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 “mass*velocity", in real unit, this is "g/mol*A/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.

Steve

2010/3/24 gesong <[email protected]>: