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

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October 29, 2012, 14:37
LES, FSI, CFX, Yplus, VIV ... and problem!
#1
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Pouya
Join Date: Feb 2012
Posts: 17
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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.

I think something is wrong with it. Could you please please please help me solve this problem?!!!
Attached Images
 Mesh.jpg (75.9 KB, 61 views) Boundary Layer Thickness.jpg (8.7 KB, 38 views) Near Wall Spacing.jpg (44.6 KB, 33 views)

 October 29, 2012, 14:42 #2 Senior Member   Filippo Maria Denaro Join Date: Jul 2010 Posts: 6,675 Rep Power: 71 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....

 October 29, 2012, 18:30 #3 Super Moderator     Alex Join Date: Jun 2012 Location: Germany Posts: 3,373 Rep Power: 46 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.

 October 30, 2012, 11:31 #4 New Member   Pouya Join Date: Feb 2012 Posts: 17 Rep Power: 13 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? I really appreciate your help.

 October 30, 2012, 13:04 #5 New Member   Pouya Join Date: Feb 2012 Posts: 17 Rep Power: 13 & one more question! Maybe it sounds crazy, but I want to know if it is possible to use a few layers in spanwise direction (and do a 2D study for each layer!). I know we can't call it a full 3D study, but it could reduce the computational expense.

October 30, 2012, 13:12
#6
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Vieri Abolaffio
Join Date: Jul 2010
Location: Always on the move.
Posts: 308
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Quote:
 Originally Posted by Pouya & one more question! Maybe it sounds crazy, but I want to know if it is possible to use a few layers in spanwise direction (and do a 2D study for each layer!). I know we can't call it a full 3D study, but it could reduce the computational expense.
no, you won't get VIV from this. the results depend on the length of the object and there are 3d interactions that you won't capture. also, you need at least a u+ and z+ values around 50-100 top in LES, as FMdenaro said in an earlier post.
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 Tags cfx, fsi, les, viv, yplus

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