Pressurized Flow
The typical gravity sewer network is dominated by circular pipe segments. These pipes are all closed and characterized by a converging top where the hydraulic top width approaches zero as flow transitions from free surface to pressure. The Preissmann slot method is used for simulating pressure or surcharged flows by adapting the conceptualization of pressurized flow to fit a free surface model. The slot extends vertically from pipe crown to infinity and over the entire length the pipe, and the width of the slot is usually 1% of the characteristic pipe dimension (diameter for a circular pipe) but not large than 0.02 ft.
Transition of a Circular Pipe to the Slot
Since a circular conduit width changes dramatically near the crown and in order to maintain a smooth transition between conduit width and the slot width, the Drainage and Utilities model adapted a transitional function of the conduit width:
where 0.98<y/d<1.2b/d = 0.001 and y/d>1.2, and
The maximum width allowed in the slot is 0.01 ft. Also, when the flow depth is above the diameter d the area remains the full circular section area therefore the slot will have no impact on the flow continuity.
The significant advantages in using this hypothetical slot are apparent in simulating the moving transitional interface between open-channel flow and pressure flow, which can happen anywhere at any time in a sewer system. Since the model applies a unified set of consistent equations and numerical schemes, it makes no special switches between open-channel flows and pressure flows, giving rise to a robust solution.
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