[lammps-users] Is there an error in shear stress (pxy) calculation in LAMMPS example file "in.wall.2d.lmp"?

Greetings to all developers and helpers,

I am calculating the viscosity by using the Couette flow concept as in the LAMMPS viscosity example “in.wall.2d.lmp”. My concern here is the calculated “pxy” by the “thermo_style” command which later used for viscosity calculation. “pxy” is one of tensor components computed by “compute pressure” which uses the simulation box volume rather than the using the volume occupied by the fluid particles in the group “flow”. The simulation box volume will include the volume of groups “upper” and “lower” which just serve as boundary conditions and we set them as zero force and zero velocity so, their virial and kinetic energy terms are zero. Why the developer did not use the volume of the group “flow” which is a fixed volume (simple to assign !) over the simulation run. Sorry if simple question but this confuses me, and I did not find an answer. I attached the input file “in.wall.2d.lmp” which is a part of LAMMPS version at Jan. 2021.

Thanks all for your previous and future help about.

Mohamed, Ph.D. student, NTU, Singapore.

in.wall.2d.lmp (2.02 KB)

Perhaps Aidan can answer your question.