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Title: Rapidly varied Flow


1

Chapter Five
RAPIDLY VARIED FLOW (RVF)
  • 5.1 Characteristics of the Flow
  • Rapidly varied flow has very pronounced
    curvature of the streamlines.
  • The change in curvature may become so abrupt
    that the flow profile is virtually broken,
    resulting in a state of high turbulence this is
    rapidly varied flow of discontinuous profile, of
    which the hydraulic jump is an example.
  • In view of the contrast with-gradually varied
    flow, the following characteristic features of
    rapidly varied flow should be noted
  • 1.The Curvature of the flow is so pronounced that
    the pressure distribution cannot be assumed to be
    hydrostatic.
  • 2. The rapid variation in flow regime often takes
    place in' a relatively
  • short reach. Accordingly, the boundary friction,
    which would play
  • primary role in a gradually varied flow, is
    comparatively small and in most.
  • Cases insignificant.

2
5.1
Characteristics of the Flow
  • 3. When rapidly varied flow occurs in a sudden
    transition structure, the physical
    characteristics of the flow are basically fixed
    by the boundary geometry of the structure as
    well as by the state of the flow.
  • 4. When rapid changes in water area occur in
    rapidly varied flow the The velocity distribution
    coefficient alpha and beta are usually far
    greater than unity and can not be accurately
    determined.

3
Cont
  • 5. Separation zones, eddies, and rollers that may
    occur in rapidly varied flow tend to complicate
    the flow pattern and to distort the actual
    velocity distribution in the stream.
  • In such cases, the flow is actually confined by
    one or more separation zones rather than solid
    boundaries.

4
5.2 FLOW over spillways 
  • Reading Assignment
  • Refer Open channel hydraulics books.
  • You will learn in dam eng.II

5
Hydraulic Jump
  • Hydraulic jump is one subject which has
    extensively been studied in the field of
    hydraulic engineering .
  • The Italian engineer Bidone (1818) first credited
    the first experimental investigation of this
    phenomena.
  • Since the n considerable research has undergone
    intio the study of this study.
  • The main reason for such continued interst in
    this topic is its immense utility in Hydraulic
    engineering and allied fields.

6
Hydraulic Jump
  • The theory of jump developed in early days is for
    horizontal or slightly inclined channels in which
    the weight of water in the jump has little effect
    upon the jump behavior and hence is ignored in
    the analysis.
  • The results thus obtained, however, applied to
    most channels encountered in engineering
    problems.
  • For channels large slope, the weight effect of
    water in the jump may become so pronounced that.
    it must be included in the analysis.

7

Practical applications
  • A hydraulic jump primarily serve as energy
    disssipator to dissipate the excess energy of
    flowing water down stream of hydraulic
    structures, such as spillways, and sluice gates.
  • Practical applications of the' hydraulic jump are
    many some of the other uses are
  • 1.dissipate energy in water flowing over dams,
    weir and other hydraulic structures mid thus
    prevent scouring of channels downstream from the
    structures.

8

Practical applications
  • 2. to recover head or raise t he water level all
    the dovvn stream side of a measuring flume and
    thus maintain high water level in the channel for
    irrigation or other water-distribution purposes
  • 3. to increase weight on an a. pron and thus
    reduce uplift pressure under a masonry structure
    by raising the water depth on the apron

9

Practical
applications
  • 4. to increase the discharge' of a sluice by
    holding back tail water, since the effective head
    will be reduced if the tail water is allowed to
    drown the jump.
  • 5. indicate special flow conditions, such as the
    existence of supercritical flow or the presence
    of a control section 0 that a gauging station
    may be located
  • 6. to mix chemicals used for water purification
    and so forth
  • 7.to aerate water for city water supplies and
  • 8. to remove air pockets from water-supply lines
    and thus prevent air locking

10
Jump in horizontal rectangular channel
  • For supercritical flow in a horizontal
    rectangular channel, the energy of flow is
    dissipated through frictional resistance along
    the channel, resulting in a decrease in velocity
    and an increase in depth in the direction of
    flow.
  • A hydraulic jump will form in the channel if
    the -Froude number Fl of the flow, the flow
    depth YI and a downstream depth y2 satisfy the
    equation

11
Jump in horizontal rectangular channel
  • This equation may be represented by the curve
    in Fig. 5-1. has been ,verified satisfactorily
    with many experimental data and will be found
    very useful in the analysis and design for
    hydraulic jumps.

12
Jump in horizontal rectangular channel
FIG. 5-1. Relation between F1 and y1,y2,for a
hydraulic jump in a horizontal rectangular channel
13
Jump in horizontal rectangular channel
14
Cont
15
Cont
16
Cont
17
Cont
18
Cont
19
Cont
20
Cont
21
Cont
22
EXAMPLE
  • A rectangular channel carrying a supercritical
    stream is to be provided with a hydraulic jump
    type of energy dissipator.If it is desired to
    have an energy loss of 5.0m in the jump when the
    inlet Froude number is 8.5 determine the sequent
    depths.
  • Solution
  • F1 8.5, and El 5.0m
  • By Eq.6.4

23
solution
24
Types of jump
  • Hydraulic jumps on horizontal floor are of
    several distinct types.
  • According to the studies of the U.S. Bureau
    of Reclamation (34,35, these types can be
    conveniently classified according to the Froude
    number' F 1 of the incoming flow (Fig. 5-2), as
    follow's
  • For FJ 1, the flow is critical, and hence no
    jump can form.
  • For Fl 1 to 1.7, the water surface shows
    undulations, and the jump
  • is called an undulation ' Jump .
  • For Fl 1.7 to 2.5, a series of small rollers
    develop on the surface of the jump, but
    downstream water surface remains smooth. The
    velocity through out is fairly uniform and energy
    loss is low .This jump may be called weak jump.

25
Types of jump
  • For F1 ' 2.5 to 4.5 there is Oscillating jet
    entering jump bottom to surface and back again
    with no periodicity.
  • Each oscillation produces a large wave of
    irregula, r period which, very commonly in
    canals, can travels for miles doing unlimited
    damage to earth banks and ripraps. This Jump may
    be called an oscillating jump.

26
Cont
27
Cont
28
Cont
29
Types of jumps
Fig 5.2 various types of hydraulic jump
30
Flow under gateFlows over spillways
  • Spill way is structure over or through a dam
    for discharging flood flows, over channel
    opening built into a dam or side of a reservoir
    to release to spill excess flood water.
  • Reading Assignment refer open channel hydraulics
    reference materials.

31
The end!
  • Thank you!
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