NUMERICAL ANALYSIS AND VALIDATION OF SOUTH VALENCIA SEWAGE COLLECTION SYSTEM DIVERSION

摘要

The old Sewage Collection System (SCS) of south Valencia, Spain, had to be recently diverted to avoid interference with the city high-speed railway works. Certain aspects of the proposed modification of this channel are complex from both the geometric and the hydraulic point of view. Besides, the structure is a unitary collector and must handle torrential flow rates up to 100m~3/s. In the present work, the stretch most prone to suffer from hydraulic issues is numerically analyzed. This part of the SCS diversion project involves complex sections, critical and subcritical flow regimes, hydraulic jumps, flow expansions and contractions and other singularities not always easy to address. In order to analyze the hydraulic performance of the described design, a three-dimensional model based on the open-source CFD platform OpenFOAM is presented. In this model, turbulence is treated using three widely used RANS approaches, namely: Standard k-epsilon, RNG k-epsilon and SST k-omega, whose accuracies are compared by means of a sensitivity analysis. Structured meshes of different element size are tested, and the model sensitivity to this parameter is also studied. The air-water interface must be accurately defined, as the flow under study is gravity-driven and water depths have to be controlled to avoid flow pressurization. To do so, an Eulerian-Eulerian volume method is used. As a result, a full physically consistent description of the flow is achieved and so a wealth of interesting variables can be computed. In order to assess the accuracy of the numerical model, some variables regarding the water surface level are compared to experimental data. These experimental results are obtained from a 1/20 scale physical model, where Froude similarity is achieved and scale effects are proved not to be significant. Good agreement between numerical and experimental results is achieved despite the complexity of the system under study. In the light of the results, the model proposed can be applied to accurately model similar hydraulic structures.