materi_density

The density $\rho$ 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

$$\dot{\rho} + \frac{\partial \rho v_j}{\partial x_j} = 0$$

where $v_j$ is the velocity in the direction of the space coordinate $x_j$. Don't forget to initialize the density by specifying node_dof records. Condition: the -materi differential group_type should be present.