Commit 41329b68 authored by Christoph Rettinger's avatar Christoph Rettinger
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Added docu to PdfField class

parent e0058121
...@@ -9,7 +9,7 @@ in particular the fluid-particle coupling. ...@@ -9,7 +9,7 @@ in particular the fluid-particle coupling.
It scales from laptops to current and future supercomputers while maintaining It scales from laptops to current and future supercomputers while maintaining
near-perfect efficiency. 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 ## Documentation and Tutorials
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...@@ -52,6 +52,21 @@ namespace lbm { ...@@ -52,6 +52,21 @@ namespace lbm {
* z-coordinates! * z-coordinates!
* Also, particle distribution functions (i.e., the values stored in the field) can be accessed using stencil * 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 )". * 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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