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Ahmed Khattab February 25, 2003 13:54

How can I specify different permeability?
In my case, which is a rectilinear flow in a mold, filled with carbon fiber, I want to change the permeability near the mold wall so the flow front will be faster near the edge (higher permeability near the edgs). So how can I specify different permeability to FLUENT?

Murali March 2, 2003 22:39

Re: How can I specify different permeability?
<font face = "courier new">I think you should use the Macro DEFINE_PROPERTY


Ahmed Khattb March 2, 2003 23:27

Re: How can I specify different permeability?
Thanks for your help. I'm new user to FLUENT, so if you please can you help me with more details

Murali March 4, 2003 18:50

Re: How can I specify different permeability?
<font face = "courier new">Since you say that you are new to UDFs, I suggest you peruse the UDF manual which will help you greatly. But let me provide you some tips which I am familiar with:

(1) Since permeability is a 'property' or a number just lilke say diffusivity or viscosity, it can be calculated through a UDF and be passed to FLUENT while running.

(2) The way to do is to use by accessing DEFINE_PROPERTY macro.

(3) If you are completely unfamiliar with UDFs, then these are the preliminary concepts:

cell = computational cell
face = face of the cell
thread = a collection of faces or cells but not both

(4) Since the 'property' of the material is a 'cell value' dependent upon the 'material' of the cell, the DEFINE_PROPERTY Macro would compute your required value and send it to the FLUENT solver while running

(5) Section 4.3.13 of UDF manual of FLUENT 6 gives you this information.

DEFINE_PROPERTY(cell_permeability, c, t)
real perm;
real x[ND_ND];


if (x[0] == 1)
perm = 5;
perm = 10;

return perm;


The above example I think should tell you how this works. It first obtains the centroidal information and stores in x, a real array. x[0] has the x co-ordinate and this value is checked. IF x[0] = 1, then the value of permeability to be specified is 5 or else 10.

(6) Once you have written this udf and compiled properly as given in chapter 8 of the UDF manual, you shall see the UDF name appearing in the drop-down list. Select cell_permeability in the appropriate place.

(7) Lastly, all these have been written under the assumption that you have to specify permeability as just a number (and not a relation)

Hope this helps


Ahmed Khattab March 10, 2003 17:39

Re: How can I specify different permeability?
I'm working to fully understand the UDF manual, but in the example defining the viscosity, and also other examples, there is a drop-down list to select UDF from it, which is not the case for permeability, is that a problem?


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