Rapidly varied Flow

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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.

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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.

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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.

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5.2 FLOW over spillways 

• Reading Assignment
• Refer Open channel hydraulics books.
• You will learn in dam eng.II

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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.

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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.

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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.

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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

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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

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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

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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.

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Jump in horizontal rectangular channel
FIG. 5-1. Relation between F1 and y1,y2,for a
hydraulic jump in a horizontal rectangular channel
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Jump in horizontal rectangular channel
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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

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solution
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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.

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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.

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Types of jumps
Fig 5.2 various types of hydraulic jump
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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.

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The end!

• Thank you!