Evaluation of the solute transport characteristics of surcharged manholes using a RANS solution

Abstract

Urban drainage networks contain a large number of structures; the most common of which is a manhole. Previous studies investigating solute transport through manholes identified a threshold level of surcharge at which the hydraulic conditions in the manholes sharply altered. At high levels of surcharge, a dead zone formed above the main jet, potentially retaining solute for later discharge. This phenomenon could have significant impacts on the results obtained from 1D urban drainage network models which only consider pure advection. Previous research (including other CFD based studies) concentrated on steady flow conditions through these structures which are not thought to be representative of the dynamic flow conditions experienced in practice. During large storm events, the capacity of combined sewer networks may be exceeded. This leads to the discharge of untreated sewage to receiving watercourses. Discharges such as these are governed by legislation and, therefore, an understanding of pollutant transfer through urban drainage networks is required. Using existing laboratory and Computational Fluid Dynamics (CFD) data, this study produced a primary and secondary validated methodology for meshing and modelling manholes using CFD. This was then extended to free-surface multiphase modelling allowing for more realistic unsteady conditions to be considered. This included an in-depth comparison of suitable turbulence models and solute-modelling options. It was found that, whilst it was possible to model multiphase free-surface flow within such a structure, it was unnecessarily computationally expensive as temporal changes in the flow field are rapid when compared with a practical time-step for use with 1D models. The validated modelling methodology was then used to recreate the original study that identified the hydraulic threshold. This included a selection of manholes with a range of inlet to manhole diameter ratios. However, this was extended further to include manholes with a ratio less than five (commonly found in UK sewer systems). It was shown that the hydraulic threshold broke down below this limit, removing the need for this to be modelled in 1D systems.EThOS - Electronic Theses Online ServiceGBUnited Kingdo