ORIFICES AS ASCERTAINABLE BY

This article appears in Volume V14, Page 39 of the Encyclopedia Britannica.

Encyclopedia Britannica - Main :: ORC-PAI

ORIFICES AS ASCERTAINABLE BY EXPERIMENTS
16. When a liquid issues vertically from a small orifice, it forms a jet which rises nearly to the level of the free surface of the liquid in the

vessel

VESSEL (O. Fr. vaissel, from a rare Lat. vascellum, dim. of vas, vase, urn)

from which it flows. The difference of level hr (fig. 14) is so small that it may be at once suspected to be due either to air resistance on the surface of the jet or to the viscosity of the liquid or to friction against the sides of the orifice. Neglecting for the moment this small quantity, we may infer, from the

elevation

ELEVATION

of the jet, that each molecule on leaving the orifice possessed the velocity required to lift it against gravity to the height h. From

ordinary

ORDINARY (med. Lat. ordinarius, Fr. ordinaire)

dynamics, the relation between the velocity and height of projection is given by the equation
v=sl2gh. (I) As this velocity is nearly reached in the flow from well-formed orifices, it is
sometimes called the theoretical velocity of discharge. This relation was first obtained by Torricelli.
If the orifice is of a suitable conoidal form, the water issues in filaments normal to the plane of the orifice. Let w be the area of the orifice, then the discharge per second must be, from eq. (I),
Q=wv=enj2gh nearly. (2)
This is sometimes quite improperly called the theoretical discharge for any

kind

KIND (O. E. ge-cynde, from the same root as is seen in " kin," supra)

of orifice. Except for a well-formed conoidal orifice the result is not approximate even, so that if it is supposed to be based on a theory the theory is a false one.
Use of the

term

TERM

Head in Hydraulics.The

term

TERM

head is an old millwright's term, and meant primarily the height through which a

mass

of water descended in actuating a hydraulic

machine

. Since the water in fig. 14 descends through a height h to the orifice, we may say there are h ft. of head above the orifice. Still more generally any

mass

of liquid h ft. above a

horizontal

HORIZONTAL

plane may be said to have h ft. of

elevation

ELEVATION

head relatively to that datum plane. Further, since the pressure p at the orifice which produces outflow is connected with h by the relation p/G = h, the quantity p/G may be termed the pressure head at the orifice. Lastly, the velocity v is connected. with h by the relation v2/2g = h, so that v2/2g may be termed the head due to the velocity v.
17. Coefficients of Velocity and Resistance.As the actual velocity of discharge differs from J 2gh by a small quantity, let the actual velocity
= va = cvJ 2gh, (3) where c is a coefficient to be determined by experiment, called the coefficient of velocity. This coefficient is found to be tolerably constant for different heads with well-formed simple orifices, and it very often has the value o97.
The difference between the velocity of discharge and the velocity due to the head may be reckoned in another way. The

total

TOTAL

height h causing outflow consists of two partsone

part

PART

h. expended effectively in producing the velocity of outflow, another hr in over-coming the resistances due to viscosity and friction. Let
hr = crhr,
where cr is a coefficient determined by experiment, and called the coefficient of resistance of the orifice. It is tolerably constant for different heads with well-formed orifices. Then
1'a='/2gkr=1 [2gh/(1+cr)}. (4)

End of Article: ORIFICES AS ASCERTAINABLE BY

If you wish, you can link directly to this article.

<a href="http://jcsm.org/StudyCenter/Encyclopedia/ORC_PAI/ORIFICES_AS_ASCERTAINABLE_BY.html">
ORIFICES AS ASCERTAINABLE BY
</a>

(Previous)
ORIENTE, OR LA REGION ORIENTALE

(Next)
ORIGEN (c. 185-c. 254)

JCSM was founded in 1997 and exists to help the community and bring people into a life-changing and productive relationship with Jesus Christ. JCSM offers over 200,000 free web pages, including its weekly inspirational emails that were sent continuously for over a decade.

Jesus Christ Saves Ministries
P.O. Box 9297
San Diego, CA 92169
1-888-887-0417 or Email