Some Experiences with Numerical Modelling of Overflows

Torben Larsen, Lasse Nielsen, Bjarne Nielsen, Erik Damgaard Christensen

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Abstract

Overflows are commonly applied in storm sewer systems to control flow and water surface level. Therefore overflows play a central role in the control of discharges of pollutants from sewer systems to the environment. The basic hydrodynamic principle of an overflow is the so-called critical flow across the edge of the overflow. To ensure critical flow across the edge, the upstream flow must be subcritical whereas the downstream flow is either supercritical or a free jet. Experimentally overflows are well studied. Based on laboratory experiments and Froude number scaling, numerous accurate and reliable formulas for the estimation of overflows have been derived. Numerical modelling of overflows is significantly more complicated than standard 1-dimensional river or sewer modelling. The problem is usually managed by incorporating the mentioned empirical formulas in the numerical models. If there are no standard formulas for a specific geometry, physical experiments have to be carried out. The present study uses laboratory experiments to evaluate the reliability of two types of numerical models of overflows in sewers systems: 1. 1-dimensional model based on the extended Saint-Vernant equation including the term for curvature of the water surface (the so-called Boussinesq approximation) 2. 2- and 3-dimensional so-called Volume of Fluid Models (VOF-models) based on the full Navier-Stokes equations (named NS3 and developed by DHI Water & Environment) As a general conclusion, the two numerical models show excellent results when compared with measurements. However, considerable errors occur when inappropriate boundary conditions and grid resolutions are chosen. The paper describes the physical and numerical models and summarises the results.
Original languageEnglish
Publication date2007
Publication statusPublished - 2007
Externally publishedYes
EventThe fifth International Symposium on Environmental Hydraulics - Arizona, USA
Duration: 1 Jan 2007 → …
Conference number: 5

Conference

ConferenceThe fifth International Symposium on Environmental Hydraulics
Number5
CityArizona, USA
Period01/01/2007 → …

Cite this

Larsen, T., Nielsen, L., Nielsen, B., & Christensen, E. D. (2007). Some Experiences with Numerical Modelling of Overflows. Abstract from The fifth International Symposium on Environmental Hydraulics, Arizona, USA, .