Re: [ESPResSo-users] Particle in Fluid, periodic boundary conditions

Hello everybody,

I performed simulations as Ulf proposed to help to track the problem down. In detail three different simulations are performed, graphs on that can be found attached:

1) I removed the boundaries, putting a particle with an initial velocity v_0y. I can see the exponential decrease of the velocity in y-direction (as expected). The z-position is pretty stable, nevertheless I see an (uncorrelated (?)) change of the velocity in z direction. It seems to be directed towards the –z direction, although it is very small (of the orders of 1E-24 ), so I guess it is an numerical artifact (?). Nevertheless I don’t see the jump in the z-position nor in the z-velocity.

2) A simulation as in 1), in addition the particle has an initial velocity in the +z direction (of the order I got for the lift force). I still see the exponential decrease in both the z- and y-velocity. No jumps of the z-position at the boundaries can be found.

3) The same as simulation 1), but this time with boundaries. I see the migration towards the middle of the channel as expected (notice, that this time, the initial z-velocity of the particle is set to zero). Neither a jump in the z-velocity nor in the z-position can be seen.

In combination of the first script I posted (which was the same as in simulation 3 here, but furthermore an external force is exerted to the fluid) I see the jump in the z-component of the position and the z-component of the velocity only if there is a combination of boundaries and an external force acting on the fluid. What is missing is a simulation with no boundaries and an external force acting on the fluid.

I furthermore played around with the friction coefficient. If I lower it by three orders of magnitude, I still notice a jump in the z-component of the velocity, although it is not as steep as before. I am not sure what this means.

I hope this helps to isolate the problem.

Kind regards

Markus

-----Ursprüngliche Nachricht-----
Von: Ulf Schiller [mailto:address@hidden
Gesendet: Montag, 1. Dezember 2014 16:41
An: address@hidden
Cc: Joost de Graaf; Wink, Markus
Betreff: Re: [ESPResSo-users] Particle in Fluid, periodic boundary conditions

Hi Joost, Markus, all,

after a quick inspection I can't see any relevant changes to the coupling on that branch (I've not gone through the CUDA implementation though). After merging and running the script Markus provided, I'm afraid the issue persists. I don't have time to track this down, but according to my experience it is likely to be due to one of the following:

- problem with pos-lattice mapping (less likely since only halo affected)

- halo not up-to-date

- forces missing in halo upon redef of fluid momentum

Note that this bug potentially affects any system with particle-fluid coupling and periodic boundary conditions.

Cheers,

Ulf

On 28/11/14 15:27, Joost de Graaf wrote:

> Dear Markus,

> Sorry about that. The ENGINE branch on hmenke's git account

> https://github.com/hmenke/espresso/tree/engine

> has a fix for the bug. As I said, I recalled seeing something like it,

> and at that time we apparently fixed the problem (which I cannot

> remember doing). We are still working on one last testcase for the

> ENGINE branch, before we have it pulled into the Master, which will

> take about a week. Then a lot of the LB bugs should be fixed.

> Kind Regards, Joost

> On 28 November 2014 at 16:24, Joost de Graaf <address@hidden

> <mailto:address@hidden>> wrote:

> Dear Markus,

> On 24 November 2014 at 21:24, Joost de Graaf

> <address@hidden <mailto:address@hidden>> wrote:

> Dear Markus,

> I remember spotting something like this in one of the older

> versions of the master, but I tried to find it and could not

> reproduce the bug with the latest version, which one are you using?

> KR, Joost

> On 24 November 2014 at 19:35, Ulf Schiller <address@hidden

> <mailto:address@hidden>> wrote:

> Hi Markus,

> On 24/11/14 18:12, Wink, Markus wrote:

> > can explain it to me? It seems, that the periodic boundary condition in

> > the LB Fluid doesn’t work. If I remove the fluid I don’t see that.

> Yes, looks like something is at odds with the periodicity

> here. Can you

> plot the z-velocity over x-position? That may point towards

> what is

> going wrong.

> Cheers,

> Ulf

Dr Ulf D Schiller

Centre for Computational Science

University College London

20 Gordon Street

London WC1H 0AJ

United Kingdom

Phone: +44 (0)20 7679 5300

From:	Wink, Markus
Subject:	Re: [ESPResSo-users] Particle in Fluid, periodic boundary conditions
Date:	Mon, 1 Dec 2014 17:08:19 +0000