## Fluids

#### Mass balance VOF

• prebenjs
Subscriber

Hi,

I'm simulating a t-junction where i have a pressure outlet, and two velocity inlets (left side of junction = air, down side = water). I'm simulating water entering from the vertical junction, with the domain inital being filled with air. The simulations are done transient. Now, as water is being filled I'm monitoring the Mass Balance, but it the results i get are not in balance. The mass flows are as follows:

Velocity inlet_air = 2.45

Pressure inlet_rightside = -2.466

velocity inlet_water = 13.309

Now, the water is "growing" through the domain but is not near of reaching the outlet.

The questions are the following:

• How do i evaluate the mass balance / imbalance in a VOF transient simulation as this?
• Is the difference between the velocity inlet_air and the pressure inlet, due to the mass flow from the water injection ? If so, why is it so small when comparing it to the water inflow?
• Is it possible to evaluate mass balance when both phases are not in contact with the pressure outlet?

• Rob
Ansys Employee
If you're adding water at one one, and air is leaving the other the mass won't balance as water is somewhat denser than air. If the solution is converged then the volume should be about equal. The other option is to monitor mass in, mass out and mass in the domain noting that you need the change in the latter.

• prebenjs
Subscriber
I'm sorry i forgot to mention that they are incompressible, but you're suggestions still is accurate i believe. So in general if it's shows good convergence it should hold?
So monitoring the mass in, out and in the domain (interior) i should be able to get a "approximate" mass balance ?

• Rob
Ansys Employee
Yes, with single density systems mass in = mass out in steady state. For a transient system, or where density differs through the domain it's a bit more complicated.
• prebenjs
Subscriber
So in general, having multiphases with different densities one should avoid using the mass balance as a measure ?
• Rob
Ansys Employee
No, but you need to appreciate that you need to account for density before thinking it's not working.
• prebenjs
Subscriber
Okay, So accounting for density for the water mass flow, with respect to air, one should get the value of the mass balance ? I'm just a little bit confused to why the domain mass flow (interior) is not taken into account for the net result in Fluent? Should this be excluded when calculating for my transient case ?
I really appreciate the knowledge that you provide!
• Rob
Ansys Employee
The monitors do just that: monitor what you tell them to. So, the main fluxes are checked on the inlets & outlets (usually). For the fluid domain we use the volume integrals etc. It can get more complicated when inlet & outlet don't tie up as the volume monitor is the current value not the change from the last time step. Fortunately we can also write the monitors out as text and Excel has improved significantly in the last 25 years..... No comment on Windows!
In a case I have now it's even more fun as I'm adding DPM droplets that hit a wall to form a film, that gets thicker and switches to VOF to then accumulate until something overflows. I'm thinking of writing that one up as a study question for the next intern!