Modelling water trap seal boundary conditions in building drainage systems: Computational fluid dynamics analysis of unsteady friction to improve accuracy

The safe removal of disease carrying human waste is the objective of all sanitation systems; shown today in design regulations which require systems to self-cleanse. The water trap seal offers fundamental protection and is the system’s sole barrier between the public sewer network and habitable space inside a building. Modelling water trap seal responses to air pressure fluctuations offers an opportunity to analyse whole system performance, but the quality of the data depends on the accuracy of the modelling technique and that of the defining inputs. AIRNET, a 1-D Method of Characteristics based model, enables rapid whole system testing, however the present boundary condition for the water trap seal within the model is based solely on steady state conditions, ignoring system dynamics. CFD offers an opportunity to numerically evaluate the flow patterns within the trap seal in response to applied air pressure transients. This unique approach to the definition of water trap response has led to new boundary conditions which more accurately reflect actual water movement in response to pressure waves of varying rise time (frequency). The outcome of this research also provides AIRNET with the ability to assess dynamic conditions across a variety of water trap seals.
Practical Application
Whole system modelling can greatly improve the ability of design engineers to fully simulate the operation of a building drainage system in a realistic way. The work described in this paper improves the accuracy of whole system models by evaluating water dynamic responses to air pressure transients using a range of techniques including CFD and more traditional 1-D finite difference method of characteristics models. The work also paves the way for more robust evaluation of building drainage products through in-depth investigation of the fluid mechanics associated with their operation.