And if so, Does it mean that ONLY STRUCTURED mesh is available in DES BL region?

4. So, What is the final solving procedure for DES? Is it in this way, a) URANS with coarse mesh first; b) map the field to the mesh prepared for DES.

5. Another thought, is it possible to use Dynamic SGS models in DES?

6. Concerning E(k)~k plot, if LES can give the full plot very similar to the real E(K)~k? what about DES, I guess since DES is RANS in near wall region, DES' E(k)~k will be like URANS one, though a little better than it, right?

It would be very nice to hear your ideas, thanks!

Regards, \Daniel

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