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

[Ulf Schiller]

Hi Markus,

I tried another thing and replaced the boundaries by plane walls normal
to the x and z direction. With that, I do not see any peculiarities.
This suggests that the issue is related to the more complex boundaries
in your system. I'll have a look at your boundaries.tcl tonight.

Cheers,
Ulf

On 01/12/14 17:08, Wink, Markus wrote:
> 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
> <mailto:address@hidden: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
> 
>  
> 


-- 
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