Re: [ESPResSo-users] Multi-realaxation time model

[Ulf Schiller]

On 30/06/15 10:10, Wink, Markus wrote:
> Hello Ulf,
> 
> perfect, that was exactly what I was looking for! Thanks a lot. 
> 
> One quick question. Does ESPResSo use the collide-stream or stream-collide 
> algorithm? In the source code it is written, that it uses stream-collide if 
> the macro "PULL" is defined, otherwise it uses the collide-stream. 
> Nevertheless, I cannot find, whether this macro is defined, neither in the 
> source code, nor in the configure- or in the Make-files.  

Collide-stream is the default, and I don't know if anyone has used PULL
as of late, so try it at your own risk...

Cheers,
Ulf

> -----Ursprüngliche Nachricht-----
> Von: Ulf Schiller [mailto:address@hidden 
> Gesendet: Dienstag, 30. Juni 2015 10:23
> An: Wink, Markus; address@hidden
> Betreff: Re: AW: [ESPResSo-users] Multi-realaxation time model
> 
> On 30/06/15 09:10, Wink, Markus wrote:
>> Hello Ulf,
>>
>> thanks a lot.
>>
>> But I am still puzzled concerning the matrix. I thought that the collision 
>> operator  Omega should look something like that in MRT:
>> Omega = M^-1*S*(m_eq-m)
>>
>> whereas M transforms the distribution function f into momentum space: m = 
>> M*f.
>> S is a diagonal matrix of the form = diag(s_0,s_1,....,s_18) consisting of 
>> the relaxation parameters. So basically in D3Q19 the matrix S should has 19 
>> parameters.
>> I assume that, for instance, s_0 should be equal to 1 (incompressible limit 
>> of the LBM ???) and that some are equal.
>>
>> The question I have is: If there are four relaxation parameters in ESPResSo 
>> (`gamma_shear`, `gamma_bulk`, `gamma_odd`, `gamma_even`), how is the 
>> diagonal matrix S explicit built out of them?
> 
> The application of the matrix S is performed in `lb_relax_modes(...)` where 
> you can read off how the relaxation parameters relate to the matrix. Due to 
> symmetries there are only four relaxation parameters, and often only 
> gamma_odd and gamma_even are used (TRT - two relaxation time model).
> 
> Cheers,
> Ulf
> 
>> -----Ursprüngliche Nachricht-----
>> Von: address@hidden 
>> [mailto:address@hidden
>> org] Im Auftrag von Ulf Schiller
>> Gesendet: Dienstag, 30. Juni 2015 09:23
>> An: address@hidden
>> Betreff: Re: [ESPResSo-users] Multi-realaxation time model
>>
>> On 30/06/15 08:16, Wink, Markus wrote:
>>> Dear ESPResSo users and developers,
>>>
>>>  
>>>
>>> I have a question concerning the MRT Lattice Boltzmann model used in 
>>> ESPResSo. In MRT the linearized collision operator is transformed to 
>>> momentum space via the collision matrix M. I have found the collision 
>>> matrix in the file lb-d3q19.hpp.
>>>
>>> But I didn't find the relaxation parameters.
>>>
>>>  
>>>
>>> Shouldn't there be some kind of diagonal matrix S containing the 
>>> relaxation rates for the different moments?
>>>
>>> Does anybody know where I can find them or tell me which relaxation 
>>> rates ESPResSo is using for the MRT algorithm?
>>
>> The relaxation rates are `gamma_shear`, `gamma_bulk`, `gamma_odd`, 
>> `gamma_even` and are set in `lb_reinit_parameters()`.
>>
>> 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