From 41329b68ac14fc367dd42d6d754d901593f5ec8c Mon Sep 17 00:00:00 2001
From: Christoph Rettinger <christoph.rettinger@fau.de>
Date: Tue, 26 Mar 2019 16:51:56 +0100
Subject: [PATCH] Added docu to PdfField class
---
README.md | 2 +-
src/lbm/field/PdfField.h | 15 +++++++++++++++
2 files changed, 16 insertions(+), 1 deletion(-)
diff --git a/README.md b/README.md
index e9f71cd44..47542b596 100644
--- a/README.md
+++ b/README.md
@@ -9,7 +9,7 @@ in particular the fluid-particle coupling.
It scales from laptops to current and future supercomputers while maintaining
near-perfect efficiency.
-See http://walberla.net/ for more information and a showcase of applications.
+See https://www.walberla.net/ for more information and a showcase of applications.
## Documentation and Tutorials
diff --git a/src/lbm/field/PdfField.h b/src/lbm/field/PdfField.h
index b2e0bce5b..5b4429877 100644
--- a/src/lbm/field/PdfField.h
+++ b/src/lbm/field/PdfField.h
@@ -52,6 +52,21 @@ namespace lbm {
* z-coordinates!
* Also, particle distribution functions (i.e., the values stored in the field) can be accessed using stencil
* directions, e.g. "pdfField( x, y, z, stencil::NE )".
+*
+* Note that the behavior is different for compressible and incompressible lattice models.
+* In the compressible case, the behavior and formulas are as expected from common LBM literature and the density
+* is taken as the 0-th order moment of the PDFs and this value is used.
+* In order to make LBM quasi-incompressible, it was suggested, e.g. in
+* Q. Zou, S. Hou, S. Chen, G.D. Doolen, J. Stat. Phys. 81(1–2), 35 (1995)
+* X. He, L.S. Luo, J. Stat. Phys. 88(3–4), 927 (1997)
+* that the density is implicitly assumed to have a constant value of 1 in many cases (e.g. in getVelocity()),
+* and only the deviation from this value enters some of the formulas, like the equilibrium distribution functions.
+* Additionally, the PDFs are normalized around 0 in the incompressible case to increase the numerical accuracy,
+* i.e. only the deviation of the PDF values from their respective lattice weight is stored.
+* As a result, manually summing of the PDF values will yield the density deviation in this case.
+* But the getDensity() function reverts this normalization (by adding 1) and will yield the physical density.
+* This normalization, however, usually doesn't affect the implementation of functions like LBM sweeps.
+*
*/
//**********************************************************************************************************************
--
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