Mathematical basis for the simulation of low amplitude air pressure transients in vent systems
The mechanism by which entrained airflow is established within drainage and vent systems is well understood, Figure 3.1. The annular water flows present in the ‘wet’ stack entrains an airflow due to the condition of ‘no slip’ established between the annular water and air core surfaces. This results in the expected
pressure variation down a vertical stack, falling from atmospheric above the stack entry due to friction and the effects of drawing air through the vent entry and the water curtains formed at each successive discharging branch junction. Within the lower wet stack region the pressure recovers to above atmospheric due to the traction forces exerted on the airflow and the necessity to discharge air to the downstream drain through the water curtain formed at the stack base. These mechanisms may be used as a basis for a mathematical simulation of air entrainment provided that the relationships linking applied waterflow to entrained airflow are known, together with the influence of stack diameter, roughness and building height. These relationships have been identified and utilised in the development of the simulation model AIRNET, to be described in this chapter, which allows the application of the finite difference Method of Characteristics to predict the time dependent air pressure regime in the system as a result of appliance discharge.