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-   -   LES, FSI, CFX, Yplus, VIV ... and problem! (http://www.cfd-online.com/Forums/main/108655-les-fsi-cfx-yplus-viv-problem.html)

 Pouya October 29, 2012 14:37

LES, FSI, CFX, Yplus, VIV ... and problem!

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Hi dear cfd-online members!

I'm doing an FSI problem using Ansys CFX. I want to study vortex induced vibration (VIV) of a riser model.
The riser model has a length of 9.63 m & a diameter of 0.02m (L/D=482).
The velocity of incident current and Reynolds number are 0.84 m/s and 18737 respectively.

I want to use LES turbulence model and Yplus should be below 1. It seems that I have to use too fine elements!
I've made a mesh. In this case the areaAve value of Yplus is 119.071.

And according to formulas of "boundary layer thickness" and "Near Wall Spacing", extracted from "ANSYS CFX-Solver Modeling Guide",
I have to use elements of order 0.0000185 m. This means a huge number of elements just in the boundary layer!!! This is while I have a rather big domain.

 FMDenaro October 29, 2012 14:42

why are you surprised of the required number of elements? you need to solve the BL so at least 3 - 4 cells must be within y+<1. You can have a dx+ and dz+ of order 30 - 40 for LES.

Note that LES is a formulation of the governing equations, not a turbulent model....

 flotus1 October 29, 2012 18:30

Yes, wall-resolving LES are computationally expensive.

You may want to drop the block structured mesh you are currently using.
This enables you to resolve the wall-normal direction better than the other directions, like FMDenaro said.
But I would say that X+ and Z+ of 40 seems a bit high. Fröhlich (the only good german book about LES) for example recommends values around 5 for the stream- and cross-streamwise direction.
Additionally, you might encounter some difficulties when a non-conformal mesh interface lies directly downstream of your obstacle.

 Pouya October 30, 2012 11:31

Thanks for your answers. So using LES will impose high CPU demands. What if I just want to study the vibrations (to get the Frequency or amplitude of the vibrations)? Should I solve for the boundary layer?
What about SAS-SST or DES or other models?