Can we close the left boundary in X1t and X2t?

[HUAN]

Hi,

I am planning to run a reactive transport model with only diffusion. The ideal model is shown in below. The water in shale will be transported to sandstone by only diffusion. Therefore, I do not want an external water to flux in. So, I want to close the left boundary. However, I did not find a way to do so. Can we implement this model? If yes, by what way can I do do?

Appreciated.

Huan

[Jia Wang]

Hello Huan,

You can specify flow conditions for your reactive transport models on the Flow pane of your X1t or X2t input file.

For a pure diffusion problem in X1t, I think you need to be setting the discharge or head potential drop to be 0. I would suggest that you check the boundary conditions set if you had altered them. The program by default uses “inlet/free outlet” boundary conditions: where fluid flows into the domain, solute mass can advect, diffuse, and disperse across the boundary, but where it exits the domain, transport occurs by advection alone. If you have set discharge in an X1t 0 but manually changed the program to treat the left boundary as an inlet, you would still allow the effects of diffusion to cross that boundary. Please see more information about boundary conditions in section 2.9 of the GWB Reactive Transport User's Guide.

For X2t, you can set the left and right boundary as open to flow, closed to flow (no-flow), or specified discharge across the bound. A no-flow boundary is a special case of a specified discharge boundary, in which the discharge is set to 0. Again, you would want to check the boundary conditions set. To see more information regarding setting flow in an X2t simulation, please refer to section 4.3 of the GWB Reactive Transport Modeling User's Guide.

Hope this helps,
Jia Wang
Aqueous Solutions LLC

 

[HUAN]

Hi, Jia:

Appreciation for your help. But I still have some questions. Could you give me some ideas?

1. First, my ovjective is to construct a model without both the injection of external water at the left boundary and the export of mass at the right boundary. So, technologically, it is a strictly closed system without any interactions with the external system.

2. My first question is what the boundary condition 'normal' in the 'Flow' pane mean?

3. In X1t, to consutrct such a model, I need to do three steps: (1) setting the discharge as 0, (2) setting the left boundary condition as 'outlet', and (3) setting the right boundary as 'outlet'? Because in 'outlet' condition, diffusion and dispersion are precluded. Therefore, the mass will not cross such a boundary. If this is correct?

4. In X2t, I tried to set both the left and right boundaries as 'closed'. However, the simulation returns errors and does not run. Therefore, to my understanding, to construct such a model, I need to do three steps: (1) setting the left boundary as 'closed' or setting the discharge as '0', (2) setting the left boundary as 'outlet', (3) setting the right boundary as 'open', but the discharge as '0', and (4) setting the right boundary as 'outlet'. If this is correct.

Can you help me make sure if above methods are correct?

Cheers

Huan

 

[Jia Wang]

Hello Huan,

Thank you for providing more information about your conceptual model. Here are some answers I hope are helpful.

The "normal" boundary conditions in the Flow pane are what is described as the default "inlet/free outlet" conditions in the user guides. This means that when fluid is entering the domain, that bound set as an "inlet", where mass is allowed to advect, diffuse and disperse across the boundary. On the other hand, where flow is exiting the domain, the bound is set as a "free outlet", where mass can only cross the boundary by advection. When a boundary is set for "normal" conditions, it can vary from being an "inlet" to a "free outlet" throughout the course of the simulation depending on the flow direction.

In your case, I think you are correct in that you should set boundaries with the "outlet" condition since you do wish to diffuse or disperse across and to set the discharge to zero. In X2t, the open flow boundary uses the potential drop across your domain to calculate the flow field and discharge. It sounds like you should be able to set both boundaries to closed and select "outlets" for your left and right bounds. Setting the boundary as "closed" should set the discharge automatically to 0. On the other hand, you can set both left and right boundaries to open and set the potential drop to be 0, keeping the "outlet" condition for both sides and see if that helps?

Hope this helps,
Jia

[HUAN]

Hi, Jia:

I appreciate your assistance. It works well now.

Cheers

[Jia Wang]

Hello Huan,

Glad to hear that your model is up and running. I hope the rest of your project goes smoothly.

Best regards,
Jia