BasinFlow: Orifice Structure
Input Parameters:
name anything can go in here
D diameter or depth of the orifice opening in inches
w width of orifice opening in inches. This is only used for rectangular openings. A value of zero assumes the D dimension was a diameter for a circular opening.
Co orifice coefficient of discharge: examples
sharp edge 0.62
round edge 0.98
square edge tube 0.61-0.82
round edge tube 0.97
Inv invert, or low point of the orifice
in riser option to select if the orifice is for flow into a riser structure
out option to select if the orifice is for flow out of a riser
dam option to select if the orifice allows flow directly through a dam

The formula used for orifices is given below, where "h" is the head measured from the center of the orifice, and "g" is the gravitational constant of 32.174 ft2/sec. (Roberson, Cassidy, and Chaudhry)

The above formula is fine for relatively small orifices, when flow is not below the center of the orifice for any significant amount of time. However, large openings tend to have significant periods of weir flow before the water level rises above the opening, or above the center of the opening where the orifice formula can reasonably be used. Therefore, an approximation is used to model a weir at lower depths. The weir equation is used, with a coefficient of 3, and a length equal to the width of flow at the given depth, which is the computed chord for a circle, or width for a rectangle. This approximation is used for heights over the invert of 1.6 times the orifice radius, at which point the orifice equation takes over. Below, a sample graph is provided showing the curves for weir and orifice flow. The intersection point is about 1.6 x radius. For graphing purposes on the structure performance curves below, the depth is from the orifice invert. The left side of the graph is the orifice bottom, and the right side is the orifice top.

(return)