Comparing unsteady modeling approaches of surges caused by sudden air pocket compression

Closed-pipe, unsteady flows involving the presence of air pockets are present in different hydraulic systems, such as water mains, penstocks, and stormwater tunnels. The study of such flows is relevant, particularly in the case when air pockets become compressed by water due to potential for large, damaging surges. Various approaches have been used to date to assess the magnitude of these surges, such as lumped inertia models (Li and McCorquodale, 1999; Zhou et al., 2001), method of characteristics based models (Zhou et al. 2011, Bousso and Fuamba, 2013), shock-fitting models (Leon et al. 2010), and shock-capturing models (Vasconcelos et al. 2009). In order to handle air phase in the formulation MOC models assume well-defined interfaces between air-water, in some cases vertical-shaped. This work aims to evaluate the accuracy of MOC models in simulating surges caused by air compression following total or partial closing of outflow valves. Various combination of key modeling parameters are considered in this study, particularly pipeline length/diameter ratios, air pocket volumes, initial flow rates and degree of valve maneuvering. Modeled results are compared with experimental results collected by the authors in 50 mm and 100 mm pipelines. Further comparison is performed with a lumped inertia modeling results. These investigations aim to assess the benefits and limits of MOC models in performing such computations in two-phase, closed conduit flows.