Computational approaches to hydraulic modeling of air entrainment in a steep raw water pipeline

Pipelines with slopes that vary significantly over the pipeline length can create unique design challenges because the flow can transition from open channel to closed conduit. These transitions can cause air entrainment, bulking flow, blow back and surging which can limit the flow capacity and create operational challenges. For steep reaches, the flow can become supercritical which can change rapidly based on upstream or downstream pipe conditions. Due to these changing conditions, typical hydraulic calculations using full pipe or normal depth assumptions are not sufficient to properly assess the pipeline performance and risks due to air entrainment and hydraulic jumps. Analysis for these conditions require the use of gradually varied flow (GVF) models to determine the water surface and velocity along the length of the pipeline. In addition, computational fluid dynamics (CFD) can be used to provide additional insights in complex flow conditions at critical locations such as bends where roll over can occur. This presentation will describe the analysis for several pipelines on a steep slope that historically have experienced flow capacity limitations along with air geysering and blowback. The focus of the analysis for these projects was to evaluate individual reaches of the pipeline where hydraulic conditions indicate a potential for open channel flow and transition to closed conduit flow. Gradually varied flow (GVF), 3D computational fluid dynamics (CFD), and bubble movement and entrainment analysis were used to determine the root causes of flow restrictions. The combination of GVF and CFD modeling with an understanding of bubble movement provided the clients with answers and solutions to a known problem that had persisted for years. This presentation will explain the causes of potential flow restrictions for these types of pipelines, the analysis to identify problem areas, and the potential solutions to achieve desired flow capacities.