UNIFORM FLOW IN OPEN CHANNELS

BySaif Ur Rehman

PG-15-43

CONTENT

What is Open Channel Types of Open ChannelTypes of flowing water and controlSolution of uniform flow in Open Channel

OPEN CHANNEL

An open channel :a)The stream not completely enclosed by solid

boundaries b)It has free surface subjected only to

atmospheric pressure.c) Referred as free-surface flow or gravity flow

Open Channel

TYPES OF OPEN CHANNEL

Natural ChannelIrregular shape

i.e : river, hillsides rivulets, tidal etuaries

Artificial ChannelRegular shape

i.e : drains, culverts, sewer, tunnels

TYPES OF OPEN CHANNEL

TYPES OF FLOWING WATER AND ITS CONTROL

Rapidly Varied Flow

i.e : hydraulic jump

Non-Uniform Flow

≠ 0

Uniform Flow

= 0

Gradually Varied flow

i.e : upstream of obstruction

Open Channel

Steady Flow

= 0

Open ChannelUnsteady Flow

≠ 0

TYPES OF FLOWING WATER AND ITS CONTROL

Steady• y (depth of water) and v (velocity) remain constant

with respect to time

Unsteady Flow• y and v change with time

TYPES OF FLOWING WATER AND ITS CONTROL

Uniform• y (depth of water) and v (velocity) remain constant

along the channel.

Non - uniform Flow• y and v change along the length of the

channel

• Flow Classifications1) Depending on the Reynolds number, Re

– Laminar Flow (if Re < 2300): very slow and shallow flowing water in very smooth open channels.

– Turbulent Flow (if Re > 4000): ordinary flow in ordinary open channels.

– Transition Flow (if 2300 < Re < 4000)

TYPES OF FLOWING WATER AND ITS CONTROL

V = average channel velocityL = length of channelv = kinematic viscosity of fluid

2) Depending on Froude number, Fr

– Fr = 1 : Critical Flow

– Fr < 1 : Subcritical Flow – slow flowing water

– Fr > 1 : Supercritical Flow – fast flowing water

TYPES OF FLOWING WATER AND ITS CONTROL

V = average channel velocityg = gravity acceralationD = hydraulics water depth

TURBULENT

LAMINAR

Bernoulli’s Equation

Lhzg

Vpzg

Vp 2

222

1

211

22

Open Channel

GEOMETRIC PROPERTIES OF OPEN CHANNELS

RECTANGULAR CHANNEL TRAPEZOIDAL CHANNEL

• The terminology of geometric elementsy : depth of flow T : top width b : bottom channel widthm : side slope So : channel bottom slope v : average flow densityFr : Froude number Re : Reynold number L : length of channelQ : flow rates , Q = AV A : area of the flow P : wetted perimeterR: hydraulic radius at cross section

D : hydraulics water depth V : volume

ν : velocity E : specific energy ∆z : weir height : flow temperature q = discharge over width

(m)

GEOMETRIC PROPERTIES OF OPEN CHANNELS

UNIFORM FLOW IN OPEN CHANNEL

Uniform flow is an equilibrium condition that flow tends to if the channel :

a)constant slopeb)constant cross sectionc)constant roughnessd)depth, water area, velocity and discharge at every section of channel are constant

• CHEZY FORMULA :

UNIFORM FLOW IN OPEN CHANNEL

Where ; V = mean velocity (m/s)C = Chezy’s factor of flow resistance (m1/2/s)R = hydraulic radiusSo = slope of energy lineQ = Discharge (m3/s)

• MANNING FORMULA :Where ; V = mean velocity (m/s)

n = Manning’s coefficient of roughness (s/m1/3) R = hydraulic radiusSo = slope of energy lineQ = Discharge (m3/s)