Developments in the definition of fluid traction forces within building drainage vent systems

With developments in the understanding of the pressure regime generated within single-stack building drainage systems, analysis by numerical modelling of complex and detailed systems serving a wide range of sanitary appliances has progressed notably. Computer-based finite-difference simulations incorporate both the complex unsteady fluid flow principles inherent within such systems and the specific design details of the drainage network, thereby allowing the time-dependent pressure response of the system to be assessed and the appliance trap seal retention or depletion to be predicted. This paper develops a new approach to the way in which the fluid flows present within the system are modelled, by providing an enhanced analysis of the waste water and entrained air flow interaction. It also extends the applicability of empirical data and examines resultant friction factor data. This approach can be applied to both single and multiple inlet vertical stack flow and can significantly enhance the designer's ability to predict system response. Combined with unsteady free surface horizontal pipe flow theory, this enables the complete drainage network, from appliance trap through branch connections, vertical stacks and the sewer connection, to be analysed.