Size Does Matter – Just Ask DEM
As I sit down to write this week’s blog entry I can’t help but think about my early years as a senior mechanical engineering student in the early 2000’s starting to use this novel tool that I had never heard of before: DEM (Discrete Element Modeling). I used it to study the packing behavior of granular materials. Those of us following this blog (and if you are not yet following it, read the earlier posts and then start following it!) know that as it turns out DEM was nothing new back then, in fact it had been 30+ years in the making. But as a mechanical engineering student I knew fluid mechanics and thermodynamics – I was blissfully ignorant about granular mechanics. And so I spent my entire summer before senior year running numerical simulations using a FORTRAN code and writing a report that allowed me to graduate the following spring.
While writing that report, I got very interested in this “new” science called granular mechanics. And so after talking to my advisor, I decided to pursue a Ph.D. degree focused in granular mechanics. The first thing I did in graduate school was to look into DEM. The potential was amazing. Note that I said potential, as there are still some serious hurdles that DEM needs to overcome. The main attraction to DEM is that we can get away with a constitutive theory for granular materials since we are tracking each individual particle in the domain of interest. Anyone that has ever run a DEM simulation will tell you that this is also one of its deterrents, as the time it takes to run a simulation can be prohibitively long. Add to this computational expense the fact that sometimes the model needs to be “calibrated” to the physical phenomena in question, and DEM can become inapplicable to “real” engineering applications.
A cheap way to speed up the simulation is to enlarge the particle size, thus reducing the number of particles for a particular mass of material. In doing so, the engineer may inadvertently change the rheology of the simulation and hence any conclusions drawn may not apply to the physical phenomena under consideration. If there was ever a situation where size matters, it is with granular materials. In a recent paper, I showed how increasing the particle diameter can increase the resulting shear stress significantly. So when using DEM, one must be very judicious in choosing input parameters. As the old adage goes: garbage in, garbage out!
DEM is a great tool. But like every great tool, it can be very dangerous in the hands of untrained users.