LAP Water

Moisture models for wood drying (and eventually wood movement)

Hardware requirements

A computer having at least a four-core CPU and 2 GB of RAM is recommended. While the simulations will run on a lesser machine, you may find that you can't use the computer for anything else while a simulation is running. And simulations typically run for hours.

Additional software required

You will need to download and install the Gmsh and GetDP packages available from http://www.montefiore.ulg.ac.be/~geuzaine/. Gmsh is the mesh generator and provides a graphical user interface. GetDP is the finite element solver, and runs within Gmsh. Both are available for Windows, Mac OS X and Linux systems.

Quick start

1. Click the Download ZIP button on this page to download all of the model files from this site.
2. After installing Gmsh and GetDP, start Gmsh.
3. Select File|Open and open one of the .pro files.
4. Click on the graphical image of the model and drag the mouse to rotate it around as desired. Use the mouse wheel to zoom in and out.
5. In the tree that's displayed on the left side of the screen, click the down-arrow next to Post-processing (near the bottom), and ensure that all three options (cutX, cutY and cutZ) are selected. This will cause GetDP to compute all three cut planes after the simulation has run.
6. Click the Run button. The first thing that happens is that the mesh is generated. Once that process has completed, the mesh will be displayed graphically. (You need to zoom way in to see the individual mesh elements.)
7. As soon as the mesh has been generated, GetDP will begin execution and solve the time-dependent behavior of the model. This will take on the order of a few hours to complete.
8. Once the simulation has completed, one of the three cut planes will be displayed, but it will be obscured by the mesh. To hide the mesh, select Tools|Visibility from the menu, click the Numeric tab at the top of the popup window, and click the Hide button next to Element in the list (you don't need to hide anything else).
9. You should now be able to see the solution on the displayed cut plane. Display the other two cut planes by checking their corresponding checkboxes (all labeled U).
10. Use the right- and left-arrow keys to run through the time steps of the simulation. Note that the time steps at the beginning of the run are very small, and get larger as the simulation runs.

That's the basic idea. I've only scratched the surface of what you can do with Gmsh and GetDP, but you'll have to read the (skimpy and sometimes confusing) documentation to learn more.

Going beyond the basics.

The .geo and .pro files are human-readable specifications of the problem to be solved. Look inside them with an ordinary text editor (e.g., NotePad) to see what they're all about.

.geo files

The .geo files are geometry files. They describe the geometrical characteristics of the problem; i.e., the shape(s) and size(s) of all of the bits and pieces that make up the model. The format of the files is extremely simplistic, but that means that specifying something as straightforward as a cube still takes lines and lines of equations. A word of caution: Gmsh is very forgiving of errors you make in your geometric specification. GetDP is the exact opposite, and if, for example, one of your edges or faces has the wrong polarity, GetDP will either crash with an undecipherable error, or give you results that have no connection to reality. Solving these kinds of problems is beyond the scope of this mini-tutorial...

diffusion.pro file

The diffusion.pro file contains all of the physics of the problem. Look inside to see various parameters that you can modify to vary the simulation conditions.

If you plan to create a new problem, copy one of the .geo files as a starting point (call the new file myProblem.geo, for example, and also create a corresponding myProblem.pro file. Inside that file, add a single line that links to diffusion.pro. (See the existing files for examples.) Once you have those two files, you can open myProblem.pro in Gmsh and run a simulation against your new problem.

To do

I'd like to eventually get to the point where this site hosts a fully interactive system that allows you to directly import geometry from SketchUp or other CAD program, and let the system figure out how to convert it into the format the Gmsh and GetDP require.

I'd also like to be able to model wood movement as well, but that might take a greater investment of effort than I have time for at the moment. We'll see...

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This work is licensed under the MIT License (MIT).