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<br /> <br /> <br /> <br />13 <br /> <br /> <br />Air Demand <br />Estimating air demand is the most important <br />component of adequate air vent sizing. Air demand <br />refers to the amount of air that the flowing water pulls <br />into the conduit (and entrains into the flow) through <br />the air vent and through the downstream exit portal (if <br />not submerged). <br />There are a number of variables that can influence air <br />demand, including: <br /> Gate opening height <br /> Head <br /> Volume flow rate and velocity of water <br /> Flow type (e.g., free surface flow, or hydraulic <br />jump that closes the conduit) <br /> Froude Number <br /> Gate geometry and roughness <br /> Conduit length, diameter, cross section shape, <br />and roughness <br /> Water surface roughness <br /> Outlet submergence <br /> Air vent geometry (e.g., entrance, bends) and <br />head loss <br /> Altitude <br />Air demand is usually greatest at small (5 to 10 percent <br />open) and large (between 50 to 100 percent open) <br />gate opening heights. Figure 3 illustrates the effect of <br />gate openings on air demand. At small gate openings <br />and when flow is not influenced by tailwater <br />conditions or by a hydraulic jump, “jet flow” occurs, <br />which entrains large quantities of air as the water jet <br />frays or breaks up. At large gate openings and free <br />surface flow conditions, air demand is caused by the <br />drag force between the water surface and the <br />overlying air column. Air demand for flow involving a <br />hydraulic jump has been shown by studies to represent <br />the lower bound of free surface flow air demand. <br />When the conduit flows full, or when the gate is at the <br />downstream end of the conduit (open to atmospheric <br />pressure), air demand is zero. <br /> <br /> <br />In addition to gate opening height and flow type, the <br />other variables bulleted above influence air demand to <br />varying degrees. Accounting for these variables in air <br />demand estimation can be challenging for the <br />practicing design engineer, as there is currently no <br />known comprehensive methodology applicable to the <br />wide range of possible outlet works configurations and <br />hydraulic conditions represented by these variables. <br />Figure 2: Outlet pipe liner collapse due to cavitation <br />Figure 3: Gate Opening, Flow Type, and Air Entrainment <br />Demand