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Re: [ESPResSo-users] Simulation of fluid using LB-method

From: Georg Rempfer
Subject: Re: [ESPResSo-users] Simulation of fluid using LB-method
Date: Thu, 30 Jan 2014 14:37:07 +0100

About the "pow(lbpar.agrid,4)*tau*tau" issue. That has puzzled us several times as well, but we are very confident that it is correct. I can not remember the specific reason (too long ago) but I remember that Owen checked that and Joost and I checked again, very thoroughly. I think if you actually track down the values from input to application it works out.

2014-01-30 Georg Rempfer <address@hidden>:
Hey all,

I remember that we fixed the issue with the missing 0.5*h*F some years ago. For a flat bounce-back wall aligned with the lattice axis, the actual hydrodynamic no-slip wall is positioned in the middle between the last fluid node and the first wall node. The (0, 0, 0) node of the LB lattice in Espresso is placed at (agrid/2, agrid/2, agrid/2). With that information you should be able to figure out the "real" width of your channel. Does that make it better?

The comment on being able to determine the position of the wall down to 1 agrid only applies to walls not aligned with the lattice axis, like the ones I used in the B.Sc. thesis.


2014-01-30 Ulf Schiller <address@hidden>:

On 01/30/2014 01:44 PM, Wink, Markus wrote:
Is there any way to avoid that problem in espresso or is it due to the
nature of the lattice Boltzmann implementation and do I have to accept
that. I am asking, since I have seen some other MD-simulations, which
also used the LB method but did not show that problem.

I suspect that the external force is not taken into account in the measurement of the hydrodynamic velocity. So what you see is the bare lattice velocity which is lower by 0.5*h*F. If you correct with the latter, do you get a better profile?


Dr. Ulf D. Schiller                        Building 04.16, Room 3006
Institute of Complex Systems (ICS-2)       Phone:   +49 2461 61-6144
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