D3Q27 Directions - Order and Naming

To really reuse code and to further unify the code lines, the directions must be matched. While doing this it could be also useful to adapt the naming scheme.

Currently, we have the following naming scheme with inconsistent order:

We need to agree on two points.

1. What order of directions should we follow?
2. Should we stick with the naming scheme or adapt it? Alternatives could be for example:

• MPZ. (X = Minus, Y = Plus, Z = Zero)
• ABC (X = (A) Minus, Y = (B) Plus, Z = (C) Zero)

added cpu gpu labels

changed the description

IMHO the CPU order is better and I like the MPZ scheme...

the "original" ABC scheme is meant that way (a=minus, b=zero, c=plus)

The advantage of CPU order is a visual difference for directions. As a compromise, I'd prefer the MPZ. I find ABC absolutely not intuitive.

Great, then we can decide on the order of the CPU.

If we also want to go with MPZ approach, I wrote down the definitions, so we can take this as a deciding-help. Thats how it would look like:

In the end, I would say that you both have to divide ;-)

If I were to classify the schemes, I would go for the original geographical notation then the MPZ then the ABC. I find the original geographical notation more intuitive and readable than both the MPZ and the ABC.

As an additional help:

I've created a google sheets document with the different notations: https://docs.google.com/spreadsheets/d/1i4X01SUYhgcw7mLch8hrWvccPH-QmeOxdDuqctZ2BxE/edit?usp=sharing

@m.schoenherr and I have adjusted the order in the GPU version so that it is the same as in the CPU version.

Right now there are four headers specifying the D3Q27 Directions:

• src/lbm/constants/D3Q27.h
• src/cpu/VirtualFluidsCore/LBM/D3Q27System.h
• src/gpu/VirtualFluids_GPU/LBM/D3Q27.h
• src/gpu/GridGenerator/grid/distributions/D3Q27.h

They all use the same order now, so we can remove some of them.

In order to have one place i suggest to keep src/lbm/constants/D3Q27.h and to remove the rest.

What we discussed so far from my understanding:

• abc is nice, for use for moment AND direction
  • as moment it is only used in the kernel, can also be an ALIAS.
• MPZ is a bit more intuitive
• E-W-T is the one we are all most used to, but has a few disadvantages.

BTW: walberla is using E-W-T as well. https://i10git.cs.fau.de/walberla/walberla/-/blob/master/src/stencil/D3Q27.h

(Personally, I have also found it the most convenient so far.)

mentioned in merge request !120 (merged)

@m.schoenherr and me removed the D3Q27.h from VirtualFluids_GPU and GridGenerator. Both parts use src/lbm/constants/D3Q27.h now.

!120 (merged)

Summary of other LBM codes:

• Walbera - EWT - https://i10git.cs.fau.de/walberla/walberla/-/blob/master/src/stencil/D3Q27.h
• Palabos - MP0 -https://gitlab.com/unigespc/palabos/-/blob/master/src/latticeBoltzmann/comprehensiveModelsTemplates3D.h#L319
• STLBM - MP0 - https://gitlab.com/unigehpfs/stlbm/-/blob/master/educational_lbm/lbm_q27.h#L163

These examples of other codes show the problem of be consistend in the complete code. ;-)

a resume could be: If we want to be exclusive, we should use the ABC notation. If out code schould be readable for other groups, we should stick to the MP0 notation. :-)

suggestion:

d3q27.cpp

I am not sure about the underscore... this causes some effects by using Mathematica...