CHAPTER 2  BASIC CONCEPTS RELATED TO FLOWING WATER AND MEASUREMENT
Equations 29 and 213 can be used to develop a equation for flow through an orifice, which is a sharpedged hole in the side or bottom of a container of water (figure 22a). To find the velocity of flow in the orifice, use equation 213, then multiply by area to get AV, or discharge, Q, resulting in:
(214)
The subscript t denotes theoretical discharge through an orifice. This equation assumes that the water is frictionless and is an ideal fluid. A correction must be made because water is not an ideal fluid. Most of the approaching flow has to curve toward the orifice opening. The water, after passing through the orifice, continues to contract or curve from the sharp orifice edge. If the orifice edges are sharp, the jet will appear as shown on figure 22. The maximum jet contraction occurs at a distance of onehalf the orifice diameter (d/2) downstream from the sharp edge. The crosssectional area of the jet is about sixtenths of the area of the orifice. Thus, equation 214 must be corrected using a contraction coefficient, C_{c}, to produce the actual discharge of water being delivered. Thus, the actual discharge equation is written as:
(215)
For a sharpedged rectangular slot orifice where full contraction occurs, the contraction coefficient is about 0.61, and the equation becomes:
(216)
A nonstandard installation will require further calibration tests to establish the proper contraction coefficient because the coefficient actually varies with the proximity to the orifice edge with respect to the approach and exit boundaries and approach velocity.

