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# Open Channel Hydraulic PowerPoint PPT Presentation

Open Channel Hydraulic. Hydrology and Water Resources RG. Review of fluid mechanics. Fluid mechanics. Weight Mass Density Specific weight Specific gravity Hydrostatics Continuity equation Types of flow Energy and Energy Head Bernoulli’s Equation Flow through open channel.

Open Channel Hydraulic

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## Open Channel Hydraulic

Hydrology and Water Resources RG

## Review of fluid mechanics

### Fluid mechanics

• Weight

• Mass Density

• Specific weight

• Specific gravity

• Hydrostatics

• Continuity equation

• Types of flow

• Bernoulli’s Equation

• Flow through open channel

### Properties of a Fluid

• Weight

W = mg(kN, lb)

• m = mass of fluid (kg, slugs) g = acceleration due to gravity 9.81 m2/sec, 32.2 ft2/sec

• Mass Density

• mass of the fluid per unit volume at a standard temperature and pressure

r = m/V (kg/m3, slugs/ft3)

• V = volume of fluid (m3, ft3)

• In the case of water, neglect the variation in mass density and consider it at a temperature of 4oC and at atmospheric pressure; then r= 1,000 kg/m3

### Properties of a Fluid

• Specific Weight

• gravitational force per unit volume

• Units: kN/m3, lb/ft3

• In SI units, the specific weight of water at a standard reference temperature of 4oC and atmospheric pressure is 9.81 kN/m3

g = W/V

• Specific Gravity

• ratio of the specific weight of a given liquid to the specific weight of pure water at a standard reference temperature

• Units????

Sg (fluid) = gfluid/ gwater

• Specific Gravity of water = ?

### Problem?

• A reservoir of glycerin has a mass of 1,200 kg and a volume of 0.925 m3. Calculate

• Weight of the glycerin

• Mass density of glycerin

• Specific weight of glycerin

• Specific gravity of glycerin

g = 9.81 ft/sec2, gw = 9800 N/m3.

## Open channel flow

### Terminology

• Open channel flow – any flow path with a free surface (open to atmosphere)

• Can be classified as

• Prismatic channel

• With constant x-section and a constant bed slope

• Non-prismatic

• Varies in both the x-sectional shape and bed slope between any two selected points along the channel length

• Atmospheric pressure acts continuously, constantly and at every location on water surface therefore is neglected

### Open Channel Hydraulics

• Variables of open channel flow analysis

• Open channel flow classification based on various criteria

• Time

• Depth

• Space

• Regime (subcritical or supercritical)

### Depth of Flow

• Elevation difference between water surface and deepest part of the channel

### Channel Slope

• Difference in the channel invert elevation between two locations divided by the distance between them

• In prismatic channel the slope is often constant over a significant channel distance

### Hydraulic depth & hydraulic radius

• Hydraulic depth: average depth across the channel

### Discharge & Velocity

• Discharge or flow rate: amount of water moving in a channel or stream system

• Velocity: speed at which water moves in an open channel

V = Q/A

V= average channel velocity, Q= discharge, A = x-sec area

• Water movement adds kinetic energy to the system

• Channel velocity is not constant at any location

• Varies both horizontally and vertically for any given channel cross-section

• Velocity near the channel banks is less than the velocity in the center of the channel

### Flow Classification

• Uniform vs. non-uniform

• One-dimensional vs. multidimensional flows

• Gradually varied vs. rapidly varied

• Subcritical vs. supercritical

### Types of Flow

• Uniform Flow

• in which the flow velocity and depth do not change from point to point along any of the streamlines otherwise it is called non-uniform or varied flow

• Laminar Flow

• in which each liquid particle has a definite path and the paths of individual particles do not cross each other

• Turbulent Flow

• if each particle does not have a definite path and the paths of individual particles also cross each other, the flow is called turbulent

### Types of Flow

• in which the depth and velocity at a point are constant with respect to time

• if Q is not constant

• One-dimensional Flow

• flow, whose streamlines may be represented by straight lines as opposed to curved lines

### Subcritical & Supercritical Flow

• Classification is based on ratio of inertial to gravitational forces at a stream location – Froude number

• If Fr > 1 – flow is ‘supercritical’ and inertial forces dominate, associated with steeper slopes (high velocity and shallow depth)

• If Fr < 1 – flow is ‘subcritical’ – gravitational forces dominate usually calm and tranquil –small slope usually in natural channels - (low velocity and high depth)

• For Fr = 1 both depth and flow are call ‘critical’

## Hydrostatics

### Energy

• What is energy?

• Ability to do work?

• Moving fluids possess energy by virtue of its

• Velocity

• Position

• Pressure

• 3 kinds of energies that can be stored in a waterbody

• Potential: due to elevation/position ‘Z’ (elevation above a fixed datum)

PE = WZ= mgZ

• Kinetic: due to velocity/motion

KE = mv2 = (W/g) v2

• Pressure: amount of work done in moving the fluid element a distance equals to the segment’s length ‘d’

Force F = PA

Work done (Pressure energy) = Fxd = PAd = P(Ad) = P(Volume) = PW/ g

### Total Energy

• Total Energy = Potential + Kinetic + Pressure

TE =WZ + (W/g)v2 + PW/ g

• Energy may be expressed as ‘Head’

• divide by ‘W’ throughout

• Represents total energy per unit weight of the fluid

H = Z + v2/g+ P/ g

• Z = Elevation Head (units of length)

• v2/g = Velocity Head (units of length)

• P/ g = Pressure Head (units of length)

### Example?

• Given:

• Water in a 6 in diameter pipe with a velocity of 8 ft/s

• Fluid pressure is 4 lb/in2

• Elevation of the center of the pipe above datum is 10 ft

• Required?

• What is total energy head?

### Bernoulli’s Equation – conservation of energy

• During a steady flow of a frictionless incompressible fluid, the total energy (total head) remains constant along the flow path

Z + v2/g+ P/ g = constant

Z1 + v12/g+ P1/ g = Z2 + v22/g+ P2/ g

### Continuity equation

• Based on the conservation of mass

• Assumption: flowing fluids have constant mass density (incompressible liquid)

• States that the quantity of liquid passing per time unit is the same at all sections

Q1 = Q2 = Q3= ….

• OR A1V1 = A2V2= A3V3= ….

• Q = flow discharge [m3/s];

• V = average velocity of the liquid [m/s];

• A = area of the cross-section [m2];

• and 1, 2, 3 = the number of sections 1-3

GOOD LUCK ;-)