Christopherus Braun

Peter,

Why not start a discussion about the advantages and disadvanteges of FEFLOW and MODFLOW? I know this discussion is at least so old as groundwater modelling, but maybe it could be helpful for somebody.

Zebra

Sorry,

I did not expect to find a MODFLOW question here (Peter: would you call it a sacrilege?). But talking about the the MODFLOW-Family: path3d is able to compute transport problems on a reversed flow field (but I think this is restricted to steady state flow fields).

Zebra

Michael,

you can swith on this option that in the --> Options --> specific options menu. You will find the option on the bottom of this window. If your computer screen has a lower resolution than FEFLOW wants you will not see this button! Then you have change the resolution of your screen.

Zebra

Hallo,

I think it would be more straight forward to set up a 2D vertical variable saturated model. Here you can easily apply a fine vertical mesh. And the model will be much more intuitive.

Remind that you have to be careful about the global and local coordinate system, because you have to define the boundary conditions according to your local system (you will find some discussions about this in the forum).

Zebra

Hi Florence,

you should shift your local coordinate system to the global one. Then this problem should no longer occure. (Tanslationof your Head-BCs into the local system would also work, but this makes a model a little bit obscure).

This shifting is not very straight forward (I like to call it a graphical voodoo procedure...). You can find an explanation video on the  FEFLOW support site (http://www.wasy.de/deutsch/produkte/feflow/support.html), look at the "How to's" on the bottom of the page and have some fun with it.

I asked the devellopers several times to get rid of this nasty thing. I would appreciate it if also some other people ask the FEFLOW crew for some change in this point. Maybe this would help. I can see no reason in this, and it is an error source, especially if the difference is not so big as in your case you may accept some wrong results.

Hope this will save your problems,

Zebra

Dev,

some ideas to your questions:
[list]
[li] A practical method for obtaining better starting conditions could be to run the model in a confined mode. If this does not run, there is probably some major error in your system. In most practical cases the confined solution should not differ to much from the unconfined one. So this will give you good starting heads.[/li]
[li] If there is one thing about models: you can think of very complex and detailed models, but if they do not run they do not help anything. Especially the vertical discretization (together with a free and movable surface) has some limitations. In most cases I do work with the phreatic option. Maybe this could help you (If you have here convergance problems you could increase the residual water depth in the options settings)[/li]
[li] According to your question about solvers: I would just accept, that each solver has a limited convergance field. So some problems will be solved better with PCG and others better with SAMG. I am always lucky if I have found a running system. The rest of the problem is for mathematicians... [/li]
[/list]

Zebra

Hi,

I had also some case where setting a Transfer Rate to 0 does not restrict the flux to 0. I think this is because of some small numerical errors. Also you have to keep in mind, that you have to assign this value to all surrounding elements, which may cause some problems with neighboutingnodes.

I also would prefere the constrain method.

To my experiences the flux constrains do work quiet well for transient models. There maybe some problems in steady state model runs. The constrains are not checked every iteration. You should (maybe several times) rerun the model and check the water balance.

Zebra

Hi,
you should check 2 things:
- Have you defined the model to be an unsaturated one (in problem class menu)? Otherwise there will be no free surface in your computation.

- You should check your coordinate systems (whether the local one is equal to the global one). This is important because the values you define as BCs are related to the LOCAL system (I have no idea why they still use this in vertical 2D models).

Zebra

Amelia,

the most frequent error source in importing data from a shapefile is the current coordinate system you use in FEFLOW. By default FEFLOW switches to a local coordinate system and in most cases your shapefile will of course not match those coordinates.  To check this: click left in the field on the lower left part of your FEFLOW screen where you see the coordinates. Here you can switch between local and global coordinate system.

Hope this helps.

Zebra

Peter,

any plans to incorporate a coupled flow-transport-boundary condition? In the case of recharge the ways described above are more or less straight forward. But in some cases, e.g. river infiltration from a 3. kind boundary the user defined input of concentration may become quiet tricky.

zebra