Learning Forum

[Alberto]

Hi community, I would ask a question about density distribution with UDF file.

I'm trying to simulate the falling descent of a solid body in air with overset mesh and 6-dof solver. Everithing is well setted up and if i use a constant value for the solid density in fluent no problems occurs.

 I would like to consider a body with different density distribution, for example rho = 40 if the cell thickness is below a threshold and rho =430 if it is above. I've write the following UDF and fluent allow to compile and assign to the material density, but when i look at the density contours in CFD post the value on the solid body is uniform and equal to 1.225 (air density). How can I check the real density of the solid ? 

UDF:

#include "udf.h"

DEFINE_PROPERTY(density, c, t)
{
    real ro;
    real x[ND_ND];
    real thickness;
    int isWallObject;
    Domain* domain;

    /* Ottieni il dominio */
    domain = Get_Domain(1);

    /* Verifica se la faccia è associata alla name selection "wall-object" */
    isWallObject = (C_FACE_THREAD(c, t) == Lookup_Thread(domain, "wall-object")) ? 1 : 0;

    if (isWallObject)
    {
        C_CENTROID(x, c, t);
        thickness = C_VOLUME(c, t) / C_FACE_AREA(c, t);

        if (thickness < 0.000353)
            ro = 40;
        else
            ro = 430;
    }
    else
    {
        ro = 1.225;
    }

    return ro;
}

 

Many thanks

[Rob]

If you're setting a material property why do you need to also set that property to 1.225 kg/m3? I can understand some compression effect on a wall, so 40 or 430, but not the reason for the second IF. 

[Alberto]

This is becouse if the cell is not in "wall-object", then set the fluid density which is 1.225. I can also remove that but the results dont change.

[Rob]

OK, but you're setting a material property. For a solid (or wall) that's the solid material type, default is aluminium. So if the solid zone is of a certain thickness there's no need for the gas property. 

[Alberto]

Ok, but i don't have the aluminium as material, i've load this udf file and fluent accept it as material property. I want density = 40 when i'm moving over the object surface where the cell are susbatially 2D, then i want 430 when their 3D. This is the way I'm trying to set material properties. Does it work?

[Alberto]

in SpaceClaim i’ve set HDPE, then in Fluent aluminium

[Rob]

The wall must be a solid material, and the flowing volumes are fluid. So, the UDF will hook to the solid material and then the solid is assigned to the wall. 

[Alberto]

Sure, this is what my UDF does, right? But i have two different density values for the same object. 

[Rob]

What material is it attached to? 

[Alberto]

in SpaceClaim i’ve set HDPE, then in Fluent aluminium

[Rob]

 

OK, and you attached the UDF to the aluminium? Is the region you're setting a wall or a solid zone? A wall doesn't see the neighbouring cell volume unless you tell it to: there's a bit in the UDF manual on adjacent face/cell/node syntax.

 

[Alberto]

It is a solid zone, obtained through a boolean subtraction where the tool is the solid and the target the fluid volume.

Yes it is attached to the aluminium material as user-defined input

[Rob]

And the solid zone isn't a wall. You may need to loop over the cells and adjacent wall thread (or the other way round) to get the thickness for your material definition.