# Shear flow, velocity profile and displacement profile

Dear all,

I’m running 3D DPD simulations of a sheared system of non-bonded particles. I’m using periodic boundary conditions in all directions and fix/deform command to shear the simulation box. Velocity profiles i obtain through ave/spatial fix, and particles displacement profile along the velocity field direction i’m using ave/time compute with ave/spatial fix. Below i show the results for the situation in which i imposed a constant velocity of 1.6. The velocity profile has a nice linear shape, although i expected to obtain a velocity of 1.6 at the moving plane instead of 1.3. Is it there a way where to fix this issue? Secondly, i also expected to obtain a linear shaped displacement profile. It seems that something wrong is happening on how i should treat particles crossing the periodic boundary.

I wonder someone can help me.

All the best,

Sacha

Dear all,

i’m sending again the same message, i realized that by mistake i sent the displacement profile graph twice and didn’t send the velocity profile graph.

Dear all,
I’m running 3D DPD simulations of a sheared system of non-bonded particles. I’m using periodic boundary conditions in all directions and fix/deform command to shear the simulation box. Velocity profiles i obtain through ave/spatial fix, and particles displacement profile along the velocity field direction i’m using ave/time compute with ave/spatial fix. Below i show the results for the situation in which i imposed a constant velocity of 1.6. The velocity profile has a nice linear shape, although i expected to obtain a velocity of 1.6 at the moving plane instead of 1.3. Is it there a way where to fix this issue? Secondly, i also expected to obtain a linear shaped displacement profile. It seems that something wrong is happening on how i should treat particles crossing the periodic boundary.

I wonder someone can help me.

All the best,

Sacha

If you are using fix deform on a fluid that is keeping up with
the deformation, then you should be using the remap v option
with fix deform, and your velocity profile should be 0 at the
bottom of the box, and have the shear velocity of the upper edge/face
at the top of the box. There are various ways to screw this up -
e.g. shear too fast, don’t wait long enough to equilibrate, thermostat
incorrectly. For the latter you want to be sure you are subtracting
out the streaming velocity before applying the thermostat - see the
4.16 section of the manual for a discussion of bias when thermostatting.

Steve