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Re: [ESPResSo-users] constant velocity field in "object in fluid"

From: Ivan Cimrak
Subject: Re: [ESPResSo-users] constant velocity field in "object in fluid"
Date: Tue, 23 Jul 2019 00:34:54 +0200

Dear Gianmarco, 

This issue is more related to general Espresso's LB fluid. 

To generate constant velocity field, you can use ext_force_density parameter. Do you have specific reason why you set it to zero? Anyway, you can indeed induce uniform flow field by setting nonzero horizontal velocity at the top AND the bottom of your box. I advice to use rhomboid lbboundary making sure there are some LB points inside the rhomboid. So the rhonmboid should have thickness at least 2 (with lbgrid 1). And place two rhomboids, one on top and one on bottom of box. 

Due to periodicity, it may work also one rhomboid, but about that I am not sure. 

I hope this helps, 


On Mon, 22 Jul 2019, 17:25 gianmarco via Espressomd-users, <address@hidden> wrote:
Hello everyone,

I'm writing this messsage because I'm trying to reproduce a fluid
constant velocity field within the package "object in fluid". To do
this, I created a fluid at rest, therefore by setting the parameter
"ext_force_density" to a zero 3d vector. To impose a constant velocity
field I inserted a wall on the top of the simulation  box, with the command


wall_shape = shapes.Wall(normal=[1, 0, 0], dist=Lz)



where vcell is a parameter representing the boundary velocity. The
resulting effect is that a mesh inserted in the fluid "feels" the effect
of the velocity field, i. e. it moves at constant velocity. However, the
configuration files produced with the command

lbf.print_vtk_velocity("fluid_%04i.vtk" %i)

show a uniform and constant velocity field, equal to zero. Does anyone
know how to fix this problem?

Thank you in advance for you response.

Best regards


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