explosions are supposed to inject both thermal and mechanical energy in the interstellar medium. Past investigations have shown that, when heated, the nearby gas cools quite rapidly and the injected thermal energy is simply radiated away. However, when energy is injected in the form of kinetic energy, the star formation process is affected (Navarro & White 1993). In the present simulations, supernova explosions were supposed to inject essentially mechanical energy into the interstellar medium through a ”piston” mechanism, represented by the momentum carried by the ejected stellar envelope. The distance $D_p$ covered by such a “piston”, ejected with a typical velocity $V_{ej} \sim 3000 km.s^{-1}$ in a time interval $\Delta t$ is $V_{ej}\Delta t$. Under this assumption a ”stellar cell” is defined including all gas particles inside a spherical volume of radius $D_p$ that will be affected by the ”piston”. The released energy is redistributed non-uniformly among these particles. In this process, it is expected that the closest gas particles receive more energy than the farthest ones. This was achieved by assigning to each gas particle $j$ a distance dependent weight $w_j(r) = A/r^n_{ij}$ , where $r_{ij}$ is the distance between the ”i-stellar" particle (host of the SN explosion) to 