Learning Forum

[Sergio92]

Hello,

I am trying to specify overall scalar flux through a wall boundary. According to Fluent's user guide:

If you select Specified Flux, your input will be the value of the flux at the boundary (that is, the negative of the term in parenthesis on the left-hand side of in the Theory Guide) dot [as in the dot product of] [as in the vector, n], where is the normal into the domain)

The dimensions of the term which the guide is referring to are [kg m^-2 s^-1]. However, Fluent only lets me enter dimensionless values (see screenshot below). I think I am missing something, but not sure what it is. Could anyone help me?

[Rob]

From memory User Scalars don't strictly have units, so you generally set up the maths so the value is in the SI units of your choice but don't specify them. This has the advantage that we can use them for all sorts of things, but does make it a little less straightforward. It's also why scalar diffusion needs to be double checked as there's no molecular weight or density associated with them.

[Sergio92]

Thank you very much for your reply, Mr Rob. I agree the scalars themselves do not have units. But is it also true for their transport terms? The equation for scalar transport is (sorry that I couldn't resize the screenshot):
Because phi is unitless, I had thought the flux would be in the same units as density [kg/m^3] times velocity [m/s].
When I enter the expression for the boundary flux (it depends on the wall's velocity and other things), should I just follow the units in the expression above, and then divide everything by 1 kg.m^-2.s^-1?

[Rob]

If you keep everything as SI and assume the scalar is just a number it all ought to work out. However, you may find the scalar value needs scaling to suit what you're doing.