Rotating drum simulation with Mohr-Coulomb constitutive law : decrease in volume

Hi everyone,

I’m currently working on the simulation of a rotating drum in 2D. I wanted to use the Mohr-Coulomb constitutive law. The granular material must be non-dilatable and non-cohesive as describe by Zuo et al. [1] or Chandra et al. [2]. I use the following material parameters:

"materials": [
      {
          "id": 0,
          "type": "MohrCoulomb2D",
          "density": 920,
          "poisson_ratio": 0.2,
          "youngs_modulus": 2700,
          "softening": false,
          "friction": 27,
          "dilation": 0,
          "cohesion": 0.1,
          "residual_friction": 13,
          "residual_dilation": 0.05,
          "residual_cohesion": 0.05,
          "peak_pdstrain": 0,
          "residual_pdstrain": 0,
          "tension_cutoff": 1e+27,
      },

I’ve gotten some issue to define the constitutive law. Firstly, I needed to use a low cohesion value otherwise the particles are not defined as suggested by @jgiven100 here.

In this way, I was able to perform a few cycles of rotation. However , over time the volume of particles decrease and I am not sure why.
I checked and the number of particles is normally the same, but I think the volume of each particle decrease with the time. Could this be due to the way the gradient deformation tensor is calculated? Is it something that could be done to avoid this?

I’ve attached the file and a gif to show the behavior I’ve obtain.

zuo

rotating_drum.tar.gz (39.6 KB)

P.S.: I have colored the material_id just to visualize the mixing process but the two materials are the same.

References :
[1] Zuo, Zhijian, Shuguang Gong, et Guilan Xie. « Numerical Simulation of Granular Mixing in a Rotary Drum Using a Generalized Interpolation Material Point Method ». Asia-Pacific Journal of Chemical Engineering 15, nᵒ 2 (mars 2020). https://doi.org/10.1002/apj.2426.

[2] Chandra, Bodhinanda, Veronika Singer, Tobias Teschemacher, Roland Wüchner, et Antonia Larese. « Nonconforming Dirichlet Boundary Conditions in Implicit Material Point Method by Means of Penalty Augmentation ». Acta Geotechnica 16, nᵒ 8 (août 2021): 2315‑35. https://doi.org/10.1007/s11440-020-01123-3.