Abstract
The discrete element method (DEM) is a well-established approach to study granular materials in numerous fields of application; each granular particle is modelled individually to predict the overall behaviour. This behaviour can be then extracted by averaging, or coarse graining, the sample using a suitable method. The choice of appropriate coarse-graining method entails a compromise between accuracy and computational cost, especially in the large-scale simulations typically required by industry. A number of coarse-graining methods have been proposed in the literature, and these are reviewed and categorized in this work. Within this contribution, two novel porosity coarse-graining strategies are proposed including a voxel method where a secondary dense grid of “pixel cells” is implemented adopting a binary logic for the coarse graining and a hybrid method where both analytical formulas and pixels are utilized. The proposed methods are compared with four coarse-graining schemes that have been documented in the literature, including the particle centroid method, an analytical method, a method which solves the diffusion equation and an approach which employs averaging using kernels. The novel methods are validated for problems in both two and three dimensions through comparison with the “accurate” analytical method. It is shown that, once validated, both the proposed schemes can approximate the exact solutions quite accurately; however, there is a high computational cost associated with the voxel method. The accuracy of both methods can be adjusted allowing the user to decide between accuracy and computational time. A detailed comparison is then presented for all six schemes considering “accuracy”, “smoothness” and “computational cost”. Optimal parameters are obtained for all six methods, and recommendations for coarse-graining DEM samples are discussed.
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Supporting data are available on request: please contact moriska@mail.ntua.gr.
Abbreviations
- \(a,{ }b,{ }c\) :
-
Distances between the particle centroid and each bin face
- \(A_{{{\text{cap}}}}\) :
-
Area of disc cap
- \(A_{i}\) :
-
Disk segment area \(i\)
- b :
-
Bandwidth (width of averaging kernel)
- \({\text{CFD}}\) :
-
Computational fluid dynamics
- \({\text{Cu}}\) :
-
Coefficient of uniformity
- \(D\) :
-
Diffusion coefficient
- \({\text{DEM}}\) :
-
Discrete element method
- \(d_{{\text{p}}}\) :
-
Particle diameter
- \(d_{{{\text{coarse}}}}\) :
-
Coarse particle diameter
- \(d_{{{\text{fine}}}}\) :
-
Fine particle diameter
- \(F_{{{\text{fine}}}}\) :
-
Fines content (%)
- \(G\) :
-
Shear modulus
- \(h\) :
-
Disk segment height
- \(n\) :
-
Porosity
- \({\text{No}}_{{\text{Eulerian-cells }}}\) :
-
Number of Eulerian cells
- \(N_{{\text{p}}}\) :
-
Number of particles
- \({\text{PCM}}\) :
-
Particle centroid method
- \({\text{Pi}}\) :
-
Number of pixels along each direction of the sample
- \({\text{PPM}}\) :
-
Particle meshing method
- \(r_{{\text{p}}}\) :
-
Particle radius
- \(t\) :
-
Time
- \(V_{{{\text{cap}}}}\) :
-
Spherical cap volume
- \(V_{{{\text{cell}}}}\) :
-
Eulerian cell volume
- \(V_{{{\text{edge}}}}\) :
-
Overlapping edge volume
- \(V_{{\text{p}}}\) :
-
Particle volume
- \(V_{{{\text{segm}}}}\) :
-
Particle segment volume
- \(X\) :
-
Multiplier that controls the Eulerian cell size
- \({\varvec{x}}\) :
-
Vector containing coordinates of Eulerian cell centroids
- \(x_{{\text{c}}} , y_{{\text{c}}}\) :
-
Coordinates of particle centroid
- \({\varvec{x}}_{{{\text{c}},k}}\) :
-
Vector containing coordinates of particle centroid
- \(a\) :
-
Magnitude of diffusion coefficient
- \(\Delta s\) :
-
Fluid cell size
- \(\Delta x\) :
-
Fluid cell size (x-dimension)
- \(\Delta y\) :
-
Fluid cell size (y-dimension)
- \(\Delta z\) :
-
Fluid cell size (z-dimension)
- \(\varepsilon^{p}\) :
-
Particle volume fraction
- \(\zeta_{{i,{\text{cell}}}} \) :
-
Coefficient of volume fraction
- \(\mu\) :
-
Interparticle friction coefficient
- \(\nu\) :
-
Poisson’s ratio
- \(\rho\) :
-
Particle density
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Acknowledgements
Dr. Edward Smith was funded by EPSRC grant EP/P010393/1 as well as by the embedded CSE programme of the ARCHER UK National Supercomputing Service (http://www.archer.ac.uk). He would also like to acknowledge support under both the Distributed and Embedded Computational Science and Engineering Program (dCSE and eCSE, respectively). Original DEM data were generated by Ms. Freya Summersgill and Dr. Thomas Shire.
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Kalderon, M., Smith, E. & O’Sullivan, C. Comparative analysis of porosity coarse-graining techniques for discrete element simulations of dense particulate systems. Comp. Part. Mech. 9, 199–219 (2022). https://doi.org/10.1007/s40571-021-00402-4
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DOI: https://doi.org/10.1007/s40571-021-00402-4