Applying volume fraction constraint

[rnpratoori]

I am working on a reaction-diffusion framework with volume fractions as the variables. Obviously, each volume fraction should be between 0 and 1 and the sum of all the volume fractions should be 1.
The easiest way to implement this is to replace one of the variables using this constraint, like in most phase-field problems. But in my framework, all the variables have a specific evolution behavior and I cannot make any of them an auxvariable. How can I enforce the constraint in my case? Running the simulation without the constraint gives the expected physics, but the variable values go beyond the bounds.

Here is the complete code - https://github.com/rnpratoori/phoenix/blob/main/problems/pstv.i

[GiudGiud]

Hello

There are multiple ways to do this.
For the sum of fractions going to 1:
You could add a lagrange multiplier to your simulation (done with a scalar single-dof variable in MOOSE) and apply it as a constraint
Ideally though, the conservation from the source term in one equation being the sink in the other would conserve the sum

you could use ConstantBounds object object that enforces the [0, 1] interval.
https://mooseframework.inl.gov/source/bounds/ConstantBounds.html

[GiudGiud]

Is phoenix your MOOSE app? How would you describe it in one sentence? It's to add it to the list of knows apps

[rnpratoori]

Yes, Phoenix is my moose app I am currently developing. It is an acronym for Polymer-to-HOst Evolution via INterface eXchange is a reaction-diffusion based framework to study the evolution of polymer scaffold into a neotissue under various conditions.