Page Menu
Home
c4science
Search
Configure Global Search
Log In
Files
F99781415
fix_viscous.html
No One
Temporary
Actions
Download File
Edit File
Delete File
View Transforms
Subscribe
Mute Notifications
Award Token
Subscribers
None
File Metadata
Details
File Info
Storage
Attached
Created
Sun, Jan 26, 14:38
Size
2 KB
Mime Type
text/html
Expires
Tue, Jan 28, 14:38 (1 d, 23 h)
Engine
blob
Format
Raw Data
Handle
23811156
Attached To
rLAMMPS lammps
fix_viscous.html
View Options
<HTML>
<CENTER><A
HREF =
"http://lammps.sandia.gov"
>
LAMMPS WWW Site
</A>
-
<A
HREF =
"Manual.html"
>
LAMMPS Documentation
</A>
-
<A
HREF =
"Section_commands.html#comm"
>
LAMMPS Commands
</A>
</CENTER>
<HR>
<H3>
fix viscous command
</H3>
<P><B>
Syntax:
</B>
</P>
<PRE>
fix ID group-ID viscous gamma keyword values ...
</PRE>
<UL><LI>
ID, group-ID are documented in
<A
HREF =
"fix.html"
>
fix
</A>
command
<LI>
viscous = style name of this fix command
<LI>
gamma = damping coefficient (force/velocity units)
<LI>
zero or more keyword/value pairs can be appended
<LI>
keyword =
<I>
b
</I>
or
<I>
a
</I>
or
<I>
t
</I>
or
<I>
m
</I>
<LI>
zero or more keyword/value pairs may be appended to the args
<PRE>
keyword =
<I>
scale
</I>
<I>
scale
</I>
values = type ratio
type = atom type (1-N)
ratio = factor to scale the damping coefficient by
</PRE>
</UL>
<P><B>
Examples:
</B>
</P>
<PRE>
fix 1 flow viscous 0.1
fix 1 damp viscous 0.5 scale 3 2.5
</PRE>
<P><B>
Description:
</B>
</P>
<P>
Add a viscous damping force to atoms in the group that is proportional
to the velocity of the atom. The added force can be thought of as a
frictional interaction with implicit solvent. In granular simulations
this can be useful for draining the kinetic energy from the system in
a controlled fashion. If used without additional thermostatting (to
add kinetic energy to the system), it has the effect of slowly (or
rapidly) freezing the system; hence it is a simple energy minimization
technique.
</P>
<P>
The damping force F is given by F = - gamma * velocity. The larger
the coefficient, the faster the kinetic energy is reduced. If the
optional keyword
<I>
scale
</I>
is used, gamma can scaled up or down by the
specified factor for atoms of that type. It can be used multiple
times to adjust gamma for several atom types.
</P>
<P>
In a Brownian dynamics context, gamma = kT / mD, where k = Bolztmann's
constant, T = temperature, m = particle mass, and D = particle
diffusion coefficient. D can be written as kT / (6 pi eta d), where
eta = viscosity of the frictional fluid and d = diameter of particle.
This means gamma = 6 pi eta d, and thus is proportional to the
viscosity of the fluid and the particle diameter.
</P>
<P>
In the current implementation, rather than have the user specify a
viscosity (in centiPoise or some other units), gamma is specified
directly in force/velocity units. If needed, gamma can be adjusted
for atoms of different sizes (i.e. sigma) by using the
<I>
scale
</I>
keyword.
</P>
<P>
Note that Brownian dynamics models also typically include a randomized
force term to thermostat the system at a chosen temperature. The
<A
HREF =
"fix_langevin.html"
>
fix
langevin
</A>
command adds both a viscous damping term
and this random force to each atom; hence if using fix
<I>
langevin
</I>
you
do not typically need to use fix
<I>
viscous
</I>
.
</P>
<P><B>
Restrictions:
</B>
none
</P>
<P><B>
Related commands:
</B>
</P>
<P><A
HREF =
"fix_langevin.html"
>
fix langevin
</A>
</P>
<P><B>
Default:
</B>
none
</P>
</HTML>
Event Timeline
Log In to Comment