  The expansion and collision of two wind-blown superbubbles is investigated
numerically. Our models go beyond previous simulations of molecular cloud
formation from converging gas flows by exploring this process with realistic
flow parameters, sizes and timescales. The superbubbles are blown by
time-dependent winds and supernova explosions, calculated from population
synthesis models. They expand into a uniform or turbulent diffuse medium. We
find that dense, cold gas clumps and filaments form naturally in the compressed
collision zone of the two superbubbles. Their shapes resemble the elongated,
irregular structure of observed cold, molecular gas filaments and clumps. At
the end of the simulations, between 65 and 80 percent of the total gas mass in
our simulation box is contained in these structures. The clumps are found in a
variety of physical states, ranging from pressure equilibrium with the
surrounding medium to highly under-pressured clumps with large irregular
internal motions and structures which are rotationally supported.
