Numerical investigations of flow behavior and energy losses in open channel junctions

Understanding the behavior of combining flows in open-channel junction is of interest in environmental and hydraulic engineering. The complexity of the junction flow problem stems from flow mixing, secondary circulation, post-confluence flow separation, contraction and backwater effects. These effects in turn result in a large number of parameters required to adequately describe the energy losses and flow structures associated with the different flow behavior and mechanisms due to flow merging. This thesis presents results from a detailed numerical investigation of the hydrodynamics of open-channel junctions, with emphasis on adequately predicting energy loss due to junction. Energy losses in junctions is an important factor to be considered when designing sewer pipe network so that the system can store excess flow without flooding and overflows. The thesis work first applied the state of art 1-D dynamic model to examine quantitatively the uneven upstream water depth increase as well as the post-confluence energy loss. The model is based on applying the momentum principle in the stream-wise direction to two control volumes in the junction by considering the interfacial shear force between the two control volumes, the boundary friction force, and the separation zone shear forces downstream of the lateral channel entrance in conjunction with overall mass conservation. Superior to conventional 1-D models, the nonlinear system, consisting of two equations of dimensionless variables without assuming equal upstream widths allowing the two upstream tributaries to be solved respectively. The numerical solution of the nonlinear system in conjunction with the derivation of a new formula of energy loss facilitate the impact assessment of three major controls including total post-confluence hydraulics, upstream momentum ratios, and junction planform. The relative increase of upstream water surface over downstream water surface as well as the relative difference between the upstream tributaries were found to increase proportionally to the junction angles and downstream Froude number. Similarly, higher upstream momentum ratios and larger junction entrance angles lead to larger energy losses. A parabolic shape relation was found to exist between the upstream depth ratios and the discharge or momentum ratios, while the symmetry of the parabola decreased as the junction angle increased and diminished when approaching 90. Previous 1-D and 2-D numerical attempts cannot account for the highly 3-D nature of CHZ (Confluence Hydrodynamic Zone), hence, resulted in an incorrect estimation of energy losses and other associated hydraulic parameters. In fact, the shear induced and reach scale pressure driven turbulence give rise to complex flow structure alteration and energy dissipation that could only be analyzed adequately by means of complete 3D numerical models. Thus the thesis also describes the application of commercially available three-dimensional computational fluid dynamic (CFD) codes-Ansys Fluent 15.x and 16.x to simulate the mean flow structure and turbulent terms in open-channel junction flow. The CFD model adopted PISO (pressure implicit splitting of operator algorithm) in conjunction with finite volume discretization. The 3-D URANS (unsteady Reynolds-Averaged Navier-Stokes equations) were solved in multi-block computation domain. The well known SST k-w two equation turbulence model was employed to model the eddy viscosity in the momentum equation. VOF (Volume of Fluid) was applied to numerically solve for the dynamically and spatially varied water surface. To visualize the time evolution of flow patterns and fluid dynamics in open channel junction, a time-dependent calculation was performed using dual time level implicit time marching scheme of second order accuracy. The model was validated on the basis of agreement with experimental data from the literature, grid independent study and comparison with other turbulence models. The validated model was thereafter utilized as a virtual lab to facilitate flow visualization under two major categories of controls that were identified in 1-D investigation. Detailed numerical solution facilitates the mappings of 3-D flow structures to show the implications of such three-dimensional nature of the flow on integral quantities such as Coriolis and Boussinesq coefficients as well as the downstream flow contraction and upstream water depth increase, and the specific energy loss. The effects of different controls were then analyzed quantitatively in comparison with experiment data available from the literature as well as 1-D numerical predictions. Such a comparative 1-D and 3-D study also quantifies the deviation of 1-D approximations and associated underlying assumptions from the ’true’ resulting flow field. Finally, this study examined the effect of flow unsteadiness on the energy losses in junctions. Due to the fact that the velocity profiles in unsteady-flow conditions show greater gradients, and thus greater shear stresses than the corresponding values in steady-flow conditions, we hypothesize that the associated energy loss could vary temporally according to the unsteadiness of the incoming flows. The SAS (Scale adaptive simulation) was implemented in conjunction with SST k-w model to dynamically adapt to resolving structures in a URANS simulation, yielding LES-like performance in unsteady region and reserving RANS capabilities in relative stable region. The 3-D model was scaled up to the prototype dimension of connecting tunnels in dropshaft MDS15, which connects to the downstream branch of Mainstream system of Chicago's Tunnel and Reservoir Plan (TARP) system. The unsteady flow simulations utilized hypothetical hydrographs characterized by different overall unsteadiness to reflect the temporal variation of flow behavior during the passage of hydrographs. The specific energy loss and associated flow features in the vicinity of confluence were evaluated accordingly to quantify the effects of unsteadiness. The numerical investigations enclosed may help to better understand the planform steering and reach scale pressure driven combining flow, free surface and other flow diversion problems. The numerical modeling strategies and findings can further facilitate the modifications of conventional 1D models and to realize full coupled 1-D and 3-D modeling for a complex open channel system