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

[Ulf Schiller]

[redirected to address@hidden

This also looks a bit fishy in lb_lbfluid_get_interpolated_velocity(...)

          local_j[0] =
lbpar.rho[0]*lbpar.agrid*lbpar.agrid*lbpar.agrid*lb_boundaries[lbfields[index].boundary-1].velocity[0];
          local_j[1] =
lbpar.rho[0]*lbpar.agrid*lbpar.agrid*lbpar.agrid*lb_boundaries[lbfields[index].boundary-1].velocity[0];
          local_j[2] =
lbpar.rho[0]*lbpar.agrid*lbpar.agrid*lbpar.agrid*lb_boundaries[lbfields[index].boundary-1].velocity[0];

On 28/11/14 13:45, Ulf Schiller wrote:
> 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