[alpha2rock]

I am working on a precipitation problem, and for that I need to define an expression:
f=sqrt(C1*C2)-sqrt(Ksp)

where C1 and C2 are molar concentrations for 2 species.

How can I define f?

Also, Ksp is a constant that I need to define with units i.e.
Ksp=1.1*10^-11[kmol^2/m^-6]

How can I define Ksp?

Do I need to use UDF or is it possible using fluent alone?
Also I need to use this expression (f) in a UDF.
So, how to go about it?

[Micael]

When you say you "also" need it in UDF, what is the first use then? Visualization?

As for the UDF purpose, that should be easy to do. Below is not a complet UDF and it assumes you use it in a macro that passes c and t, but might be useful as a starting point.

int species_1_index; species_2_index;
real Ksp;
real y1, y2;
real mw1, mw2;
real rho;
real f;

Ksp = 1.1e-11;

/* you got to figure out index for your species */
/* I arbitrary put 0 and 1 as index */
species_1_index = 0;
species_2_index = 1;

/* species molecular weight [kg/kmol] */
/* I put some arbitrary values */
mw1 = 18.;
mw2 = 32.;

/* Get species mass fraction */
y1 = C_YI(c,t,species_1_index);
y2 = C_YI(c,t,species_2_index);

/* mixture density */
rho = C_R(c,t);

/* molar concentration [kmol/m3] */
C1 = y1*rho/mw1;
C2 = y2*rho/mw2;

f = sqrt(C1*C2)-sqrt(Ksp);

[alpha2rock]

Thank you for your reply Michael.

Yes, defining the parameters/expressions directly into the UDF as variables can be done.
But, alternatively I wanted to call parameters defined in defined as "Named Expressions" in the Fluent interface, which gives me an option to mark them as "use in UDF". Can you help me on how to call these while writing my udf?