Page FLUID MECHANICS Discharge from an Orifice Let A cross ‐ sectional area of the orifice π 4)d and A c cross ‐ sectional area of the jet at the vena contracta π 4)d c then A c C c A or C c        c c A d A d where C c is the coefficient of contraction At the vena contracta the volumetric flow rate Q of the fluid is given by c c v Q area of the jet at the vena contracta actual velocity AV or Q C AC gh     The coefficients of contraction and velocity are combined to give the coefficient of discharge C d d c v d i.e C C C and Q C A gh   Typically values for C d vary between . and .6 Circular Orifice Q C d A gh where Q flow m s) A area m h head m Rectangular Notch Q / d CB g H where B breadth m) H head m above sill Triangular Notch Q  / d C g tan H where H head m above sill) θ notch angle