how to add gravity body force, b_i, into biot.py

Hi all,

I would like to add gravity body force, b_i, into biot.py: http://sfepy.org/doc-devel/examples/biot/biot.html I could find any examples.

The body force, b_i, in balance equation can be expressed by

(\sigma_{ij} - p \delta_{ij}),j + \rho b_i = 0

where \sigma_{ij} is stress, p is pore pressure and \rho is the density of solid phase. \delta_{ij} is Kronecker delta.

Should I add a new term from scratch?

Thanks in advance.

ouyang