Re: [ESPResSo-devel] A bug in Espresso

[Ivan Cimrak]

Hello,

I noticed some strange behaviour. I created an lbfluid, initiated it
with some nonzero velocity and let it evolve (EXTERNAL_FORCES were
switched off). I printed the velocity profile of the fluid throughout
the simulation. Then I put a particle in the fluid and again printed
the velocity of the fluid. I supposed, that the outcome should be
different, since any particles influence the behaviour of the fluid.
But the output was identical.

Then I did the same with EXTERNAL_VELOCITY switched on and the outputs
were different.

I think that in file lb.c, in procedure lb_viscous_coupling, after
local_f is increased by

local_f[0] += delta[3*x+0]*delta[3*y+1]*delta[3*z+2]*delta_j[0];
local_f[1] += delta[3*x+0]*delta[3*y+1]*delta[3*z+2]*delta_j[1];
local_f[2] += delta[3*x+0]*delta[3*y+1]*delta[3*z+2]*delta_j[2];

also the flag
        lbfields[node_index[(z*2+y)*2+x]].hasforce
shold be set to 1. Otherwise, in procedure lb_collide_stream, the
procedure lb_apply_forces is not called

            if (lbfields[index].has_force) lb_apply_forces(index, modes);

When EXTERNAL_FORCES are switched on, this problem does not occur.


Thanks,

Ivan