Dear all,

I came across an inconsistency in units in OGS which is worrying me.

We ran a coupled H2T model (PP model for multiphase flow), where T is defined in [°C]. We found that when the density function is called from CalCoefMass2() for example, T is converted to [K] by

if(diffusion)

TG = interpolate(NodalValC1) + T_KILVIN_ZERO;

Then Density(variables) is called. In our model, rho=f(P,T) (model 6) and the reference T_0 = 21 °C is mfp-input value in [°C], which is intuitive, as her whole model runs with BCs and IC T in [°C].

In this case

delta_T = variables[1] in [K] - T_0 in [°C]

which gives quite a large delta_T (~274°) and thus a very large density change (-110 kg/m³).

So in order to correctly use the PP model and T-dependent density,

it seems to me we have to define IC, BC etc. in [°C], but the mfp-density parameter for rho(T) in [K]. If this is true, I think we should generally think about the way we handle T-units in OGS, as user errors are very likely with the current implementation.

I also found that for other processes the + T_KILVIN_ZERO temperature transformation is applied:

- CalCoefMass()

- Richards

- CalCoefLaplace()

- Richards (if(MediaProp->permeability_stress_mode > 1))

- Liquidflow (if(MediaProp->permeability_stress_mode > 1))

- Airflow

- CalCoefLaplace2()

- CalCoefAdvection()

- Heat (if(FluidProp->density_model == 14)

- CalcAdvection

-(if(multiphase))

and several CalCoef_RHS... functions

These seem to be mostly special cases (but I'm not completely sure) but some warning messages should probably be be added as long as there is no standard internal T unit.

What do you think?

Have a nice evening,

Christof

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_______________________________________

Dr. Christof Beyer

Institute of Geosciences

Geohydromodelling

Christian-Albrechts-University Kiel

Ludewig-Meyn-Str. 10

24118 Kiel

Germany

phone: +49(0)431-8803172

fax: +49(0)431-8807606

mobile: +49(0)176-24297908

email: christof.beyer@gpi.uni-kiel.de

home: http://www.ifg.uni-kiel.de/

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