The -B.div(B) source term in the momentum equation was dropped for
the isothermal MHD case. Apparently, its lack causes some numerical
instabilities related to the accumulation of the divergence of B.
Therefore, take it into account for the isothermal case too.
Signed-off-by: Grzegorz Kowal <grzegorz@amuncode.org>
Grzegorz Kowal
a45a380d0c