Simple question about YoungsModulus...

Submitted by hbc on Fri, 10/23/2015 - 04:19

Hi, I'm running the simulation of steel particle sedimentation.
In the input file of LIGGGHTS, I think that the unit of 'YoungsModulus' is [Pa] in SI unit.
But, when I write the steel's mechanical property(about 200GPa = 200.e9) on particle and aluminium property (about 68.9GPa= 689.e8) on the wall,
the calculation results is so weird. (The particle's rebounded force is very large...)
I want to know the definite unit of YoungsModulus in the input file.
Thank you

bc

Forums:

aaigner's picture

aaigner | Fri, 10/23/2015 - 10:22

Hi hbc,

If you use
units si
Yes, it is Pascal, but in DEM simulation the Young's modulus is usually artificially reduced to reduce the required timestep.

In your case, most probably the time step is too big, thus the resulting forces are very large.

Bests
Andreas

Daniel Queteschiner | Fri, 10/23/2015 - 13:51

You can, for example, estimate the required time step size in the LIGGGHTS script, by using the following formula
variable youngsModulus equal 5.0e6
variable poissonsRatio equal 0.45
variable radius equal 0.005
variable density equal 2000
variable shear_mod equal ${youngsModulus}/(2.0*(${poissonsRatio}+1.0))
variable t_rayleigh equal PI*${radius}*sqrt(${density}/${shear_mod})/(0.1631*${poissonsRatio}+0.8766)
variable dt equal 10.0^(floor(log(${t_rayleigh}))-1)

hbc | Fri, 10/23/2015 - 15:50

I studied the rayleigh time, and I knew that the time step should be 10% of rayleigh time (Tr).
So, I want to know the information (or derivation) about the 'dt'

variable dt equal 10.0^(floor(log(${t_rayleigh}))-1)

Thank you

bc

Daniel Queteschiner | Tue, 10/27/2015 - 12:04

That line is basically a rounding procedure that gives you a "nicer" time step in the form 10^n instead of a "messy" number like 0.00012345

esmaeilyazdani | Tue, 10/27/2015 - 19:02

Dear All

when I change artificially young modulus to reduce time step, is my simulation valid yet? I think this matter can affect my results.

Regards

Esmaeil

ckloss's picture

ckloss | Thu, 12/10/2015 - 22:22

Hi Esmaeil,

yes you are right. Typically most people would use 5e7 Pa as Young's Modulus, but this will of course effect the physics - e.g. wave propagation speed will be different

Best wishes
Christoph