Bennett, P., Stovin, V. R. and Guymer, I. (2011) Improved CFD simulation approaches for manhole mixing investigations. In: 12th International Conference on Urban Drainage, Porto Alegre, Brazil, Sep 11-16, 2011

Abstract

Previous work studied the effects of manhole diameter on longitudinal dispersion in surcharged manholes. A surcharge level at which the solute transport characteristics of the manholes sharply altered was identified; at low surcharge (termed 'pre-threshold') the travel times varied linearly with surcharge, at high surcharge (termed 'post-threshold') the travel times dropped to a low constant level. Subsequent research focused on a 218 mm manhole, using laboratory Particle Image Velocimetry (PIV) to validate Computational Fluid Dynamics (CFD)-based simulations of the flow fields. The study included a parametric study of turbulence and other modelling options. However, all tests were carried out using a fixed lid assumption—a non-slip wall to represent the free surface. Furthermore, the k-ε Realizable turbulence model (recommended for use with jets) was not considered. This paper will therefore provide an evaluation of the k-ε Realizable turbulence model, under both fixed lid and free surface modelling assumptions. Primary validation—based on qualitative flow field visualisation—will be supported by secondary validation, based on solute traces. This work ultimately concludes that although the k-ε Realizable fixed lid model appears to more accurately reproduce the mean flow field under both hydraulic regimes, secondary quantitative validation shows the improvement to be less clear. The free surface model appears to represent the mean flow field only moderately well whilst still offering an acceptable fit to laboratory solute trace data. Importantly, the two contrasting hydraulic regimes were both replicated.

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