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

[Ulf Schiller]

Brief question:

Currently it is not guaranteed that the delta in
lb_lbfluid_get_interpolated_velocity(...) and lb_viscous_coupling(...)
are the same, that is, the interpolation and spreading operators are not
conjugate and thus momentum conservation is not guaranteed. Is this on
purpose?

Cheers,
Ulf

On 28/11/14 11:26, Ulf Schiller wrote:
> Hi Markus,
> 
> this looks like a bug. I haven't touched the code in a while and after a
> brief look at the master I think the coupling needs to be revisited.
> There's a MPI call somewhere in the velocity calculation which is
> inefficient and defies the locality of the algorithm. I couldn't spot
> anything in particular which might cause your problem, and I don't have
> time right now, sorry. I'll try to come back to this in the afternoon or
> over the weekend.
> 
> Cheers,
> Ulf
> 
> On 28/11/14 11:07, Wink, Markus wrote:
>> Hello everybody,
>>
>>  
>>
>>  
>>
>> 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?
>>
>> -       I am using version 3.3.0, so the latest I guess
>>
>>  
>>
>>  
>>
>> 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.
>>
>>  
>>
>> -          Attached you will find the z-velocity as a function of the
>> x-position (by accident I named it y-position). The velocity of the
>> particle looks awkward when it gets close to the boundary (marked with a
>> black line). I furthermore performed a simulation, where I checked the
>> interpolated velocities of the fluid at the origin of the particle. The
>> v_z component of the fluid looks also awkward close to the periodic
>> boundary. Even if I don’t put particles in the fluid, I get a strange
>> velocity of the fluid close to the periodic boundary. I was wondering,
>> if the periodic boundary condition for the LB fluid has some bug. I
>> played around a bit with the variable tau, but it still shows that
>> strange behavior close to the boundary.
>>
>>
>> Does anyone has a clue where this could come from?
>>
>>  
>>
>> Attached I put some graphs of the velocity and the forces acting on the
>> particle as a function of the position in the channel as well as the
>> interpolated velocities of the fluid. Maybe that helps people spotting
>> the error.
>>
>>  
>>
>> Thanks in advance.
>>
>> Greetings
>>
>>  
>>
>> Markus
>>
>>  
>>
>>  
>>
>> *Von:address@hidden
>> [mailto:address@hidden
>> *Im Auftrag von *Joost de Graaf
>> *Gesendet:* Montag, 24. November 2014 21:25
>> *An:* Ulf Schiller
>> *Cc:* address@hidden
>> *Betreff:* Re: [ESPResSo-users] Particle in Fluid, periodic boundary
>> conditions
>>
>>  
>>
>> 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
>>
>>  
>>
> 
> 


-- 
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