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Normal Hydraulic Jumps NHJ
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28.1 Introduction
When the depth of flow changes rapidly from a low stage to a
high stage, it results in an
abrupt rise of water surface. This local phenomenon is known as
'hydraulic jump'. It
occurs in a canal below a regulating sluice, at the toe of a
spillway or at the place where
a steep channel slope suddenly turns flat. It is well known that
a large amount of
air entrains in the roller portion of the jump due to the
breaking of the water surface.
Consequently a large amount of energy loss occurs in the jump
through dissipation in
the turbulent body of water. A considerable amount of
investigations, both theoretical
and experimental, have been carried out on the jump (See box -
History).
SluiceGate
1
Hydraulic jump
2
3
Fig. 28.1 - Rapidly varied flow with Hydraulic jump (1 and 3
subcritical flows,2 Super critical flow)
Fig. 28.2 - Formation of Hydraulic jump at the toe of the
spillway
Hydraulic jump
Toe
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History
The earliest description of the hydraulic jump appears to be by
Leonardo da Vinci
in 1452- 1519. Bidone was the pioneer to conduct investigations
on the hydraulic
jump in 1818 -1819. Belanger in 1828 developed the momentum
equation
connecting the sequent depths. Then onwards innumerable
contributions have
been made towards the understanding of the basic mechanism of
the hydraulic
jump. The following are some of the significant contributors
amongst several
investigators responsible for present state of knowledge of the
jump: Bresse (
1860 ), Darcy and Bazin ( 1865 ), Uniwin ( 1875 ), Ferriday and
Merriman ( 1894
), Gibson ( 1913 ), Kennison ( 1916 ), Woodward and Riegel Beebe
( 1917 ),
Koch and Cartstanjen ( 1926 ), Lindquist ( 1927 ), Safranez (
1917 ), Einwachter
( 1933 ), Smetana ( 1934 ), Bakhmeteff and Matzke ( 1936 ) ,
Escande ( 1938 ),
Citrini ( 1939 ), Nebbia ( 1940 ), Kindsvater (1944 ) ,
Blaisdell (1948 ), Forster
and Skrinde (1950 ), Moore and Morgan ( 1957 ), and Rouse et al.
(1958 ). A
detailed mathematical treatment of hydraulic jump was made by
Flores (1954 ) .
Rajaratnam's contributions to the knowledge of hydraulic jumps
during 1960s are
outstanding. For a comprehensive bibliography on the jump,
reference may be
made to the following references: 'The standing wave or
hydraulic jump ( 1950 ),
(Central Board of Irrigation and Power)' ; ' A bibliography on
hydraulic jump
(Central Board of Irrigation and Power) ( 1955 )'; ' Hydraulic
energy dissipators (
1959 )' ( Elevatorski - The Hydraulic jump, May 1955); Hydraulic
Energy
dissipators Elevatorski - Mcgraw Hill, 1959) 'Open Channel
Hydraulics Chow
V.T., McGraw Hill ( 1959 )';'Advances in Hydroscience (Hydraulic
jump byRajaratnam.N edited by Chow.V.T. Vol. - 4 , Academic Press
New york and
London, Page 197 to 280 ( 1967 )' ; Self Aerated flow
characteristics in
developing zones and in Hydraulic jumps, (Thandaveswara Phd
Thesis, Indian
Institute of Science, Bangalore, June- 1974).
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Hydraulic Jump has many practical applications, for example (a)
to dissipate the high
kinetic energy of water near the toe of the spillway and to
protect the bed and banks of
a river near a hydraulic structure (b) to increase the head in
the power channel (c) to
remove air pockets from pipes (d) for mixing of chemical in
water supply system.
Figure below shows a schematic view of the classical hydraulic
jump on a horizontal
floor. The details in this unit are confined to the case of the
hydraulic jumps on level
floors in rectangular channels and this type of jump is referred
to as the Normal
Hydraulic Jump (NHJ). The supercritical Froude number of the
approach flow is the
major parameter that influences the characteristics of the
hydraulic jump.
L jLrj
Toe
1 2
y2V2
F1 y1 V1 y
28.3 SCHEMATIC VIEW OF THE HYDRAULIC JUMP
1 2
x
Hydrostatic pressure distribution Roller zone
yr
Super critical to sub critical
