# Rename force to forceDensity in LBM force models

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.