Dear Lammps users,
I know that the velocity profile can be calculated using the following command, as shown in the document:
“fix 1 flow ave/chunk 100 10 1000 molchunk vx vz norm sample file vel.profile”.
However, when I use “norm all” instead of “norm sample” for plane poiseuille flow simulation, I found that the maximum velocity is one to two orders greater than the value computed using “norm sample”. Also, with the velocity profile computed using “norm sample”, the viscosity of the flow particles calculated from the Hagen-Poiseuille equation (with the slip velocity) is much greater than the bulk viscosity (calculated from MD). On the other hand, the viscosity calculated from the velocity profile using “norm all” is within the same order of the bulk viscosity. I wonder why this is the case, and I am not sure which velocity profile I should use now. Could you please share your ideas or experiences on this?