ABSTRACT
The physical processes encountered in all branches of sciences and engineering can be classified into two major categories: time-dependent processes and stationary processes. Time-dependent processes describe evolutions in which quantities of interest change with time. If the quantities of interest cease to change in an evolution then the evolution is said to have reached a stationary state. Not all evolutions have stationary states. The evolutions without a stationary state are often referred to as unsteady processes. Stationary processes are those in which the quantities of interest do not depend upon time. For a stationary process to be valid or viable, it must correspond to the stationary state of an evolution. Every process in nature is an evolution. Nonetheless it is sometimes convenient to consider their stationary state. In this book we only consider non-stationary processes, i.e. evolutions that may have a stationary state or may be unsteady.
