Units and set parameters to Algorithms

[angelswat]

Hi @jgostick , @TomTranter.

I have a question about the units of parameters setted in Algorithms. For example, there're algorithms like ohmic conduction, stokes flows, fourier conduction, etc. In all of them, there are two parameters to set, BCVALUE in inlets and outlets. But I do not understand what units and what type of parameter I have to set. Only I know that I can set Dirichlet and Neumann parameters. I want to apply these algorithms but I need to know it.

Please, help me. Thanks.

[jgostick]

Units are not easy to implement in python...we've looked at the available packages at several points over the past few years and they just arent' suitable for one reason or another. So, the end results is that users must keep track of their own units. You are solving for n = gX, then the units you chose for g and X will dictate the rate n. That said, openpnm tries to use SI for all internal 'predefined' calculations.

[angelswat]

Hi,

In all algorithms, there are boundary conditions. Then, what´s the difference between these? That is, example, in ohmic conduction, how set the boundary conditions?? or in stokes flow.

The comments of code, have not helped me.

Thanks.

[ma-sadeghi]

@angelswat You might find this table useful:

Algorithm	Dirichlet BC	Units	Neumann BC	Units
Ohmic Conduction	Voltage	Volts	Current	Amperes
Stokes Flow	Pressure	Pascals	Flow rate	m^3/s
Fickian Diffusion	Concentration	kg/m^3	Mass flow rate	kg/s
Fourier Conduction	Temperature	Kelvins	Energy flow rate	Watts

These are the default BC types and their corresponding units. Of course, you can technically use any units that you prefer. But, it could turn out to be tricky as you need to make sure every model (for instance, those returning thermal conductivity, diffusivity, etc.) you add to your simulation uses the same units. So, we strongly encourage you to stick to the SI units. Let me know if you need more clarification.

[angelswat]

@ma-sadeghi Thanks so much!

Sorry, I´ve another question. When I work with these algorithms, there´re values that I´ve to set, example, thermal conductance, diffusive conductance, electrical conductance, etc. The format is:

phys_water['throat.electrical_conductance']=XXXX

What units have all them? I want to set those parameters but I don´t know the units. Help me please.

Thanks.

[ma-sadeghi]

@angelswat No worries. If you use OpenPNM's built-in models for calculating these parameters, the units are automatically resolved (and assumed to be SI). In the latest version 2.0 (not yet released, but available on dev-V2-alpha1 branch) the physics-related models are located in openpnm.models.physics. For instance, you can use hagen_poiseuille model for calculating throat.hydraulic_conductance by the following command:

import openpnm as op
hagen_poiseuille = op.models.physics.hydraulic_conductance.hagen_poiseuille
phys_water.add_model(propname='throat.hydraulic_conductance', model=hagen_poiseuille)

Similarly in the latest stable version (1.6.2), you have:

import OpenPNM as op
hagen_poiseuille = op.physics.models.hydraulic_conductance.hagen_poiseuille
phys_water.add_model(propname='throat.hydraulic_conductance', model=hagen_poiseuille)

Back to your original question, at any rate, if you want to manually set the these parameters, here's the general rule for figuring out their units:

Units of (conductance) * Units of (Dirichlet BC) = Units of (Neumann BC)

For instance:

Units of (electrical_conductance) * Volts = Amperes.

So the units of throat.electrical_conductance must be Amperes per volts. Similarly in FourierConduction:

Units of (thermal_conductance) * Kelvins = Watts.

So the units of throat.thermal_conductance must be Watts per Kelvins.

[jgostick]

ok, so I will close this now. We cannot really support units and we're basically hoping that numpy decides to do this someday. Until then, users must mind their own units....sorry.