[lammps-users] How to calculate viscosity

Thanks Steve. Here is my follow up questions.

fix viscosity command just do exchanging momenta between two particles in different regions of the simulation box every N steps. This induces a shear velocity profile in the system. This enables a viscosity of the fluid to be calculated.
But which command can alculate the total momentum transferred?

Can you help me check the following command, if I want to output the momentum transferred by these swaps by the fix?

fix visco mobile viscosity 100 x z 20
fix 1 mobile ave/time 1 10 50 f_visco[1] f_visco[2] f_visco[3] file momentum.mobile

Thanks.

Yang

09年4月15日，周三, Steve Plimpton [email protected] 写道：

> 发件人: Steve Plimpton [email protected]
> 主题: Re: [lammps-users] How to calculate viscosity
> 收件人: “YANG LU” <luyang0309@…215…>
> 抄送: [email protected]
> 日期: 2009年4月15日,周三,下午9:17
>
> I would read the original Muller-Plathe paper for the
> formula. Read section 4.15 of the manual for how
> to get quantities out of fixes. Also the thermo_style
> custom command doc page.
>
> Steve
>
> 2009/4/14 YANG LU <luyang0309@…215…>:
> > Hi All
> > I want to use “fix viscosity” command to calculate polymer’s shear
> > viscosity.
> > It is mentioned in documentation that “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.”
> >
> > But I still can’t figure out how to do it.
> > Which argument is calculated in this command? How to get the momentum flux?
> > How to out put the result of a “fix” command?
> > Is there any easy way to get the shear rate mentioned above?
> > Thanks.
> >
> > Yang
> >
> >
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|

Read the fix viscosity doc page. It tells you the fix stores a scalar
quantity which is the momentum you want. It also tells
you where else to read about how to extract that quantity from
the fix.

Steve