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Re: [ESPResSo-users] fluid-particle coupling in different LB resolutions

From: Markus Gusenbauer
Subject: Re: [ESPResSo-users] fluid-particle coupling in different LB resolutions
Date: Thu, 12 Mar 2015 11:05:48 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.5.0

Hi Iva,

I am facing a similar problem with the parameters grid size, friction, mass and viscosity. And I still don't understand how these parameters correlate.
Right now I am experimenting with a single point with external force (gravitation) and its final velocity in the fluid.

A line got my attention in the userguide, maybe there is the problem/solution:

"Internally the LB implementation works with a different set of units: all lengths are
expressed in agrid , all times in tau and so on. Therefore changing agrid and tau, might
change the behaviour of the LB fluid"

And it certainly changes the behaviour!


On 2015-03-11 20:07, Iveta Jancigova wrote:
I realized I didn’t attach a simulation script to my question, so I’m attaching it now.
Please, let me know what could be the issue.
Thank you,


On 11 Mar 2015, at 13:20, Iveta Jancigova <address@hidden> wrote:


I want to ask about a strange behavior I encountered:

When I put a single particle in the center of a channel 20x10x10 with fluid (ext_force on fluid 0.0001 0 0, dens 1, visc 1) and look at the velocities of the particle and fluid at the same position, I get the following:
for grid=1: 	v_fluid ~ 0.000728167	v_part ~ 0.000723167
for grid=0.5: 	v_fluid ~ 0.000737281	v_part ~ 0.00018307
for grid=0.25: 	v_fluid ~ 0.000738749	v_part ~ 4.58593E-05
While the fluid velocity is roughly the same, the particle velocity seems to decrease ~4 times when the grid size is halved.

I have tried varying the friction parameter (even by orders of magnitude) but it only made minimal difference. I suppose that is because the particle is simply carried by the fluid.
So my questions are: Is it possible there is an error in the coupling? Or what can I do to get the proper velocity of the particle?

Thank you,
Iva Jancigova
Iveta Jancigova, PhD student
Faculty of Management Science and Informatics
University of Zilina, Slovakia
cell-in-fluid research group:

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