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ELF

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Originally appearing in Volume V14, Page 47 of the 1911 Encyclopedia Britannica.
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ELF ==_ If the orifice is small in __~ r - - dimensions compared with h, the filaments will all have nearly the same velocity, and if h is measured to the centre of the orifice, the See also:

equation above gives the mean velocity of the See also:jet. See also:Case of a Submerged Orifice.—Let the orifice See also:discharge below the level of the tail See also:water. Then using the notation shown in fig. 41, we have at M, vi=o,pi=Gh;+p. at m, p=Gha+pa. Inserting these values in (3), § 29, o+hi+pa/G—hi =v2/2g+ha—h2+pa/G ; v2/2g =h2-ha = h, n~ - DD' But r s. See also:iron i ssZee/wasirter,vr, t o+pa/G+hi—hi =v2/2g+pa/G—h ; v2/2g=h; v=J (2gh) =8.o2511 h. (2) (2a) (3) where it is the difference of level of the See also:head and tail water, and may be termed the effective head producing flow. Case where the Pressures are different on the See also:Free See also:Surface and at the Orifice.—Let the fluid flow from a See also:vessel in which the pressure is po into a vessel in which the pressure is p, fig. 42. The pressure po will produce the same effect as a layer of fluid of thickness po/G added to the head water; and the pressure p , will produce the same effect as a layer of thickness p/G added to the tail water. Hence the effective difference of level, or effective head producing flow, will be and the velocity of discharge will be v = V [2g{ho+(po-p)/G}]. (4) We may See also:express this result by saying that See also:differences of pressure at the free surface and at the orifice are to be reckoned as See also:part of the effective head. Hence in all cases thus far treated the velocity of the jet is the velocity due to the effective head, and the discharge, allowing for contraction of the jet, is Q =cwv =cwJ (2gh), (5) where w is the See also:area of the orifice, See also:cat the area of the contracted See also:section of the jet, and h the effective head measured to the centre of the orifice.

If h and w are taken in feet, Q is in cubic feet per second. It is obvious, however, that this See also:

formula assumes that all the filaments have sensibly the same velocity. That will be true for See also:horizontal orifices, and very approximately true in other cases, if the dimensions of the orifice are not large compared with the head h. In large orifices in say a See also:vertical surface, the value of h is different for different filaments, and then the velocity of different filaments is not sensibly the same.

End of Article: ELF

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ELF (O. Eng. aelf; cf. Ger. Alp, nightmare)

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