Friday, January 24, 2014

Spin Around & Aligning with the Hertz Model

By changing the 'Material model' in the Woo-DEM from 'Linear' to 'Hertz' it is possible to make the particles go round, be it Clock-Wise or Counter Clock-Wise, the direction is incidental.

“The Hertz-Mindlin model begins by assuming that contacting solids are isotropic and elastic, and that the representative dimensions of the contact area are very small compared to the various radii of curvature of the undeformed bodies. Another assumption of the Hertz-Mindlin model is that the two solids are perfectly smooth. Only the normal pressures that arise during contact are considered (the extensions of Hertz theory for the tangential component of traction will be discussed later). The Hertz-Mindlin contact-force-displacement law is nonlinear elastic, with path dependence and dissipation due to slip, and omits relative roll and torsion between the two spheres. Strictly speaking, the simplified contact force-displacement law is thermodynamically inconsistent (i.e., unphysical), since it permits energy generation at no cost.”

If you want you could check out the action yourself in Woo-DEM,  the program is free (, and it’s just a 5 to 10 minute download & installation, copy these lines into your Ubuntu Terminal and you're good:

sudo add-apt-repository ppa:eudoxos/woo-daily
sudo apt-get update
sudo apt-get install python-woo

or use the singlecore-version:

sudo add-apt-repository ppa:eudoxos/woo-daily
sudo apt-get update
sudo apt-get install python-woo-singlecore

To start the program simply type in ‘woo’ and next you can press the F10-key to launch the control panels. Chose at the Preprocess-tab the preset plugin ‘EllGroup (woo.pre.ell2d)’, select Hertz (highlighted in yellow) to make ‘m go round and press the arrow in the lower right corner to process these  settings.

Press Play (>) at ‘Simulation’ to start all the action, you can switch ‘Trace particles’ on/off at the Trace-tab; for coloring the particles look at the Display-tab where you set ‘colorBy’ to ‘angVel’ and select ‘Z’ at ‘vecAxis’ or chose some other settings, have fun!

btw notice that the 'Restitution coefficient' (COR) is set to 0.7 if you boost it up to 0.99 then there's less disipation, and the particles will move more dynamic and for a longer time, but by doing so the subtle aligning interaction is gone and they become again chaotic, acting similar to the ‘Linear’ method ... and that *special* circular motion is gone. Hopefully this can be fixed.

For more info on Contact Models and Coefficient Of Restitution in Woo: