【正文】 accumulated air inflow reaches its assumed 40s. Fig. 5 illustrates the air pressure profile from the stack base in both stacks 1 and 4 at mm. In addition, the simulation replicates local appliance trap seal oscillations and the operation of active control devices, thereby yielding data on network airflows and identifying system failures and consequences. While the simulation has been extensively validated [10], its use to independently confirm the mechanism of SARS virus spread within the Amoy Gardens outbreak in 2003 has provided further confidence in its predictions [12]. Air pressure transient propagation depends upon the rate of change of the system conditions. Increasing annular downflow generates an enhanced entrained airflow and lowers the system pressure. Retarding the entrained airflow generates positive transients. External events may also propagate both positive and negative transients into the network. The annular water flow in the ‘wet’ stack entrains an airflow due to the condition of ‘no slip’ established between the annular water and air core surfaces and generates the expected pressure variation down a vertical stack. Pressure falls from atmospheric above the stack entry due to friction and the effects of drawing air through the water curtains formed at discharging branch junctions. In the lower wet stack the pressure recovers to above atmospheric due to the traction forces exerted on the airflow prior to falling across the water curtain at the stack base. The application of the method of characteristics to the modelling of unsteady flows was first recognized in the 1960s [13]. The relationships defined by Jack [14] allows the simulation to model the traction force exerted on the entrained air. Extensive experimental data allowed the definition of a ‘pseudofriction factor’ applicable in the wet stack and operable across the water annular flow/entrained air core interface to allow bined discharge flows and their effect on air entrainment to be modelled. The propagation of air pressure transients in building drainage and vent systems is defined by the St Venant equations of continuity and momentum [9],(1)(2)These quasilinear hyperbolic partial differential equations are amenable to finite difference solution once transformed via the Method of Characteristics into finite difference relationships, Eqs. (3)–(6), that link conditions at a node one time step in the future to current conditions at adjacent upstream and downstream nodes, Fig. 2.. St Venant equations of continuity and momentum allow airflow velocity and wave speed to be predicted on an xt grid as shown. Note , . For the C+ characteristic:(3)when(4)and the C characteristic:(5)when(6)where the wave speed c is given byc=(γp/ρ).(7)These equations involve the air mean flow velocity, u, and the local wave speed, c, due to the interdependence of air pressure and density. Local pressure is calculated as(8)Suitable equations link local pressure to airflow or to the interface oscillation of trap seals.The case of the appliance trap seal is of particular importance. The trap seal water column oscillates under the action of the applied pressure differential between the transients in the network and the room air pressure. The equation of motion for the Ubend trap seal water column may be written at any time as(9)It should be recognized that while the water column may rise on the appliance side, conversely on the system side it can never exceed a datum level drawn at the branch connection.In practical terms trap seals are set at 75 or 50 Trap retention。s vertical stacks. This paper presents a simulation based on a fourstack network that illustrates flow mechanisms within the pipework following both appliance discharge generated, and sewer imposed, transients. This simulation identifies the role of the active air pressure control devices in maintaining system pressures at levels that do not deplete trap seals. Further simulation exercises would be necessary to provide proof of concept, and it would be advantageous to parallel these with laboratory, and possibly site, trials for validation purposes. Despite this caution the initial results are highly encouraging and are sufficient to confirm the potential to provide definite benefits in terms of enhanced system security as well as increased reliability and reduced installation and material costs. Keywords: Active control。s and 300s onwards, the AAV on pipe 12 opens fully and an increased airflow from this source may be identified. The flutter stage is replaced by a fully open period from to s, the increasing and decreasing phases of the transient propagation being presented sequentially. The traces illustrate the propagation of the positive transient up the stack as well as the pressure oscillations derived from the reflection of the transient at the stack termination at the AAV/PAPA junction at the upper end of pipe 11..(a) Sequential air pressure profiles in stack 1 during initial phase of stack base surcharge. (b) Sequential air pressure profiles in stack 1 during final phase of stack base surcharge. 8. Sewer imposed transientsTable 2 illustrates the imposition of a series of sequential sewer transients at the base of each stack. Fig. 8 demonstrates a pattern that indicates the operation of both the PAPA installed on pipe 13 and the selfventing provided by stack interconnection. airflows as a result of sewer imposed pressure transients.As the positive pressure is imposed at the base of pipe 1 at 12文章通過四根立管提出一種模擬實驗，說明了瞬時產(chǎn)生和加強的氣壓在排水管中的流動機制。命名原則C+——特征方程c——波速, m/s D——分支或堆積直徑, m f——摩擦因子， 英國定義通過Darcy Δh=4fLu2/2Dg g——重力加速度, m/s2 K——損失系數(shù)L——管長, m p——壓力, N/m2 t——時間, s u——空氣速度, m/s x——距離, mγ——比熱率Δh——水頭損失, m Δp——壓力差, N/m2 Δt——時間間隔, s ρ——密度, kg/m3目錄命名原則――瞬時氣壓的控制和抑制。復(fù)雜的通氣系統(tǒng)需要大量費用且于空間有密切聯(lián)系[8]。正壓衰減器[10]被開發(fā)用來吸收瞬時正壓產(chǎn)生的氣流，這種衰減器完成了必要的設(shè)備供應(yīng)，為劇烈的瞬時氣壓