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lostking18 May 7, 2018 08:04

Sign of source term
Hi, there:

Is there any one who can tell me if the mass source S_m, Momentum source S_Mom, Energy source S_ht on the right hand side or the left hand side of the governing Equation? I.E.
Is it looks like:
d Phi / dt + F(phi) + Sm = 0
or like:
d Phi / dt + F(phi) = Sm
In CFX theory guide Chapter 11.1.1. Discretization of the Governing Equations:

I saw the source term S_Mom and S_ht are written at the right side of the governing Equation. But recently when I try to assign the mass source term Sm with a positive value, the net mass flow rate over the whole computational domain is negative. This indicate the mass source term Sm should be at the right side of the governing equation, right?

Thank you very much!

Opaque May 7, 2018 14:12

Positive source terms increase the transported quantity, negative values decrease its value regardless where they are in the equation (which can be written in multiple ways)

How did you determine your positive mass source had a negative contribution to the total mass in the system?

Say system with one inlet plus one outlet, add a mass source on the wall. Outlet mass flow = Inlet mass + mass source at the wall.

Hope the above helps,

lostking18 May 8, 2018 17:49

Hi, Opaque
Thank you for your answer! I now have solved the problem.

Opaque May 9, 2018 08:43


Would you mind sharing the solution for others in the future?

lostking18 May 11, 2018 12:05

Hi, Opaque, Thanks! I will keep that in mind. This time I'm just embarrassed by the stupid reason: I made mistake on the sign convention. T_T
So generally, when the mass source is positive, we will always get the positive net mass flow rate. Vice versa.

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