[zobekenobe]

What are the units of the User defined scalars (UDS) ?


The mass flow rate for the UDS is always in kg/s.


Are there any ways to set the UDS units?

[sahoo]

As the name suggests its a scalar. So, whatever will be the unit of your scalar same is the unit of your UDS. When you will operate on it you will get either a scalar or a vector but unit remains same.

[obscureed]

Hi Zobekenobe,

Fluent has no opinion on the units for a UDS -- it is user-defined, so Fluent stays out of it, and (as far as I know) there is no way to tell Fluent the units. It would not use the information for anything anyway, except maybe labelling contour plots etc.

It is worth comparing the transport equation for a UDS with the equivalent for a species mass fraction. You should find that the UDS behaves like a mass fraction -- hence, if you think of a UDS as being something like a concentration, you should think about it as "per unit mass of fluid" rather than "per unit volume of space". This difference is significant only if the fluid density changes. (I'm assuming that you select Mass Flow Rate as the flux function. Without that, there is even less connection to the flowfield, and the UDS does not even need to be concentration-like.)

While I'm on the subject: the UDS diffusivity is missing a factor of density compared to a species diffusivity. Also, UDS diffusivity does not include turbulence unless you include it in a UDF.

You say that the mass flow rate for a UDS "always" has units of [kg/s]. I guess that you're referring to the boundary condition at a velocity inlet, or a reported mass-weighted flux somewhere, for example. Well, I think you are correct *if* you are regarding the UDS as a mass fraction -- a very sensible choice, but not the only choice. I doubt that it is referred to as a mass flow rate inside Fluent. A massflowrate-weighted average will be reported with units [kg/s], with an implicit factor of the UDS unit. If a UDS has units of [mmol/kg], for example, then this kind of total flowrate will end up having units of [mmol/s] as far as the user is concerned, with Fluent remaining unconcerned.

I hope this helps.
Ed