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!!!

Hi,

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

Bests,

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".

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!!!

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).

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

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.

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.