Using residence time for the extraction of recirculation regions

This paper introduces the concept of residence-time, from the Eulerian view point, in a rigorous manner. The equations for various flow regimes are derived and a numerical solver is introduced based on Lax-Wendroff integration. An implementation is discussed that allows the coupling of this solver to any explicit CFD code. Examples of this concept are shown for extracting recirculation regions by segregating old fluid from fluid that has not been in the simulation for much time. The comparison of iso-surfaces generated using this procedure and separation surfaces are examine-d.