How do you consider viscosity?

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1

Dear sir

I want to simulate the fluid flow considering viscosity. However, my input file does not take into account the viscosity. Considering the velocity profile, the velocity profile at the bottom is 0 (Vbottom = 0 ), and I want it to rise gradually.

What commands should I enter and what should I consider?

Thank you for reading my mail.

2d flow

dimension 2

boundary p p p

atom_style atomic

neighbor 0.3 bin

neigh_modify delay 5

create geometry

lattice hex 0.7

region box block 0 80 0 10 -0.5 0.55

create_box 2 box

create_atoms 1 box

mass 1 1.0

mass 2 1.0

LJ potentials

pair_style lj/cut 1.12246

pair_coeff * * 1.0 1.0 1.12246

define groups

region 1 block INF INF INF 1.25 INF INF

group lower region 1

group boundary union lower #upper

group flow subtract all boundary

set group lower type 2

initial velocities

compute mobile flow temp

velocity flow create 1 482748 temp mobile

fix 1 all nve

fix 2 flow temp/rescale 20 1.0 100.0 150 1.0

fix_modify 2 temp mobile

flow

velocity boundary set 0.0 0.0 0.0

fix 3 lower setforce 0.0 0.0 0.0

fix 6 flow addforce 5.0 0.0 0.0

Run

timestep 0.003

thermo 1000

thermo_modify temp mobile

dump 1 all atom 100 dump.flow.*

run 25000

2

Dear sir

I want to simulate the fluid flow considering viscosity. However, my input
file does not take into account the viscosity. Considering the velocity
profile, the velocity profile at the bottom is 0 (Vbottom = 0 ), and I want
it to rise gradually.

What commands should I enter and what should I consider?

​viscosity is not ​an input parameter for point particles, but a property
of the model (potential function and choice of parameters).
if your system is not exhibiting the expected behavior, you may need to
check the parameters or perhaps check whether it has reached equilibrium.
please also see the "examples/VISCOSITY" folder and the accompanying
sections of the LAMMPS manual for examples showcasing different methods to
compute viscosity of a material (i.e. model) from MD simulations.

axel.