Pressure dependent porosity

[nlosacco]

Hi,

by reading the docs https://mooseframework.inl.gov/modules/porous_flow/groundwater_models.html it is clear that an evolving porosity can be easily implemented, e.g. through PorousFlowPorosityLinear to include porewater pressure dependency. In this way It seems that the porosity will depend on the difference wrt a reference pwp. What I would like to do, instead, is to have an initially homogenous porosity throughout the domain and have it vary with the increment of pwp, as described later on in the docs, at paragraph ”Evolution of porosity for an isothermal, mineral-free, situation”. It is not clear how this can be achieved.

tnx

[GiudGiud]

Hello

The coupleable interface, used to retrieve variable values, can easily be used to compute the increment in time.
The current value is obtained with coupledValue, as can be seen in numerous kernels & materials (look for Coupled...), and the previous value with coupledValueOld. Then the difference is the increment

[nlosacco]

Thanks for your reply @GiudGiud ,
I am not sure I fully understand how to proceed, though.
I implicitly assume that there is no incremental evolution law coded in MOOSE.
So, following your suggestion

1. I calculate the pwp increment;
2. I have to update the porosity wrt to the previous timestep.

How can I do 2) ?

I need a simple linear incremental relation like

d\phi = \beta d\P_f

[nlosacco]

If such a thing doesn’t exist I can use the initial pwp as reference pwp

[GiudGiud]

I have to update the porosity wrt to the previous timestep.

what is porosity in your simulations? A material property? an auxiliary variable?
I am assuming it is a material property because PorousFlowPorosityLinear is a material.

So in that case to achieve 2), you need to modify the material defining the porosity (in the source code) to use the increment.

Alternatively, if the modification is simple enough, you may be able to write a parsed expression in the ParsedMaterial to compute the new porosity value. this expression can depend on the previous porosity value.
However, in that route, you will still need to compute the increment manually.
For that, you can use an auxiliary variable that is a copy of the pwp, set by a ProjectionAux or a CopyAux, and that copy should be performed with execute_on = timestep_begin

[josebastiase]

Hi,

This may be helpful

[nlosacco]

By trying to be very specific on what I thought the problem is, I think I am not making things clear.
I will pause this post and create a new one: #31696 (comment), starting from the beginning of the story.