Calibrating parameter inputs using simple bench-test comparisons
Published on: July 1, 2020
Even the best engineering simulation products give inaccurate results if the parameters you define at the start don’t reflect the real-life material properties of what you’re investigating. R&D teams readily turn to material databases for structural, fluid, and electronics input parameters with consistent success. Unfortunately, this is not the case for most granular material analysis. There is no discrete element modeling (DEM) material library, no matter what you hear from anyone else. And there likely will never be.
Whether you are performing simulation at the North or South Pole, in Germany or Japan or the U.S., the properties of steel are always the properties of steel, period. This is not true for granular media. Copper ore mined from north Brazil is not exactly the same as that from a South-African mine; the same applies to coal, coffee beans and sand. It is careless (and potentially expensive) to assume that the material properties would match up. The factors at play include fracture patterns, moisture, size and microstructure, among others.
So where do you start without a global material library or reference sheet, since particles are so diverse? Some chemical and pharmaceutical teams have created reference sets with detailed historical data, but most of the industry is not so fortunate. You can take an educated guess for density, friction-factor, adhesion, cohesion, particle stiffness and moisture parameters, but you still won’t know if your solution is accurate — or even how inaccurate it is.
The proper way to determine accurate DEM parameters is by conducting preliminary calibration or validation bench experiments. Rocky DEM offers an automated solution called Calibration Suite that helps you quickly and easily narrow down the parameters that realistically represent your investigational material.
Calibration entails performing several test cases to reach an ideal, specific set of parameters. If you have ball-park measurements from a reference paper or previous simulations, validation is just as easy. Rocky needs no external software to run the calibration. All processing is done within Rocky’s single interface. And because it requires minimal user interaction, it especially benefits new users. You simply press start and follow the prompts in the pop-up menu. Once you enter parameters, the calibration starts automatically. As you gain experience in a specific granular material, you can build up your own parameter data set.
For advanced users, the Rocky Calibration Suite incorporates an automatic post-processing script that uses advanced techniques to perform a high-level analysis of data. Plots show all values that need to be compared, such as angle of repose, average angles, standard deviation, statistical error. If the simulation result doesn’t match your experiment, you know that one of the parameters is incorrect — the friction factor is too high for the angle of repose, for example — so repeat the calibration with an adjusted value.
Rocky DEM’s Calibration Suite tests
The Rocky Calibration Suite includes these tests:
Remember, a calibration simulation will produce results even if you input incorrect parameters. But there is little meaning in those results — you cannot be sure that it is accurate!
Leon White Nogueira
CAE Applications Specialist
Leon White Nogueira is a mechanical engineer with an M.Sc. from the Polytechnic School at the University of São Paulo (POLI-USP). He joined ESSS as a CAE applications specialist, first in the computational fluid dynamics (CFD) group and currently in the discrete element method (DEM) group on the Rocky DEM technical team.