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-   -   Transpiration velocity? (http://www.cfd-online.com/Forums/fluent/30101-transpiration-velocity.html)

Riccardo July 31, 2002 11:11

Transpiration velocity?
 
Hi everyone! I'm doing some runs on Fluent 6.0 to compute aeroelastic loads; i have to impose little normal modes motion of my wing inside fluent. for this target i could use the moving and deforming mesh option, in fluent 6.0, but is very time consuming. there are some reports that suggests to use for these cases the transpiration velocity, that is to impose instead of the real motion of the wall, a source of normal velocity like the velocity of the wall, while however the wall will remain fixed. is there a way to do this? maybe changing the wall boundary to a velocity inlet boundary? or to a porous media boundary? Does anyone work with this kind of problem?

thank you all! Riccardo

Lanre August 1, 2002 02:05

Re: Transpiration velocity?
 
Use a moving reference frame. This is how it is done to model the blade motion of pumps, mixers, etc. See the FLUENT Users Guide for more on the modelling approach.

Riccardo August 1, 2002 04:44

Re: Transpiration velocity?
 
Thank you Lanre, but the moving reference frame (sliding mesh, rotating frame....) and also the moving wall don't pass normal velocity from a zone to the other: they can trasmit only tangential velocity component. And my deformation motion is almost normal to the boundary! I'm trying with velocity inlet: what do you think?

Laika August 5, 2002 10:40

Re: Transpiration velocity?
 
Velocity inlet???? Are you trying to model a moving wall as a velocity inlet? Do you expect to have meaningful results with such an unphysical boundary condition?

Laika, still orbiting

Riccardo August 7, 2002 08:20

Re: Transpiration velocity?
 
My wall is vibrating as the normal modes of the wing. this kind of vibration is of low amplitude, so you can see the moving wall as a little perturbation of the flow field. the flow is unviscid, and over the wall the air has to follow the normal velocity of deformation of the wing: is kind of source of velocity for the flow field, this is the reason way i'm trying to use velocity inlet (the concept of transpiration velocity is studied since a long time, and seems to work fine for this kind of aeroelastic problem). the problem for velocity inlet is on how to set the boundary condition on it (they change face for face, as in all the deformations). Why are you talking in so "terroristic" way? Velocity inlet can give problem if you have stagnation point in your flow field (and i've not) while you're modelling compressible flows!!!

Did you have any problem with it? with similar problems? Why do you call it "non fisical" boundary condition?

Thank you!!! Riccardo

Riccardo August 7, 2002 08:22

Re: Transpiration velocity?
 

Lanre August 7, 2002 10:18

Re: Transpiration velocity?
 
I think you might want to review my earlier suggestion more closely. Review the FLUENT Users Guide on moving zones. You have options with the moving reference frame and even the sliding mesh. Now that you mention your wall is vibrating, you'll need a UDF to address the oscillatory source term and grid motion, respectively. Alternately, and more elegantly, you can use the deforming mesh. If the deformation is small, it make the solution even simpler. Don't bother with a velocity condition...how will you address the shear at the wall?

Riccardo August 7, 2002 15:03

Re: Transpiration velocity?
 
I will re-read the manual on sliding mesh and MRF, and i will look.... for the shear it's no matter because my flow is inviscid. Thank you Lanre!


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