[lammps-users] The pdim direction must be periodic in fix viscosity?

Dear all LAMMPSers,
I try to use fix viscosity command to compute the shear viscosity of some fluid.
Is the “pdim” direction in this command is restricted to be periodic?
There is no restrictions about this in Manual. However I can not get a linear velocity profile if the “pdim” direction is non-periodic.

Any sugguestion is appreciated.

Thank you all very much.