Note: Orifice and Weir Flow Computations The orifice flow calculation proceeds as follows: 1. Initially and whenever the setting (i.e., the fraction opened) changes, flow coefficients for both orifice and weir behavior are computed as follows: a. For side orifices: Define Hcrit = h/2 where h is the opening height. b. For bottom orifices: i. For a circular orifice, compute area over length (i.e., circumference) as AL = h /4. ii. For a rectangular orifice compute AL = h*w/(2*(h+w)) where w is the opening width. iii. Compute Hcrit = Cd*AL/0.414 where Cd is the orifice discharge coefficient. At step 1b, the critical head for the bottom orifice, where orifice flow turns into weir flow, is found by equating the result of the orifice equation to that of the weir equation Cd*Area*sqrt(2g)*sqrt(Hcrit) = Cw*Length*sqrt(Hcrit)*Hcrit or Hcrit = (Cd * Area) / (Cw/sqrt(2g) * Length) The value of Cw/sqrt(2g) for a sharp crested weir is 0.414. ...
Robert E Dickinson 
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