Hi, I am working on UDS modeling for steady state, no convection case. In my model I have taken diffusivity of 1 Kg/m-s and constant UDS source of 100 in a sphere and boundary condition on the surface of sphere for user defined scalar to be zero. I have compared the solution from fluent against analytical solution.

From analytical method- I am getting the user defined scalar at the center of sphere to be 416.667 From Fluent â€" I am getting it to be 0.04

On using different source values I see that the user defined scalar at center varies by a factor of 10,000 compared with the analytical solution. Can anyone explain me the reason for this difference and how does fluent scale user defined source term / volumetric source?

UDS is nothing but a generalised model for species transport. Basically diffucivity for a species is defined in m2/s, whereas UDS diffusivity is defined in Kg/m-s. The difference in units is due to multiplication of density term in UDS equation. If you look at the equation 14.1.1 and 14.1.2 in species transport model and UDS equation 11.2.2 in Fluent6.2.16 user guide manual and udf manual respectively, you would find the difference.

I am not sure if you are taking care of this difference.

Please see if this helps you.

Thanks Akshay for the help....but it didn't really help....

From which institute/organisation you are? IF you ahve licensed version you can take help of fluent support.