PFC - Energy commands reference

PFC
1

Hello,

I have calculated the different energy components after a triaxial compression test on a rock specimen in PFC using the following FISH functions:

fish define get_kinetic_energy
return ball.energy(‘energy-kinetic’)
end

fish define get_boundary_energy
return wall.energy(‘energy-boundary’)
end

fish define get_parallel_bond_strain_energy
return contact.energy.sum(‘energy-pbstrain’)
end

fish define get_slip_energy
return contact.energy.sum(‘energy-slip’)
end

I would like to know how these energies are actually calculated in PFC — specifically, the equations or formulations used for each type of energy so that I can properly reference them in my work. I was able to find the commands in the documentation, but not the underlying equations.

Additionally, I would like to extract the timestep value in order to plot the different energy components versus timestep. Could you please provide the exact command that returns the timestep? I have tried some commands, but they did not work.

Thank you for your help.

Best regards,

2

If possible, I would also like clarification on the units of these energy calculations. Are they reported in Joules, or in another unit system?

3

Hello, this is David Potyondy. The energy computations are described in the Energy Partitions section of each contact model, for the parallel bond model they are { Linear Parallel Bond Model — Itasca Software 9.0 documentation }. I am not sure where are the equations for energy-boundary and energy-kinetic. I will ask Matt Purvance to answer this.

4

The timestep for the mechanical process is obtained from FISH via mech.timestep {mech.timestep — Itasca Software 9.0 documentation}.

5

Hello David,
Thank you for pointing me to the Energy Partitions section in the contact model documentation. I understand now where to find the information for the parallel bond model. I wasn’t sure where the equations are located, so I’ll wait to hear back from Matt Purvance on this.
Best regards,
Alaa

6

Matt has said the following about energy units. It depends on the units that the user is assuming in their model (models are unitless). For a particle, kinetic energy is 0.5*m*v*v; this is what is computed and reported. If mass is assumed to be in kg and the distance units are presumed to be meters (i.e., SI units), then the units are kg*m*m/s/s which are Joules. What if the user is presuming grams and cm instead, and assigning stiffnesses accordingly too? Then the units will be grams*cm*cm/s/s. As long as they are consistent with units, then it will turn out to be sensible. If using SI units then it will be Joules.

7

There is a discussion about monitoring PFC energy quantities during rock cutting in Section 1.2 of the memo that can be downloaded from the link {ShareFile}.

8

Thank you so much for your help.

Best regards,

Alaa