This merge request renames "force" to "forceDensity" in the LBM force models. With this change, we aim to emphasize that the user must specify a body force density in waLBerla's force models.
The force is defined as \boldsymbol{F} = m \rho \boldsymbol{a}
, where m = \Delta x^{3} \rho
is a cell's mass, \rho
is the macroscopic fluid density, and \boldsymbol{a}
is an acceleration.
The force density is \boldsymbol{F_\rho} = \boldsymbol{F} / \Delta x^{3} = \rho \boldsymbol{a}
, because we use \Delta x = 1
in waLBerla.
When using a incompressible lattice model, the density does not change and is equal to the LBM reference density \rho_{0}=1
. Therefore, \boldsymbol{F} = \boldsymbol{F_\rho}
.
In contrast, when using a compressible lattice model, the density may vary in space and time, so that \boldsymbol{F} \neq \boldsymbol{F_\rho}
.
Additionally, this merge request adds a warning when Guo's forcing model is used in conjunction with a TRT or MRT collision operator. This warning addresses #176 and should be removed once the issue is resolved in !560.