Open channels exam terms

Open Channel Flow

Free surface and gravity driven

Unsteady

Q is not constant at cross section

Steady

Q is constant at cross section

non-uniform flow

varied cross section, can have a stead Q

uniform flow

y=yo at all cross-section position and prismatic cross section (normal)

Turbulance

flocculating rotation at many scales

surface drag

along boundaries (bed and banks)

form drag

finite body shape (bridge piers and baffle blocks)

datum

subcritical

high y, low v, v

supercritical

low y, high v, v>c

critical

Fr=1

Fr

ratio of inertial forces to gravitational forces in a flow, and is used to classify flow regimes and predict wave behavior

Choking

when channel bottom change is high enough or downstream width is small enough to start to slow down upstream flow

Weirs

measure and control flow in open channel

Hydraulic Jump

supercritical flow upstream meets subcritical flow downstream

if no head loss between two cross section

Upstream E1=E2 Downstream

Flumes for Q measurements

Used to measure an open channel

Flumes for Q measurements

use for contractions and raised channels bottom to force critical depth

y1 supercritical

y1<yc, Fr>1

y1 subcritical

y1>yc,Fr<1

Surface water for long distance (jump appearances)

2.5

well behave, stable (jump appearances)

4.5

rough down stream surface (jump appearances)

Fr1>9

how to stabilize jumps

using baffle blocks

what happens when channel roughness increases

y2 and L decreases

Stilling basin

combination of baffle blocks, chute blocks, sill with concrete channel liners and wall

Bridge piers

obstruct flow and reduces area

What contraction angle condition prevents choking?

θ

What are three examples of Surges?

Close Sluice Gate

Close a turbine

Tidal bare hurricanes

What is Surges?

Waves that move back upstream (Vs)

where, Vs = surge wave velocity

What 5 variables are typically known when working with a stilling basin?

Qdesign

b

y1

V1

# of blocks

what is the purpose of a stilling block?

Energy dissipation after a supercritical spillway

In a hydraulic jump, what happens to the Froude Number from upstream to downstream?

Fr1> 1 to Fr2< 1

How does a measuring flume work?

It constricts the channel causing the flow to accelerate and pass through critical flow at a control section

What important flow condition occurs over the crest of a broad-crested weir?

The flow reaches critical depth over the crest

y=yc

What is a sharp crested V-notch weir?

A weir with a V-shaped opening used to measure open-channel discharge

What is a suppressed rectangular weir?

A rectangular weir where the crest length equals the channel width

L=b

What causes head loss in an expansion?

flow seperates

eddies ( eats up energy)

energy dissipation

What happens during choking?

When channel bottom change is large enough or downstream width is small enough it starts to slow down the upstream flow.

y=yc @ downstream cross section

Define what Specific Energy (E) is?

Relative to horizontal datum

What is the difference between Riverbed and Riverbanks?

Riverbed - the bottom surface of the river

Riverbank- the sides of the river

What correction is made for 2D and 3D effects?

α, the kinematic energy correction factor

What is a streamtube?

Tube or conduit formed by a bundled group of neighboring streamlines.

7 Types of Open Channels

Streams

Rivers

Storm Sewers

Sanitary Sewers

Ditch

Canals

Aqueducts