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Re: [ESPResSo-users] Multi-realaxation time model
From: |
Ulf Schiller |
Subject: |
Re: [ESPResSo-users] Multi-realaxation time model |
Date: |
Tue, 30 Jun 2015 10:17:00 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0 |
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