Alex Costall PhD Student

Hi again,

Sorry Bjorn, I have managed to confuse myself once again.

Given a 2D system with a Richards-style equation, I want to model a known inflow of fresh water from one side of the model.

Do I apply the known value of recharge to each cell on the border?
i.e. The same number regardless of number of cells or total length?

Or..

Do I need to account for the number of cells, and the depth range which those cells span?

For example, in the picture below - do I need to apply -0.5 m/d for each of the nodes in the target area, or do I need to apply -(0.5/8) to each of the nodes, to make a total of -0.5 m/d?

I suspect this is a fundamentally obvious question, and I apologise in advance.

Great - Thanks Bjorn.

I am fairly confident I have covered all of the points mentioned in that thread - I will cover them here again for posterity..

[list][li][b]Is the mesh fine enough?[/b][/li][/list]
I think so! Assuming the Peclet number is a valid measure of mesh approriate-ness (I think we discussed in another thread).
[list][li][b]Does the mesh contain bad shaped elements?[/b][/li][/list]
0.0% Delaunay-violating triangles
6.5% > 90 degrees
0.0% > 120 degrees
[list][li][b]What temporal discretization are you using?[/b][/li][/list]
Automatic, Maximum of 1 day.
[list][li][b]Fully implicit schemes?[/b][/li][/list]
FE/BE
[list][li][b]What error norm are you using?[/b][/li][/list]
First.

The only way I've been able to stabilise it is using the upwinding, and I've used the PGLS (least-squares) one for no other reason that it seems to work best (I didn't have much luck with Shock upwinding).

There are just so-many nodes now, it's incredibly slow to run - and it's not even 3D yet...!

[quote]There is no hard limit available to stop negative concentrations, because the amount and magnitude of negative concentrations depend on model specific settings. [/quote]

Could you elaborate on the model-specific settings...? Is this mesh-related or another parameter?

I am trying to simulate the seawater interface in a highly conductive aquifer, yet get massive numerical instability and negative mass concentrations!

I have tried to include the upwinding, but need a sharper mixing zone (i.e. lower dispersivity?) to even closely match the collected data.
If I understand it correctly, upwinding will increase the dispersivity to reduce the gradient over the cells and should resolve some of the instability issues?
The other option I have seen recommended is to refine the mesh more, however my model already has 1.5 million nodes with element diameter of < 0.4 metres, and Peclet number is less than 0.5 on average, and I can't wait all year for even finer models to run...

so there are two questions;
[list type=decimal]
[li]How can I stop getting negative solute concentrations (i.e. is there a hard limit available?)[/li]
[li]How do people [b]realistically [/b]model saline water intrusion in reasonable time-frames?[/li]
[/list]

Any real-world examples would be greatly appreciated - I'm struggling to find anything in the literature...

Thanks Bjorn - I think I was just describing my problem very poorly...!

Hopefully this image works;

[img width=200 height=71]http://i.imgur.com/bYcMsER.png[/img]

Assuming all of these boundaries are defined in supermesh, I want to select the region indicated in blue, from the 'complex boundary' to the 'bottom of model'.

The idea being that, complex geometries or boundaries presented by different geological facies can be selected and adjusted.

Sorry for delay - The problem is 2D, so unless I've totally just missed it, there's no Copy Selection button...

Thanks Bjorn - I am able to select the map-line itself, no worries, but I need to select nodes *below* that line....!

Great - thankyou!

"In transport models, the elemental distribution Péclet Number is available which can be used to check whether the spatial discretization is sufficiently fine for a specific transport problem."

I'm struggling to find more information on this topic. What dictates 'sufficiently fine' as far as Peclet numbers are concerned?

Is lower or higher better....?!

I am trying to select every node below a supermesh line, yet can't find any means to do so via the expression editor!

Surely there's a way to do it?!

For example; I have a line which has a slight curve along the length, which has been included in the meshing stage. I want to select every node below that particular line. I can't simply use the expression editor and select every node below a particular 'y' as this will miss sections of the line.

Hi Bjorn,

Thanks for looking into that.

Can you confirm that you managed to expand the tutorial example to a 3D setting? (If so, how?!)

For reference, I'm using the example 8.9 from the Introduction to groundwater modelling, from the course-notes.

I am also on the latest version!