53:071 Principles of HydraulicsLaboratory Experiment #3Analysis of Open-Channel Flow Transitions Using Specific Energy Diagram

Li-Chuan Chen, Marian Muste, and Larry Weber

Objective

To observe the changes in the behavior of the free surface when subjected to changes in the channel bed.

Principle

2

2

1

2

22

g

Vyz

g

Vyz

2

22

22 gy

qy

g

VyH

zHH 21

Principle

2

2

2gy

qyH

zHH 21

Apparatus — Flume

hQ 04.2

Apparatus — Obstruction

7.786.965.875

0.24

ybump (ft)

x (ft)

For x 5 ybump = 0For 5 < x 5.87 ybump = 0.276x – 1.38For 5.87 < x 6.96 ybump = 0.24For 6.96 < x 7.78 ybump = 2.28 - 0.293xFor x > 7.78 ybump = 0

What are we going to do?

We are going to measure the water surface elevation along a open channel.

Procedures

1. Establish a stabilized discharge in the system.2. Measure the water level difference on the

differential manometer for computing discharge.

3. Measure the water surface elevation upstream, above, and downstream of the obstruction.

4. Change the flow rate and repeat Step 2 and 3 for another flow condition.

5. Observe the ripples formed at the tip of the point gage.

Data SheetFlow Situation I Flow Situation II

h = (ft); Q = (cfs); yc1 = (ft) h = (ft); Q = (cfs); yc1 = (ft)

Track(ft)

Water Surface

Elevation (ft)

BumpGeometry (ft)

Actual depth(ft)

Track (ft)

Water Surface

Elevation (ft)

BumpGeometry (ft)

Actual depth(ft)

 

               

               

               

Analysis

Plot the water surface elevation in the vicinity of the channel transition.Construct the specific energy diagram using the measured data.Compare theoretical specific energy diagrams with the experimental curves.Comment on the flow behavior in the two flow situation.

Sample Result — Water Surface Profile

Water elevation through channel transition

Flume track (m )

Flo

w d

ep

th (

m)

11.7 11.8 11.9 12.0 12.1 12.2 12.3 12.4 12.5 12.6 12.7 12.8 12.9 13.0 13.1 13.2 13.3 13.4 13.50.00

0.05

0.10

0.15

0.20

ycy1

y2

M easurem ents q2 > q 1

Channel bedM easurem ents q 1

Sample Result — Specific Energy Diagram

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20

Dep

th(m

)

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

y c(un it d ischarge) = 0.057 m

y c (data ) = 0.056 m

y1

(subcritica l)

y2 (supercritica l)

M easurem en ts q 2 > q 1

Theoretica l d iag ram q 2

Theoretica l d iag ram q 1

M easurem en ts q 1

Specific energy (m )

Specific energy diagram

Further Considerations

What are the main assumptions associated with the use of the specific energy diagram?Comment on the differences noticed on the ripples produced by the point gage upstream and downstream the obstruction.Analyze the water surface behavior for the same unit discharges used in your experiment hypothesizing the flow enters in the obstruction as supercritical flow.