Alcuni casi di moto bifase non darciano

Nella nota vengono esaminati e discussi alcuni casi in cui il moto bifase in regime laminare nel sottosuolo non segue la classica estensione della legge di Darcy al moto polifase. Si mette inoltre in evidenza che, più che dal tipo di mezzo in cui avviene il moto (poroso o fratturato), il modello e le leggi del moto dipendono dal rapporto tra le forze presenti (viscose, capillari, di gravità e di inerzia) e quindi dai valori dei numeri di Reynolds, di Bond e Capillare. ABSTRACT Until a few years ago, the study of multiphase flow in porous or fractured media was mainly of interest for reservoir engineering. However, this study has recently acquired importance also in the case of aquifer pollution by non aqueous phase liquids (NAPLs). Depending on the circumstances, the fluids may have very different characteristics, and different may be the characteristics of the rocks where they flow: from porous media (unconsolidated or cemented) to fractured rocks (with fractures ranging from microfractures of a few microns, to megafractures which can cover thousands of metres and can have openings sometimes several centimetres wide). Consequently, the factors which affect flow can weight differently in different situations, giving rise to different laws of flow. At first sight, it would seem obvious that there are different models for flow in porous media and in fractured rocks; it can be seen, however, that the laws of flow are decisively affected, not only by the features and velocity of the fluids, but also by the size of the flow paths, a size which may be more similar between a porous medium and a fractured rock than between two very different porous media or two fractured rocks. For example, the flow in a rock with fractures having openings of several millimetres is more similar to the flow in gravel than to that in a rock with microfractures. So, we believe it is more suitable to examine the flow in fractured rocks and in porous media in a unitary manner, based on the size of the flow paths or, rather, on the weight of the present forces: viscous, gravitational, inertial and capillary and more precisely in terms of the three dimensionless numbers: Reynolds number, Capillarity number and Bond number. This feature is especially suitable in the case of flow which does not follow Darcy’s law, extended to multiphase flow, i.e., where the inertial forces are not negligible compared to the viscous forces, or the capillary forces are not prevalent compared to all the others, or the flow of the phases is not cocurrent.