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

[Kai Szuttor]

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
> > 
>

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