Expression for dashpot force

MFiX
#1

Hi

I am learning DEM theory through dem_doc_2012-1.pdf (doe.gov). In this document, the contact force Fc between two colliding particles is composed of the spring force and the dashpot force.
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The spring force, for example, its normal and tangential components are given.
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But I do not find the dashpot force. How could I know its expression?

Thanks

#2

The Dashpot force is related to the relative velocity, multiplied by a damping coefficient (both in normal and tangential directions):
image

#3

Thank you @jeff.dietiker

#4