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-   -   porous media drag fuction ? (https://www.cfd-online.com/Forums/flow-3d/81739-porous-media-drag-fuction.html)

 club68512 November 5, 2010 02:58

porous media drag fuction ?

hello everyone~

i have a program about the porous media drag fuction...
that include 1.Volume fraction 2. Fluid fraction 3. Reynolds number dependent..
but the user's guide don 't tell what situation should use which one...

i wanna model the wicking phenomena ,i use two fluid model(have phase change) so can not use the unsaturated flow model,and the fiber diameter of pouros component structure are under 0.1mm ..

does anybody have the experience??

 MuxaB November 6, 2010 20:43

i suggest using the reynolds-dependent drag model, IDRG=4, and compute the drag coefficients, OADRG and OBDRG using the equations given in the manual and the average fiber diameter in porous media.

You may also have to use the capillary pressure, OPCAP, to model the wicking, if there is a significant wetting effect at the fluid front.

 club68512 November 7, 2010 03:28

Quote:
 Originally Posted by MuxaB (Post 282481) i suggest using the reynolds-dependent drag model, IDRG=4, and compute the drag coefficients, OADRG and OBDRG using the equations given in the manual and the average fiber diameter in porous media. You may also have to use the capillary pressure, OPCAP, to model the wicking, if there is a significant wetting effect at the fluid front.
thanks!!
but if use OPCAP , the fluid will be forced to the porous media??

I think ,if don't use OPCAP, the solver may automatically compute the capillary pressure by itself?

 MuxaB November 7, 2010 13:06

Quote:
 Originally Posted by club68512 (Post 282494) thanks!! but if use OPCAP , the fluid will be forced to the porous media?? I think ,if don't use OPCAP, the solver may automatically compute the capillary pressure by itself?
No, the code will not compute OPCAP for the saturated porous media case. OPCAP accounts for the surface tension forces inside the pores and since the pores are not directly resolved by the mesh, OPCAP must be defined. The sign of OPCAP defines wetting/non-wetting conditions. I can't remember the exact correspondence - please see the manual for that.

 club68512 November 7, 2010 13:16

Quote:
 Originally Posted by MuxaB (Post 282534) No, the code will not compute OPCAP for the saturated porous media case. OPCAP accounts for the surface tension forces inside the pores and since the pores are not directly resolved by the mesh, OPCAP must be defined. The sign of OPCAP defines wetting/non-wetting conditions. I can't remember the exact correspondence - please see the manual for that.
thanks!!!!!!!!

so... if don't use opcap ,the wicking phenomena won't happen??

I was refer to this http://www.flow3d.com/resources/news...ring08_06.html
from How to Define Filters don't use OPCAP ...

 MuxaB November 8, 2010 23:05

Quote:
 Originally Posted by club68512 (Post 282536) thanks!!!!!!!! so... if don't use opcap ,the wicking phenomena won't happen?? I was refer to this http://www.flow3d.com/resources/news...ring08_06.html from How to Define Filters don't use OPCAP ...
There must be a driving force for the fluid to enter porous media: either an external pressure or the capillary pressure. Wicking to me implies that the flow is driven by capillary forces, such was when water spreads in a napkin or a sponge. The page you refer to describe the selection of drag resistance forces which are related to viscous losses and the tortuosity. Capillary forces arise at the free surface.

 club68512 November 8, 2010 23:18

thanks a lot !!

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