The density
is added to the node_dof records.
Now this density is used in the equations,
and the data item record materi_density in the data part
needs not to be specified.
The density is governed by the mass conservation law
where vj is the velocity in the direction of the space coordinate xj. Don't forget to initialize the density by specifying node_dof records. Condition: the -materi differential group_type should be present. And see also group_materi_density_minimum.