  The interstellar medium is observed in a hierarchical fractal structure over
several orders of magnitude in scale. Aiming to understand the origin of this
structure, we carry out numerical simulations of molecular cloud fragmentation,
taking into account self-gravity, dissipation and energy input. Self-gravity is
computed through a tree code, with fully or quasi periodic boundary conditions.
Energy dissipation is introduced through cloud-cloud ineslatic collisions.
Several schemes are tested for the energy input. It appears that energy input
from galactic shear allows to achieve a stationary clumped state for the gas,
avoiding final collapse. When a stationary turbulent cascade is established, it
is possible to derive meaningful statistical studies on the data such as the
fractal dimension of the mass distribution.
