Fluidization and fluid views of discrete event systems

Fluidization is an efficient relaxation technique to tackle classical state explosion problems in discrete event systems (DES), which consists in approximating the discrete states by some continuous or hybrid ones. This is not a technical work in the most classical sense. More epistemological and methodological, the purpose is to overview fluidization in several well-known modeling paradigms for DES. As a central modeling paradigm for parallel and synchronized DES, Petri nets are considered first and the most used firing semantics for timed fluid PNs are briefly discussed. Fluid queuing networks and process algebras are related to fluid Petri nets. In the last part, three modeling formalisms that by definition use continuous states are recalled with a special interest on Forrester diagrams and their connection with fluid Petri nets. We assume from the reader a basic knowledge of Petri nets [1], [2].