Re: [ESPResSo-devel] LB fluid with variable viscosity

[Ivan Cimrak]

Ok, 

I will keep this in mind. I suggest that I will wait until you are ready with 
refactoring and afterwards I will redo the variable viscosity feature starting 
from your refactored code….

Do you agree?  When do you expect to have the refactoring done? 

Ivan

> On 6 Dec 2018, at 11:30, Kai Szuttor <address@hidden> wrote:
> 
> Hi Ivan,
> 
> please note that we are currently in the process of refactoring the CPU LB and
> that the code of your development branch and espressos python branch might
> diverge (see PR https://github.com/espressomd/espresso/pull/2392).
> 
> Best,
> 
> Kai
> 
> On Mon, Dec 03, 2018 at 09:05:01AM +0000, Ivan Cimrak wrote:
>> Hello Michael,
>> 
>> 
>> I am going on with implementation of variable viscosity of lbfluid, for now 
>> only for LB. The GPU version will come later.
>> 
>> I added the necessary fields in LB_FluidNode. For now, I added only the 
>> var_visc_gamma_shear. (You can check it at 
>> github/icimrak/espresso/diff_viscosity) This field will not be updated every 
>> time_step, since the blood cells do not jump a lot and their boundaries do 
>> not cross lattice points very often. I suppose that once per 100-500 
>> timestep would be sufficient to reflag the lb nodes. I added further code, 
>> when recomputing the modes in lbfields, and on several other places.  
>> 
>> I am now implementing the reflagging of lbfluid, by setting correct 
>> lbfields[index]var_visc_gamma_shear value. To this end, I need to perform on 
>> each MPI rank 
>> 1. a procedure determining the intersections of blood cell boundaries with 
>> LB lattice grid lines parallel to x-axis (horizontal lines)
>> 2. run over these lines and according to the intersections I need to 
>> flag/reflag individual LB lattice grid points and set the correct 
>> var_visc_gamma_shear value (depending on whether the grid node is inside or 
>> outside the blood cell).
>> 
>> Assume I can determin the intersection of blood cell boundaries and 
>> horizontal LB grid lines (by an algorithm based on a loop over the 
>> particles). I need to send this data from MPI slave to MPI master. But on 
>> each slave, I could have variable number of intersections. How can I 
>> implement this? Can I use the mpi_gather_stats/mpi_gather_stats_slave 
>> functions? How do I implement the variable size od data that need to be 
>> gathered?
>> 
>> 
>> 
>> Thank you,
>> Ivan
>> 
>> 
>> 
>> 
>> 
>>> On 27 Nov 2018, at 15:05, Michael Kuron <address@hidden> wrote:
>>> 
>>> Hi Ivan,
>>> 
>>>> But according to this value I will be able to assign proper shear,
>>>> bulk, ghost even, ghost odd relaxation times. So only one real number
>>>> for each lbnode will be necessary to store.
>>> 
>>> If you update this field less often than once per timestep, it might
>>> make sense to store all relaxation times instead of the single number
>>> as that will save you some computation time (including divisions, which
>>> are rather expensive on GPUs).
>>> 
>>>> But I am not sure how to implement such a field. Could you please
>>>> point me out to some similar variable in ESPResSo, co I can implement
>>>> it in a similar manner?
>>> 
>>> I guess it would be similar to the LB force field (node_f in
>>> lbgpu_cuda.cu or lbfields[index].force_density in lb.cpp).
>>> 
>>>> Or is there a guide how to implement a global field so that
>>>> parallelization will be taken care of?
>>> 
>>> You don't need to worry about parallelization. You can calculate this
>>> field locally on each MPI rank because it only depends on particles
>>> that are on that MPI rank. Furthermore, on the GPU you don't need to do
>>> anything because the particles from all MPI ranks are present on the
>>> GPU.
>>> 
>>> Michael
>>> 
>>