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We systematically perform hydrodynamics simulations of 20 km s^-1 convergingflows of the warm neutral medium (WNM) to calculate the formation of the coldneutral medium (CNM), especially focusing on the mean properties of themultiphase interstellar medium (ISM), such as the average shock front positionand the mean density on a 10 pc scale. Our results show that the convergence inthose mean properties requires 0.02 pc spatial resolution that resolves thecooling length of the thermally unstable neutral medium (UNM) to follow thedynamical condensation from the WNM to CNM. We also find that two distinctpost-shock states appear in the mean properties depending on the amplitude ofthe upstream WNM density fluctuation (= sqrt(<drho^2>)/rho_0). When theamplitude > 10 %, the interaction between shocks and density inhomogeneityleads to a strong driving of the post-shock turbulence of > 3 km s^-1, whichdominates the energy budget in the shock-compressed layer. The turbulenceprevents the dynamical condensation by cooling and the following CNM formation,and the CNM mass fraction remains as ~ 45 %. In contrast, when the amplitude <=10 %, the shock fronts maintain an almost straight geometry and CNM formationefficiently proceeds, resulting in the CNM mass fraction of ~ 70 %. Thevelocity dispersion is limited to the thermal-instability mediated level of 2 -3 km s^-1 and the layer is supported by both turbulent and thermal energyequally. We also propose an effective equation of state that models themultiphase ISM formed by the WNM converging flow as a one-phase ISM.

Bimodal Behavior and Convergence Requirement in Macroscopic Properties of the Multiphase Interstellar Medium Formed by Atomic Converging Flows