## Fluids

#### momentum balance, physical origin of ‘Domain Src (Pos)’

• Geoff
Subscriber

I am performing a buoyancy driven two-phase flow simulation in CFX. Air (ideal gas) is injected into the bottom of a vertical pipe filled with water. Pressure boundary conditions are used for the pipe and mass flow rate of air is specified.

As part of my post processing, i am trying to perform a momentum balance on a segment of the pipe:

pressure differential force = wall shear + weight of column + momentum differential

I have been unable to get the equation to balance so i looked at the output file of my run (included below). I noticed a relatively large momentum contribution from Domain Src (Pos). I looked it up in solver modeling guide 16.10.3.2  and found the explanation unsatisfactory. I would like to understand where it came from physically and how i can properly account for it in my momentum balance.

```  +

+
|                             V-Mom-Bulk                             |
+

+
Boundary         : Periodic                               -1.0541E-06
Boundary         : air inlet                              -3.5050E-05
Boundary         : outlet                                 -7.4481E-03
Boundary         : symmetry                                5.2063E-19
Boundary         : wall                                   -8.6698E-02
Boundary Sources : wall                                   -5.1424E-05
Boundary         : water inlet                            -1.1001E+00
Domain Src (Neg) : fluid                                  -6.4758E-02
Domain Src (Pos) : fluid                                   1.3152E+00
Neg Accumulation : fluid                                  -5.5979E-02

Domain Imbalance :                                         1.8272E-04     ```
• DrAmine
Ansys Employee

Which multiphase model are you using? What about Residual behavior and other domain imbalances? Actually Pos and Neg should sum up to near zero at convergence.

• Geoff
Subscriber

I am using Homogeneous, with Continuum surface force model. It is a transient run where the flowrates in and out are fluctuating a lot . I have a max residual criteria of 1E-3 and a conservation target of 1% set. The RMS residual is typically around 5E-6. The domain imbalance is the highest for water mass, but the residual is highest in V momentum

• DrAmine
Ansys Employee

• Geoff
Subscriber

Hope this helps, if i am missing anything specifically let me know.

• DrAmine
Ansys Employee

Imablances are looking good: but what about the water phase it sill have a huge accumulation?

• Geoff
Subscriber

I do not understand what you are asking by 'what about the water phase it sill have a huge accumulation?'

It is a transient simulation, it is not a problem to instantaneously have a large accumulation of water. in fact i expect it based on the physics of them problem.

If mass is being accumulated in the domain id expect it to be represented by the 'Neg Accumulation' term.