Clarification on PorousFlowHalfCubicSink

[danielquestions]

Hi MOOSE team,

We’re trying to implement evapotranspiration boundary conditions using PorousFlowHalfCubicSink, and we're a bit confused about how the max parameter works — particularly in combination with use_mobility = true.

In the documentation (modules/porous_flow/examples/groundwater/ex02_steady_state.i), the example assumes:

ET rate = 4e-5 kg/m²/s
k = 1e-10 m²
rho = 1000 kg/m³
mu = 1e-3 Pa.s

It then states:

ET = max * k * rho / mu
⇒ max ≈ 40

However, based on our understanding of this equation, the correct value of max should actually be:

max = ET * mu / (k * rho) = 0.4

So we suspect the documentation may contain a mistake?. Could someone clarify this?

Also, we're unsure about the units of the sink: is the resulting flux expected to be in kg/m²/s (for fluid mass), and is max unitless even when used with use_mobility = true?

Also in unsaturated conditions (solving Richards), when injecting or extracting a fluid in the unsuturated zone would you always recommend to use_relperm = true?

We’d appreciate any help clarifying how to properly set this up, since we want to apply this boundary condition to simulate ET in a realistic way.

Thanks a lot in advance!

[GiudGiud]

@WilkAndy can you please help here?

[WilkAndy]

Hi @danielquestions

• your calculation of max = 0.4 is correct, given those parameters you quote. The MOOSE input file has slightly wrong documentation there. However, i note that the input file actually uses k=1E-12 m2, so its calculated value of max = 40 ends up being the correct value to use (ie, just the comment is wrong, not the actual value used) and that's what your formula would have also calculated, had you used that permeability.
• the resulting value for a fluid sink is always kg/m2/s.
  • with no use* then the value of f*g is kg/m2/s. I like to think of flux_function, f, as dimensionless, and then the dimensions of max (and g) will be kg/m2/s. Alternatively, you might like to think of f as dimensionful, and g as dimensionless.
  • with use_mobility then the value of f * g * mobility is kg/m2/s. This means, for example, f = dimensionless, g (and max) has dimensions of kg/(m2 s2) .
• I'd always use_relperm = true for extraction, otherwise you might hit convergence problems as MOOSE tries to extract fluid from a region where there is none. For injection it's less important.

[WilkAndy]

The documentation error (1E-10 to 1E-12) will be fixed when #32118 is merged