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Update How-to viscosity and put the example together with other viscosity examples.

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Zheng Gong
2020-04-30 17:05:01 +02:00
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doc/src/Howto_viscosity.rst
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Calculate viscosity
===================
The shear viscosity eta of a fluid can be measured in at least 5 ways
The shear viscosity eta of a fluid can be measured in at least 6 ways
using various options in LAMMPS. See the examples/VISCOSITY directory
for scripts that implement the 5 methods discussed here for a simple
Lennard-Jones fluid model. Also, see the :doc:`Howto kappa <Howto_kappa>` doc page for an analogous discussion for
Lennard-Jones fluid model and 1 method for SPC/E water model.
Also, see the :doc:`Howto kappa <Howto_kappa>` doc page for an analogous discussion for
thermal conductivity.
Eta is a measure of the propensity of a fluid to transmit momentum in
@ -130,9 +131,24 @@ time-integrated momentum fluxes play the role of Cartesian
coordinates, whose mean-square displacement increases linearly
with time at sufficiently long times.
The sixth is periodic perturbation method. It is also a non-equilibrium MD method.
However, instead of measure the momentum flux in response of applied velocity gradient,
it measures the velocity profile in response of applied stress.
A cosine-shaped periodic acceleration is added to the system via the
:doc:`fix accelerate/cos <fix_accelerate_cos>` command,
and the :doc:`compute viscosity/cos<compute_viscosity_cos>` command is used to monitor the
generated velocity profile and remove the velocity bias before thermostating.
.. note::
An article by :ref:`(Hess) <Hess3>` discussed the accuracy and efficiency of these methods.
----------
.. _Daivis-viscosity:
**(Daivis and Todd)** Daivis and Todd, Nonequilibrium Molecular Dynamics (book),
Cambridge University Press, https://doi.org/10.1017/9781139017848, (2017).
.. _Hess3:
**(Hess)** Hess, B. The Journal of Chemical Physics 2002, 116 (1), 209-217.