## Fluids

#### multiphase flow, vof model with passive scalar

Subscriber
I am working on a problem with multiphase flow with an air-water system in a 2D rectangular geometry. About half of the container (from bottom) is initially filled with water (I used patching) and the top-half is filled with air. We have three inlets at the bottom from where air is being injected at certain interval and for certain durations (I used UDF for this). The flow problem works fine using the VOF method.

But when I have added a user-defined scalar, I am not getting desired results. I have associated the scalar with only the water-phase, and used the flux function as "mass flow rate". Initially the scalar has a certain value at the bottom-part of the container and 0 in the upper-part as it contains air.

I think when air is being injected, the computational cells that contains air within the water domain, there should be no scalar present, but the simulation does not show this, rather it shows no-change of the scalar concentration in the bottom-part even where water-air is present.

• DrAmine
Ansys Employee

Why should the cell containing water and air not contain the scalar? I do not understand your point. The scalar is advected with the same velocity field as the bulk but weighted by the volume fraction of water.

Subscriber

After injecting air through the inlet, air forms bubble with different shapes. As a result, some part in the domain contains only air with volume fraction 1 (that means no-water). Still we don’t see any variation of the scalar. We thourght that, as you also mentioned (the scalar is wighted by volume fraction of water), the plot would look like similar to the plot of volume fraction of water, but the plot of scalar shows same everywhere even where there is only air. Is there anything we are missing?

Note that this scalar can exist only in water as it is associated with water phase.

• Rob
Ansys Employee

If a scalar is attached to a single phase you can get a false presence as a tiny (really tiny) volume fraction can exist in the whole domain: plot water fraction between 0 and 0.0001 to see what's going on.