Description types of flow, steady flow and unsteady flow, GVF, Discharge,Reynolds Number, State of flow,

1. 1
Presentation -2
Description of Types of Open channel flow :
Time as the criteria
(1) Steady flow and
(2) Unsteady flow
(1) Steady flow: Time as the criteria.
The flow in an open channel is said to be steady if the
depth of water remains constant all over the channel
during the time interval. Steady flow is divided into
two categories :
(i) Uniform flow: Space as the criteria. Open channel flow is
said to be uniform if the depth of flow in every section is
remained same. A uniform flow may be steady or unsteady
depending on whether or not the depth changes with time.

2. 2
(ii) Varied flow (space is the criteria) : is the flow where
the depth of flow changes along the length of the channel.
The varied flow may be :
(a) Gradually varied flow: is the flow where the
depth of flow changes gradually at different cross sections
(b) Rapidly varied flow: is the flow where the depth of flow
changes abruptly at different cross sections at a short
distance.
Presentation -2(contd.)

3. 3
Unsteady flow
The flow is unsteady if the depth changes with time .
Unsteady flow is of two types
(i) Unsteady flow/Unsteady varied flow:
The depth of water remains same with the change of time.
Unsteady flow is of two types.
(1) Gradually varied unsteady flow
(2) Rapidly varied unsteady flow.
ii) Unsteady uniform flow (rare): is the flow where water
level fluctuates from time to time with parallel to the bed
profile.
Presentation -2(contd.)

4. 4
Discharge:
The total flow/volume of water passes through a channel reach per
unit time is called discharge.
v
L (length)
Figure : Channel Section(channel reach )
Q = V/T ...................... Eq-1
where,
Q = Discharge
V = volume of water passing through the channel reach of length L
T = time of water passing
V = AL
A = flow cross sectional area normal to the direction of the flow
Q = AL/T = AV , where, V is mean velocity
Bank
Flow
Presentation -2(contd.)

5. 5
In steady flow the discharge remain constant throughout the
reach of the channel under consideration i.e, the flow is
continuous. Then the Eq- 1 for different channel cross sections
becomes as:
Q = V1A1 = V2A2 = V3A3 and so on ......... (2 -2)
The Eq-2 is the continuity equation for a continuous
steady flow.
Hence, Q = AV
Reynolds Number: The effect of viscosity relative to
inertia can be represented by the number which is called Reynolds
number after the name Reynolds defined as the following:
Presentation -2(contd.)

6. 6
R = (Inertia force)/(Viscous force)
R = VL/υ
where,
R = Reynolds number
V = viscosity of the flow fps
L = characteristics length in ft.- hydraulic radius
υ (neu) = kinematic viscosity of water in f2ps.
For water at 68 0 F (200C) - the kinematic viscosity,
υ (neu) = 1.08 X 10-5 f2ps
Presentation -2(contd.)

7. 7
1.Laminar flow: The flow of
water where viscous forces are strong
relative to the inertial forces and
viscosity plays a significant part in
determining the flow behavior is
called laminar flow. The water
particles move in a regular path.
Laminar flow is also sometimes called
streamline flow. It occurs for flows
with low velocity and/or high viscosity.
Reynolds number, R <500
State of flow
The state of flow i.e. the behavior of open channel flow is basically
governed by the effects of viscosity (liquid- sticky and thick) and
gravity relative to the inertial forces of the flow. Reynolds number is
the criterion used to predict whether a given flow will be laminar or
turbulent. Depending on the effect of viscosity relative to the
inertia, the flow may be classified as (1) Laminar Flow (2)
Turbulent flow (3) Transition flow
Water particle path
Presentation -2(contd.)

8. 8
2. Turbulent flow: The flow of
water where the viscous forces are
relatively weak to the inertial forces is
called turbulent flow. The turbulent
flow where water particles follow
irregular path and shape which are
neither smooth nor fixed. Reynolds
Number, R> 12500
3.Transition flow : The flow pattern in between
laminar and turbulent states is Transition flow.
Reynolds number 500 <R<12500
When hydraulic radius is taken as characteristics length, the
corresponding range for laminar to turbulent flow R is 500 to
12,500 since diameter of pipe is four times its hydraulic
radius
Hence, d0 = 4 R
Flow
Presentation -2(contd.)