Hi Steve,

# I have a question on this very old post about calculating the viscosity using Muller-Plate Method using fix viscosity. Here is the relevant post:# Either this (both are the same):

or

Here are the comments from you and the previous post:

Previous post: I also take the velocity profile from the end of the run reported by fix 2. I plot the x-direction velocity as a function of the z-height and take the slope of the linear fit. This is then dvx/dz, which has a value of 0.0098.

You: If you look in the MP paper (cited on the LAMMPS doc page), there is a factor of 2 in this calculation, which you may be missing.

My concern:

As I understand, the original paper has discussed on the PBC and that factor has already been included in the formula:

j= (f_3)/(2tLx*Ly)

Suppose, for simplicity though we can always do regression, I calculate the slope like this:

Slope = [vx(lx/2)-vx(0)]/[lz/2] i.e velocity difference at the middle and bottom layer divided by half the box size.

Initially, I thought that the total flux is through that face of the box. However, as written in the lamps website: “The scalar is the cumulative momentum transferred between the bottom and middle of the simulation box (in the pdim direction) is stored as a scalar quantity by this fix”. Do I still need the extra factor as you mentioned in that post if I calculate slope as mentioned above?

Thanks in advance for you comment.

Subas Dhakal

Syracuse University

Hi Steve,

I have a question on this very old post about calculating the viscosity
using Muller-Plate Method using fix viscosity. Here is the relevant post:

have you looked at the inputs in the examples/VISCOSITY folder? they
contain *validated* input examples for the multiple methods supported
by LAMMPS.

axel