# Derivative of a scalar or a variable through UDF

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 August 3, 2012, 16:42 Derivative of a scalar or a variable through UDF #1 Member   Fer Villa Join Date: Apr 2012 Posts: 35 Rep Power: 14 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!!!

 August 5, 2012, 11:29 #2 Senior Member     Amir Join Date: May 2009 Location: Montreal, QC Posts: 735 Blog Entries: 1 Rep Power: 23 Hi, You can store it in a UDS and retrieve its derivation. Bests, __________________ Amir

August 6, 2012, 01:41
#3
Senior Member

Join Date: Feb 2010
Posts: 164
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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 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!!!

 August 6, 2012, 09:07 #4 Member   Fer Villa Join Date: Apr 2012 Posts: 35 Rep Power: 14 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!!! Last edited by fevi84; August 6, 2012 at 09:38.

August 6, 2012, 10:40
#5
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Join Date: Feb 2010
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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 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!!!

August 6, 2012, 13:20
#6
Member

Fer Villa
Join Date: Apr 2012
Posts: 35
Rep Power: 14

Quote:
 Originally Posted by gearboy 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).

 February 3, 2015, 18:28 #7 Member   Join Date: Dec 2014 Posts: 38 Rep Power: 11 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.

 February 3, 2015, 21:39 #8 Member   Join Date: Dec 2014 Posts: 38 Rep Power: 11 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.