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-   -   Derivative of a scalar or a variable through UDF (http://www.cfd-online.com/Forums/fluent-udf/105612-derivative-scalar-variable-through-udf.html)

fevi84 August 3, 2012 16:42

Derivative of a scalar or a variable through UDF
 
Hi,

I need help!!!

I need calculate of derivative of a scalar, in my case, the derivative of the volume fraction.

Anyone knows how I can calculate this derivative through UDF??? because I only know the macros for the derivative of the velocity (C_DUDX(c,t), C_DUDY(c,t), C_DUDZ(c,t),...).

is there a way to calculate the derivative of a scalar or a variable other than velocity?

Thanks!!!

Amir August 5, 2012 11:29

Hi,

You can store it in a UDS and retrieve its derivation.

Bests,

gearboy August 6, 2012 01:41

As Amir said, store the volume fraction into a UDSI and do not solve the equation of this UDSI. Then achieve the derivative by C_UDSI_G.
Use the following code to get the volume fraction.


Material *mix_mat = mixture_material(Get_Domain(1));
Material *spe_mat = NULL;
real all_mass_fracts[MAX_SPE_EQNS];
real all_mole_fracts[MAX_SPE_EQNS];
int i = -1;
mixture_species_loop(mix_mat, spe_mat, i)
{
all_mass_fracts[i] = C_YI(c,t,i);
}
Mole_Fraction(mix_mat, all_mass_fracts, all_mole_fracts);



After this, the mole fractions of all species can be found in the array "all_mole_fracts".





Quote:

Originally Posted by fevi84 (Post 375311)
Hi,

I need help!!!

I need calculate of derivative of a scalar, in my case, the derivative of the volume fraction.

Anyone knows how I can calculate this derivative through UDF??? because I only know the macros for the derivative of the velocity (C_DUDX(c,t), C_DUDY(c,t), C_DUDZ(c,t),...).

is there a way to calculate the derivative of a scalar or a variable other than velocity?

Thanks!!!


fevi84 August 6, 2012 09:07

Thank you very much for their replies.

I don't understand how store the volume fraction in a UDS without solving this UDS. I have to create a UDS through the graphical interface?

Can you please explain more.

Thanks!!!

gearboy August 6, 2012 10:40

1 Attachment(s)
Attachment 15027

As to the usage, see chapter "Example UDF that Utilizes UDM and UDS Variables" in Fluent 6.3 UDF pdf manual(page 3-43).




Quote:

Originally Posted by fevi84 (Post 375602)
Thank you very much for their replies.

I don't understand how store the volume fraction in a UDS without solving this UDS. I have to create a UDS through the graphical interface?

Can you please explain more.

Thanks!!!


fevi84 August 6, 2012 13:20

Thank you very much again, your reply was very useful.


Quote:

Originally Posted by gearboy (Post 375614)
Attachment 15027

As to the usage, see chapter "Example UDF that Utilizes UDM and UDS Variables" in Fluent 6.3 UDF pdf manual(page 3-43).


Harry321 February 3, 2015 19:28

Hello,

Could one of you tell me difference between gradient and derivative in fluent. I used code:

fprintf(fp2, "DWDZ %f ", C_DWDZ(c, t));
fprintf(fp2, "CWG %lf \n", C_W_G(c, t)[2]);

but the values are different.

Harry321 February 3, 2015 22:39

Dear colleagues,

I don't know why in previous version of a code, fluent plotted wrong values but after I add:

fprintf(fp2, "DWDZ %f ", C_DWDZ(c, t));
fprintf(fp2, "CWG[0] %lf \n", C_W_G(c, t)[0]);
fprintf(fp2, "CWG[1] %lf \n", C_W_G(c, t)[1]);
fprintf(fp2, "CWG[2] %lf \n", C_W_G(c, t)[2]);

fluent plotted the same values for derivative of a velocity and for C_W_G(c, t)[2], I cannot see difference between this and previous one, but it works and the results are the same so it means that gradient and derivative are the same.


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