[lammps-users] about fix viscosity

Hi,dear sir,
i use “fix viscosity” command to calculate the viscosity, but i am confused about the units: the fix calculate the total momentum, the unit of the total momentum is “massvelocity", in real unit, this is "g/molA/fs”, which is “intensive”. i am confused how can i transfer this to the real momentum flux in my simulation? Thanks for any feedback.

Ge Song
PHD candidate
Engineering mechanics,
Tsinghua university,
Beijing, China

The doc page says:

The cummulative momentum transferred between the bottom and middle of
the simulation box (in the pdim direction) is stored as a scalar
quantity by this fix. This quantity is zeroed when the fix is defined
and accumlates thereafter, once every N steps. The units of the
quantity are momentum = mass*velocity.

As described below, the total momentum transferred by these velocity
swaps is computed by the fix and can be output. Dividing this quantity
by time and the cross-sectional area of the simulation box yields a
momentum flux. The ratio of momentum flux to the slope of the shear
velocity profile is the viscosity of the fluid, in appopriate units.
See the Muller-Plathe paper for details.

The momentum is simply the sum of all the swaps: sum m *
delta-velocity. It's up to you to convert
to a flux and a viscosity, using the methods in the MP paper.


2010/3/24 gesong <[email protected]>: