• Gaseous oxygen diffusion

    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?
  • Re: Overcoming convergence problems

    Pete and Panos:

    Those were both really helpful posts.  Thanks!
  • Re: Overcoming convergence problems

    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. 
  • Overcoming convergence problems

    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?
  • Re: Voids in vadose zone

    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.
  • Re: Voids in vadose zone

    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. 
  • Voids in vadose zone

    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?


  • 3D regionalization

    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.

  • Re: Processors for modelling

    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?
  • Processors for modelling

    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.