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Re: [ESPResSo-users] Any limit of the lattice Boltzmann grid number?


From: Ulf Schiller
Subject: Re: [ESPResSo-users] Any limit of the lattice Boltzmann grid number?
Date: Wed, 31 Aug 2011 10:46:03 +0200
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On 08/31/2011 10:38 AM, Florian Fahrenberger wrote:
Hallo Chunda,

first of all: Please ALWAYS use the mailing list for questions!
Otherwise everyone who encounters the same questions will not be able to
find the answer already given and ask again.

1. But the basic idea from your last reply to
choose cell number is as many as computer can handle so that the whole
"resolution" of water is high enough to avoid unphysical behavior such as
fluctuation, right?

No. That is not what I would do. Since the Lattice-Boltzmann time step
is usually larger than the MD time step, you will more probably end up
with unphysical behaviour if you choose your grid too fine. Each LB time
step, the fluid speeds can only propagate for one LB cell, and a very
fine mesh results in very small cells, which results in a very slow
propagation of hydrodynamics.
As I said before, grids of 16 or 32 cubed are usually big enough for a
senseful fluid dynamics. If you have huge or unconform systems like
tubes or pores, you might want to go with 64^3 or even bigger, but that
is usually not necessary.

It should also be noted that any change of the lattice units (i.e. grid spacing and LB time step) changes the viscosity and the equation of state (the latter being of minor importance in the incompressible limit). Consequently, with a different time step one gets different Reynolds, Schmidt, Peclet, etc. numbers.

Best wishes,
Ulf

--
Dr. Ulf D. Schiller                        Building 04.16, Room 3006
Institute of Complex Systems (ICS-2)       Phone:   +49 2461 61-6144
Forschungszentrum J├╝lich, Germany          Fax:     +49 2461 61-2850

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