Learning Forum

[adi959719]

Hi,

I have set-up a user defined scalar to be solve in fluent. It uses the default unsteady function and mass flow rate as the flux function. In other words, I have implemented a scalar transport equation with the scalar having the units of #/kg.

In my case, the scalar is actually space charge and have the unit Coulombs/kg (C/kg).

Hence, with mass flow rate as the flux function (density*velocity = (kg/(m^2.s))), the units of the advection terms are C/(m^2.s).

My questions are:

1. If I specify the BC of this UDS as "specified value", I assume the unit would be in C/kg. Is this correct?
2. If I specify the BC of this UDS as "specified flux", is the unit C/(m^2.s) or something else?
3. If I specify the BC as "specified flux = 0", on a wall (where no flow can go through), it means the wall is insulating the charge correct? But does it mean zero gradient on the charge near the wall?
4. If I specify the BC as "specified flux = 0", on an outlet (where flow can go through), it means that charge is still advected out, but the gradient of charge is set to zero correct?
5. If I specify the BC on a wall as "specified flux", and set it not equal to zero, say x, does the wall then act like a source/sink, where charge is being inserted/deleted to at a rate of x C/(m^2.s)? If so, does that then mean +x is act as sink and -x act as source?

 

 

[adi959719]

Hi, 

I would like to mention that I think that this post have similar problem I am currently having. 

In which, I basically want a wall to act like a sink for the uds and specify the rate which the uds is being depleted at that wall. I think the way to this should be specify a flux BC at that wall and not write a define_uds_source for it.

[Rob]

So, a specified flux. Remember scalars don't strictly have units so the value will depend on what's specified. Just be aware that the rate you remove the scalar at also has to account for the rate the scalar can reach that surface: that can cause some issues if poorly balanced.