Is the shear modulus equation in the doc wrong?

Submitted by xuji2010 on Fri, 10/31/2014 - 09:47

Hi all,

In the doc of DEM simulation, the effective shear modulus is:
1 2(2+v1)(1-v1) 2(2+v2)(1-v2)
-- = ------------------ + -------------------
G Y1 Y2

but, the in the global_properties.cpp, line 217:
matrix->data[i][j] = 1./(2.*(2.-vi)*(1.+vi)/Yi+2.*(2.-vj)*(1.+vj)/Yj);

The above two don't match, which one is wrong?

Appreciate any help in advance!

Forums:

ckloss's picture

ckloss | Wed, 11/05/2014 - 19:52

Hi xuji2010,

thanks for the note - I am sure the implementation is correct.
I'll check before the next release!

best wishes
Christoph

Daniel Queteschiner | Thu, 11/20/2014 - 12:19

The implementation is correct, the manual is wrong, i.e., the correct formula is
1/G* = (2 - v1) / G1 + (2 - v2) / G2 = 2 (2 - v1) (1+ v1) / Y1 + 2 (2 - v2) (1+ v2) / Y2
with
G = Y / 2 (1 + v)

(e.g. in: Di Renzo, Di Maio, Chemical Engineering Science, Volume 60, Issue 5, March 2005)

MiRa | Thu, 03/30/2017 - 13:41

Hello everyone,

I came across a similar problem when working through the LIGGGHTS documentation for the gran model hertz [A].

The calculation of the effective shear modulus, G*, states that this equation is something like:
1/G* = 2 (2 - v1) (1 + v1) /......

I tried to trace this equation back to the publication of Di Renzo et al. (2004) [B] (which is mentioned in the documentation) where an equation for G* is mentioned in the appendix. However, the equation for the effective shear modulus in Di Renzo et al. (2004) is different from the one in their 2005 article [C].

The documentation is either pointing to the wrong reference or the equation in [B] is incorrect.

Best regards,
MiRa

[A] http://www.cfdem.com/media/DEM/docu/gran_model_hertz.html

[B] Di Renzo, Alberto; Di Maio, Francesco Paolo (2004): Comparison of contact-force models for the simulation of collisions in DEM-based granular flow codes. In: Chemical Engineering Science 59 (3), pp. 525–541. DOI: 10.1016/j.ces.2003.09.037

[C] Di Renzo, Alberto; Di Maio, Francesco Paolo (2005): An improvedinte gral non-linear model for the contact of particles in distinct element simulations. In: Chemical Engineering Science 60 (5), pp. 1303-1312. DOI: 10.1016/j.ces.2004.10.004

ckloss's picture

ckloss | Sun, 04/23/2017 - 12:22

Hi,

thanks for the note! We'll check for the next release!

Best wishes
Christoph

arnom's picture

arnom | Mon, 04/24/2017 - 09:36

Hi MiRa,

thanks for the comment. Indeed it appears that reference [B] has a typo in the equation. For the sake of clarity we added ref [C] and added a note to [B] indicating this mistake.

The changes will appear in the next release.

Best regards,
Arno

DCS team member & LIGGGHTS(R) core developer