text: Depending on the gas flow characteristics in the system there are two states that can be matter of simulation:
  * Steady state – the simulation does not take into account the gas flow characteristics' variations over time and described by the system of algebraic equations, in general nonlinear ones.
  * Unsteady state (transient flow analysis) – described either by a partial differential equation or a system of such equations. Gas flow characteristics are mainly functions of time.