PowerPoint Presentation

Lecture 2

Specific Energy

1

Objectives

Upon completing this chapter, you should be able to:

Identify, compute and plot Specific Energy

Identify and compute critical depth

Compute and plot momentum

Understand Froud number

2

Specifc Energy

Specific energy (E=D+v2/2g)

Total energy head above channel bottom

Specific Energy diagram by plotting E agains D with Q constant

With high depths and very low V, E approaches the value of D (refere to equation above and Fig 1)

For high velocity, D appraches zero

3

Figure 1 Specific Energy diagram

Critical depth

Minimum value of E

Theoretical concept

Depends on channel slope and flow Q

The minimum value of E is assigned at critical depth (see fig 1)

Specif Energy (contd.)

a = cross-sectional area

T = top width of channel

Q = flow rate

g = acceleration due to gravity

7

Specif Energy (contd.)

Subcritical flow

Depths greater than critical depth

Tranquil flow

Supercritical flow

Depths less than critical depth

More rapid flow

8

Figure 2: Specific energy diagram

9

Specif Energy (contd.)

Critical velocity

Velocity of water at critical depth

Critical slope

Channel slope that causes normal depth to coincide with critical depth

10

Specif Energy (contd.)

Froude number

Used to distinguish between supercritical and subcritical flow

F = Froude number

V = average velocity

D = flow depth

g = acceleration due to gravity

11

Figure 3 Channel transition. Subcritical flow transitions to supercritical flow by passing through critical depth.

12

a

3

T

=

Q

2

g

gD

V

F

=