units in LIGGGHTS

Bias's picture
Submitted by Bias on Wed, 06/13/2018 - 08:34

Hi!
I'm a new user to LIGGGHTS and have a question about the units for cases. How does it work with units in LIGGGHTS; is the code always solving in the unit specified for solving equations or are there cases where it will automatically change to SI units instead?
Is it specified the dimensions of model parameters for example that youngsModulus should be in Force/L^2 i.e. pressure? How can i check what dimensions the model parameters should have?

When doing a modification of a contact model and using a pre existing model parameter, does it have to be taken into account then what unit this model parameter has?

Best regards
Tobias

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Daniel Queteschiner | Wed, 06/13/2018 - 12:48

LIGGGHTS interprets all given values to be in the units specified by the units command, see https://www.cfdem.com/media/DEM/docu/units.html
Any internal dimensional constants used in LIGGGHTS (in most cases none) are also used according to the specified unit system.

Bias's picture

Bias | Wed, 06/13/2018 - 14:35

Does this mean that LIGGGHTS does not change units in the computations to SI units anywhere? Or can there be circumstances when its important to use a model parameter that has the "correct" intended dimensions?
Lets say for example that I use global model property surfaceTension for my own contact model but i specify and use that property as it would actually be a Force as dimension and not energy/length*length. Would that cause a problem for the solver? (More than the obvious coding confusion of course..)

Is dimensions of model parameters specified anywhere or is that information not used?

Daniel Queteschiner | Wed, 06/13/2018 - 15:11

LIGGGHTS does what you tell it to do. It's not doing some magical dimensional analysis on your equations.
It offers some conversion factors specific to the units system given in the input script: https://github.com/CFDEMproject/LIGGGHTS-PUBLIC/blob/master/src/force.h#L80 which are set here: https://github.com/CFDEMproject/LIGGGHTS-PUBLIC/blob/master/src/update.c...
You're the one responsible for applying them correctly and for implementing your equations correctly.
If you use e.g. a density value and the units system is SI then the density value should be in kg/m³ if the units system is cgs then the value should be in g/cm³.

Bias's picture

Bias | Wed, 06/13/2018 - 15:29

Thanks Daniel! I was interpreting this also from the code but just wanted to be sure :)

Best regards
Tobias