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[Christian Jakob <question210907@xxxxxxxxxxxxxxxxxxxxx>]

Question #210907 on Yade changed:
https://answers.launchpad.net/yade/+question/210907

Christian Jakob proposed the following answer:
> and anybody has advanced method to generate numerical sample to model
real granular sand?

Hi wangxiaoliang,

I also want to simulate a sand with DEM. So our interests are nearly the same. 
To generate a sand sample there are (as far as I know) 3 methods:

1. generate with given PSD -> hard overlaps -> calm-function needed
2. radius expansion method -> also calm function needed
3. generation of sample in a higher model -> let particles fall down by gravity (settlement)

Every method has advantages and disadvantages. 2. and 3. can be done with makeCloud().
I tried all methods and found, that for my purpose (model of a loose sand) method 3 is the best one. 

In method 1 very hard overlaps occur after generating the sample and one
has to calm particles movements (by setting velocities to zero) for the
first steps. I found out, that it can lead to instabilities and
unexpected behavoir. So I tried method 2, but I also detected numerical
instabilities arising from this method. Also it is very tricky to
increase radii from clumped particles (as I have in my model).

So now I use settlement method (method 3) and I do not have
instabilities any more. But the disadvantage of this method is, that the
number of particles (that were generated) are random. Also one can not
specify a porosity. One will get loosest packing, when settlement is
finished. (But you can use ThreeDTriaxialEngine to get your
porosity/packing after the settlement procedure!)

For the problem with rolling resistance I use clumped particles and a
high friction to get the given repose angle from a pour cone test. So I
replace one sphere by hardly overlapping two spheres with same radius to
get a "non-spherical" particle (clump). I know thats not the real
particle shape, but it is more realistic than pure spherical particles.

Regards,

Christian.

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