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OPEN CHANNEL FLOW

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Title: OPEN CHANNEL FLOW


1
OPEN CHANNEL HYDRAULICS
OPEN CHANNEL FLOW
  • Department of Water Resources and Irrigation
    Engineering
  • Kombolcha Institiue of Technology

Ins.G.D
2
CHAPTER ONE OPEN CHANNEL FLOW
  • Question
  • What is the most obvious difference between pipe
    flow and open channel flow?????????????
  • (in terms of flow conditions and energy
    situation)

3
Open Channel Flow
  • Is a flow of liquid in a conduit with free space.
  • (interface between water and air)
  • is applied to understand the flow of a liquid in
  • Artificial channel (flumes, spillways, canals,
    weirs, drainage ditch, culverts) and
  • Natural channel (streams, rivers, flood plains).

4
Open Channel Flow
  • Open-channel flow must have a free surface,
    whereas pipe flow has none.

5
Open Channel Flow
  • A free surface is subject to atmospheric
    pressure.
  • In Pipe flow there exists no direct atmospheric
    flow but hydraulic pressure only.
  • it is much more difficult to solve problems of
    flow in open channels than in pipes.
  • complication comes from the position of the free
    surface which will change with time and space.
  • And also by the fact that depth of flow, the
    discharge, and the slopes of the channel bottom
    and of the free surface are all inter dependent.

6
Summary between open channel flow and Pipe Flow
Table 1
Pipe flow Open channel flow
Flow driven by Pressure work Gravity(potential energy)
Flow cross section Fixed ,known Unknown in advance because flow depth vary
Characteristics of flow parameters Velocity deduced from continuity Flow depth deduced both from continuity and moment equation
Specific boundary Atmospheric pressure at free water surface
7
Classification of open channel flow
  • are made according to change in flow depth with
    respect to time and space.

8
Classification of open channel flow
  • Steady and Unsteady Time is the criterion.
  • Flow is said to be steady if the depth of flow at
    a particular point does not change or can be
    considered constant for the time interval under
    consideration. i.e.
  • The flow is unsteady if depth (flow x-ics)
    changes with time.

9
Classification of open channel flow
  • Uniform Flow and non Uniform Space as the
    criterion.
  • Open Channel flow is said to be uniform if the
    depth and velocity of flow are the same at every
    section of the channel. i.e
  • Hence it follows that uniform flow can only occur
    in prismatic channels.

10
Classification of open channel flow
  • Open Channel flow is said to be non uniform if
    the depth and velocity of flow varies at every
    section of the channel. i.e
  • Non uniform flow classified as rapidly flow
    (R.V.F) and gradually varied flow (G.V.F)

11
Classification of open channel flow
  • For steady uniform flow, depth and velocity is
    constant with both time and distance.
  • This constitutes the fundamental type of flow in
    an open channel.
  • It occurs when gravity forces are in equilibrium
    with resistance forces.

12
Classification of open channel flow
  • Steady non-uniform flow
  • Depth varies with distance but not with time.
  • This type of flow may be either-
  • (a) gradually varied -
  • (requires the application of the energy and
    frictional resistance equations) or
  • (b) rapidly varied-
  • (requires the energy and momentum equations).

13
Classification of open channel flow
  • Unsteady non uniform flow-
  • The depth varies with both time and space.
  • This is the most common type of flow and requires
    the solution of the energy momentum and friction
    equations with time.
  • In many practical cases the flow is sufficiently
    close to steady flow therefore it can be analysed
    as gradually varied steady flow.

14
Classification of open channel flow summary
figure 2
15

Types of Open channel
  • Artificial channels
  • are channels made by man.
  • They include
  • irrigation canals, navigation canals, spillways,
    sewers, culverts and drainage ditches.

CULVERT
Subsurface drain
16

Types of Open channel
  • In the field they are commonly constructed of
    concrete, steel or earth and have the surface
    rough nesses reasonably well defined (although
    this may change with age particularly grass
    lined channels.)
  • Analysis of flow in such well defined channels
    will give reasonably accurate results.

17

Types of Open channel
  • Natural channels
  • Streams, rivers and etc.
  • are neither regular nor prismatic and their
    materials of construction can vary widely
    (although they are mainly of earth this can
    possess many different properties.)
  • The surface roughness will often change with time
    distance and even elevation.
  • It is more difficult to accurately analyze and
    obtain satisfactory results for natural channels
    than is does for manmade ones.
  • The situation may be further complicated if the
    boundary is not fixed i.e. erosion and deposition
    of sediments.

18
Velocity and
Pressure Distribution in open channel
  • The measured velocity in an open channel will
    always vary (not uniformly distributed) across
    the channel section because of the presence of a
    free surface and frictional resistance along the
    boundary.
  • Neither is this velocity distribution usually
    symmetric (as it is in pipe flow) due to the
    existence of the free surface.
  • The velocity distribution at a solid boundary
    must be zero, and in open channel it generally
    increases with the distance from the boundaries.

19
Velocity and
Pressure Distribution in open channel
  • The maximum velocity doesnt occur at free
    surface instead it occurs at usually below the
    free surface water at a distance of 0.05 to 0.25
    of depth.
  • The velocity distribution in a channel is
    measured either with the help of pitot tube or
    current meter.
  • On account of non-uniform velocity distribution
    over channel section, correction factor will have
    to be applied while calculating Kinetic energy
    and momentum.

20
Velocity and
Pressure Distribution in open channel
  • The kinetic energy correction factor ?(alpha) is
    also known as coriolis coefficient in honors of
    G.coriolis who firs proposed it.
  • The momentum correction factor ?(beta)Boussinesq
    coefficint after J.Boussinesq who first proposed
    it.
  • Experimental data indicate that in fairly
    straight prismatic channel turbulent flow the
    alpha value ranges from 1.03 to 1.36 and that of
    beta ranges from about 1.01 to 12,however, for
    simplicity the value of both alpha and beta
    assumed to be unity.

21
The figure below shows typical Velocity
Distribution across some channel cross section
. The number indicates percentage of maximum
velocity
22
THE END!
THANK
YOU
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