jefflew

I was wondering if anyone has tried modelling the diffusion of gaseous oxygen through an unsaturated clay layer.  I am investigating a clay cap that was installed over mine tailings to prevent water infiltration and leaching.

Chapter 11 of Vol. 1 of the white papers is relevant, but that is a discussion of gas flow, not diffusion.  The obvious difference it that diffusion is driven by a concentration gradient and flow is driven by a pressure gradient, so I am not sure that it really applies.

Any thoughts?  Has anyone tried this?  Is it possible in FEFLOW?

Pete and Panos:

Those were both really helpful posts.  Thanks!

Thanks Pete, that was really helpful.  I have been running a big unsaturated model and have found that convergence can be a challenge using the autostepper with no maximum limit.  When I limit the step to 0.01 days, the model takes a day to run, but I get convergence.  So what you are saying fits with what I see. 

Anyone have an opinion about the best way to overcome convergence problems?  What do you find to be most effective - refining the mesh, or reducing the time-steps (or something else)?

Is there any method for trying to resolve convergence issues that is more efficient than simply experimenting?

If anyone else is facing the same sort of problem, have a look in the FEFLOW reference manual, chapter 22.  It is not exactly the same issue, but it helped me in understanding what I needed to do to create a modelled void.

Hi Pete:

Thanks for both replies.  The discrete features won't work because the scale I am working with is small - an experimental cube of soil 5m on a side with a void that is about 1 m in diameter, so the 1D and 2D discrete features will not accurately reproduce what is happening.  I tried using a large K-value for the void, but the model showed the void as a preferential flow pathway with a higher water content than the surrounding soil.  In reality, the void was just that -empty, with no water in it.

I have thought about ganging up a series of 2D discrete features along adjacent nodes to "build" a 3D feature, but have not tried it yet. 

How would you go about modelling a void in the vadose zone?  The discrete feature elements don't seem appropriate because they are 1-D or 2-D.  I am modelling karst-like voids, which are 3-D. 

I have experimented with setting the unsaturated porosity to 1 at all nodes within the void and with setting a hydraulic head BC of 0 (atmospheric) at all nodes adjacent to the void, but I am not happy with the result. 

Any thoughts?

Thanks!

Jeff

Has anyone tried to regionalize a FEFLOW model (i.e. akima, kriging) in three dimensions?  I would like to hear your experiences and advice.  Is there any third-party software that could help?

We are working with a complex, non-layered environment.

Thanks!

Thanks for the reply, Peter.  In your opinion, is there any way to give a generalization of which attributes have the biggest impact on modelling speeds - clock rate, FSB speed, CPU cache, something else?  What hardware do you use and how often do you update?

Any thoughts on how different processors (specifically the Intel i5 vs i7) will affect modelling in FEFLOW?  I am in the market for new hardware and am not sure how to evaluate which processors are most cost effective.  Same for RAM.

I am most interested in cost effectiveness.  If the top end processor give 1% improvement in performance compared to the second tier processor, I am not that interested in paying for it.

Thanks!