# porous media drag fuction ?

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 November 5, 2010, 02:58 porous media drag fuction ? #1 New Member   liang liam Join Date: Nov 2009 Location: Taiwan Posts: 28 Rep Power: 7 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??

 November 6, 2010, 20:43 #2 Senior Member   michael barkhudarov Join Date: Mar 2009 Posts: 331 Rep Power: 9 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.

November 7, 2010, 03:28
#3
New Member

liang liam
Join Date: Nov 2009
Location: Taiwan
Posts: 28
Rep Power: 7
Quote:
 Originally Posted by MuxaB 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?

November 7, 2010, 13:06
#4
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michael barkhudarov
Join Date: Mar 2009
Posts: 331
Rep Power: 9
Quote:
 Originally Posted by club68512 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.

November 7, 2010, 13:16
#5
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liang liam
Join Date: Nov 2009
Location: Taiwan
Posts: 28
Rep Power: 7
Quote:
 Originally Posted by MuxaB 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 ...

November 8, 2010, 23:05
#6
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michael barkhudarov
Join Date: Mar 2009
Posts: 331
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Quote:
 Originally Posted by club68512 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.

 November 8, 2010, 23:18 #7 New Member   liang liam Join Date: Nov 2009 Location: Taiwan Posts: 28 Rep Power: 7 thanks a lot !!

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