Scaling dispersion processes in surcharged manholes.

Urban drainage network models are increasingly used in the water industry for hydraulic and water quality simulation. These models require inputs for energy loss and mixing coefficients to make predictions of head loss and the transport of solutes or dissolved substances across hydraulic structures, such as sewer pipes and manholes. Laboratory-derived head loss and mixing coefficients for manholes may be used in urban drainage modelling. However, the applicability of the laboratory-scale derived parameters to full scale structures in the urban drainage system, i.e. scalability of these parameters, is not clearly understood. The overall aim of the research is to derive generic scaling methodologies to describe the impact of physical scale of manholes on the hydraulic and mixing processes using laboratory- and CFD-based analyses. A 1:3.67 scale model of an 800 mm internal diameter manhole (the prototype) studied by Guymer et al. (2005) has been constructed in the laboratory. Laboratory experiments were conducted to measure head loss and solute dispersion in the scale model. The solute dispersion results were analysed using advection dispersion equation (ADE) and aggregated dead zone (ADZ) models and comparisons of the results with the prototype experimental data were made. The cumulative temporal concentration profiles (CTCPs) for the scale model were also compared with the prototype profiles. However, analysis of the laboratory-derived data failed to quantitatively identify the scale effects because the recorded data of the two manholes was not directly comparable. Computational fluid dynamics (CFD) was used to investigate the effects of scale in the surcharged manhole. A thorough validation study was conducted to provide confidence in the CFD model predictions. A standard modelling protocol for manhole simulations was developed through the validation study. Three differently sized manholes were created using CFD. The scale effects on the flow field, energy loss and solute transport characteristics were investigated. The findings of the study suggest that scale effects exist in the three manholes; however, the degree of the effects is very small. The scale effects were attributable to the dissimilarity in Reynolds number and that led to different characteristic of the jet in the manhole. Methodologies to scale the hydraulic and solute transport processes in surcharged manholes have been presented.