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". Quote:

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!!! 
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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 343). Quote:

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