Passive O2 gas diffusion on Richards mode for hypoxic riperian transect

[Bisesh Joshi]

Hi all,

This is a follow up question. Is using Millington Quirk  (MQ) for tortuosity for both gas and liquid diffusion a valid in Richards mode?. I'm simulating O2 diffusion from atmosphere into top unsaturated zone of my persistently reducing riparian transect. However, using standard value of gas diffusion  (1d-5)  for O2 equilibrated to atmosphere is causing oxygenated system, even with aerobic respiration and other reactions working.

Please let me know if you have any idea on this.

Thank you.

Best regards,

Bisesh

--

Bisesh Joshi

PhD student

Water Science and Policy

Graduate College

University of Delaware

901-297-6761

[Hammond, Glenn E]

Bisesh,

To clarify for future readers, for RICHARDS+GIRT, only diffusion is considered for the gas phase transport—advection does not occur as the gas phase is not simulated on the flow side. The Millington-Quirk model applies to transport tortuosity, utilizing the saturation of each phase. So, yes, MQ should help.

Additionally, refining the vertical grid can help minimize numerical dispersion.

Glenn

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[Bisesh Joshi]

Thank you very much  for the quick reply Glenn.

In this case is it reasonable to reduce gas diffusion coefficient  from 1d-5 to lower values like 1d-7 or 1d-8?

By reducing this value I'm getting reasonable values.

My vertical resolution is 0.15 m.

Best regards,

Bisesh Joshi

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Subject: Re: [pflotran-users: 8743] Passive O2 gas diffusion on Richards mode for hypoxic riperian transect

 

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[Bisesh Joshi]

Hi Glenn,

Does Richard's mode only take one diffusion coefficient?

It seems the first Fluid property (gas) is replaced by the second fluid property (liquid)  in Richards mode when liquid property is mentioned later than gas property.

The results change when liquid property is mentioned before the gas property. 

FLUID_PROPERTY

  PHASE GAS 

  DIFFUSION_COEFFICIENT 1.d-5 !2.4d-9

END

FLUID_PROPERTY

  PHASE LIQUID

  DIFFUSION_COEFFICIENT 1.0d-9

END

Thanks for your help.

Best regards,

Bisesh

[Hammond, Glenn E]

Bisesh,

Reducing the gas phase diffusion coefficient will diminish the impact of diffusion within the gas phase. To account for gas phase diffusion, I recommend significantly decreasing the time step size and observing whether this adjustment improves your results. If you notice an improvement, it will be necessary to continue using a smaller time step to achieve a reasonably accurate solution.

Glenn

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[Hammond, Glenn E]

Bisesh,

In contexts outside of reactive transport, the diffusion coefficient applies specifically to each fluid phase; therefore, only one diffusion coefficient exists for each phase.

I'll need to investigate the issue you've described to determine what might be causing the problem. Can you send me a small test problem that fails?

Glenn

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