California State University Sacramento Department of Civil Engineering
Department of Civil Engineering Instructor MEH
FLOW OVER A SHARP-CREASTED WEIR -- PROCEDURES
References:
Roberson, J.A. and C. T. Crowe, Engineering Fluid Mechanics, 6th ed., John Wiley and
Sons, 1997, pp. 542-546
Daugherty, R., J. Franzini, and E. J. Finnemore, Fluid Mechanics with Engineering
Applications, 8th ed., McGraw-Hill, Inc., 1985.
Objectives:
To examine the fundamental characteristics of flow over two sharp-crested weirs
To measure the coefficient discharge values of rectangular and triangular weirs
To gather pressure information for a future lab on hydrostatic forces
Apparatus:
One-half meter flume and sharp-crested weir with manometer board
V-notch weir in return-flow channel
Point gages and manometer board /piezometers
Experimental Procedure:
Set up for the experiment.
Open the surge tank valve and turn on the large pump.
Close the drain valve on the upstream side of the flume head tank.
Open the feed valve and fill the head tank until the water overflows the weir and continue until water overflows the V-notch weir.
Close the feed valve; then the downstream gate on the flume to get a small water level all along the flume.
Check that the piezometers are reading the same level. If
not, bleed air from the tubing. When you are satisfied that the air is out, open the downstream gate. (Leave this gate open for the rest of the lab.)
Forces On Rectangular Weir
When the water just covers the flume bottom, read the piezometer heights.
These tell you the
elevations of the taps.
Check that the water height is at the weir crest elevation
and that the piezometer tubing
doesn’t have any air in it.
The piezometers should read constant values at the same level as the water in the flume. If needed, add additional water to assure that the static water
level is at the height of the weir. Use the point gages to measure the water elevations at
the rectangular crest height and the vertex of the V-notch.
Forces On Rectangular Weir (cont)
Measure the flow profile and pressure on the weir at one medium flow rate
Establish the flow over the weir crest necessary to obtain a head H of about 0.3-ft .
Determine the discharge from the flow meters.
Make sure the vent is open.
Record the height of each of the piezometer columns.
Some of the piezometers record the pressure on the face of the weir;
the others record the pressure on the bottom of the
flume.
Record the point gage readings at each flume tap position.
Close the valve on the ventilation pipe at the base of the weir
Note the change in the
nappe profile and level of water backed up against
the downstream face of the weir.
Calibrate the rectangular and V-notch weirs.
Establish a steady flow and record the discharge and water levels (head) at both weirs.
Note: Storage of water in the channel may cause the V-notch weir head to stabilize
slowly,
so don’t record the height prematurely.
Repeat the previous step at least five more times.
Shut-down
Close the flume feed valve. Open the drain valve on the head tank.
Turn off the pump and close the surge tank valve.
FLOW OVER A SHARP-CRESTED WEIR -- RESULTS
Format: Memo Report [Include your team logo somewhere in the report.]
SUMMARY
Present the key results and describe any major deviations from the printed procedures and why you did them.
You do not need to turn in the lab procedure sheet with your report, but it is ok to add it as an attachment. You may use a computer sketch of the apparatus in the body of the report, but the attachment must be hand drawn.
RESULTS:
Plot the approaching head and flow data for the vented rectangular weir, and the V-notch weir on a
graph with log-log-scaled paper. Determine the unconstrained equations of the rating curve
for each device. Comment on the linearity of your plotted rating curves in terms of standard
deviation and coefficient of determination.
Compare your experimental values for the vented rectangular weir and the V-notch weir with literature equations or graphs such as the equations in Ch. 13 of Roberson & Crowe.
Plot the water surface profiles from the point gage and piezometer readings for the vented and the
unvented rectangular weirs for a single Q only. Comment on any differences you observe
between the point gage data and the piezometer data.
Plot the pressure distribution curve from your manometer data on the upstream face of the
rectangular sharp-crested weir. Calculate the force on the weir by integration of the pressure
distribution and compare this value with that obtained by assuming a linear hydrostatic
pressure distribution.
ATTACHMENTS:
Attach your sketch of the experimental set-up and copies of your input data and calculated results.
Write out the key equations used in calculating the results, with one sample calculation for each equation.
Discussion questions
Are the values of the exponent on the approaching head and the weir coefficient consistent with
theory and empirical relations in the literature? Using an exponent of 1.5 for the heads on the
rectangular weir and 2.5 on the V-notch weir, compute the weir coefficients for both weirs at each
experimental flow rate. Is the use of constants for the exponent and the coefficient acceptable?
Compare the performance of the V-notch weir with that of the rectangular weir at both high and
low flows.
For the convenience of computation, the hydrostatic assumption has been widely used by the
engineers. Do you think the assumption is acceptable in terms of accuracy and magnitude?
Overestimated or underestimated?
CALIBRATE SHARP-CRESTED WEIRS: INPUT (OBSERVED) DATA
Lab. Team: Date of Experiment:
Lab Team Members:
--------------------------------- CONSTANT DATA ----------------------------------
Water Temperature ________° F Rectangular Channel Width ________ ft
Rectangular Weir: By Manometer at Crest ________ in, & at Bottom: ________ in.
By Point Gage at Crest ________ ft, & at Bottom: ________ ft.
V-notch Weir: Channel Bottom-to-Notch ________ ft V-notch Angle ____°
------------------------------- VARIABLE DATA --------------------------------------
|
Flow |
Rectangular Weir |
V-Notch |
| Run |
Meter |
Point Gage |
Manometer |
Point Gage |
| No. |
(gpm) |
(ft) |
(in) |
(ft) |
| 1 |
|
|
|
|
| 2 |
|
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|
|
| 3 |
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| 4 |
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| 5 |
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| 6 |
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| 7 |
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Pressure Forces on Rectangular Weir At Q = _________ gpm
Note: This table assumes tap A is at the top.
|
Tap |
Manometer Heads |
| Tap |
Elevation |
No Flow |
With Flow |
|
(in) |
(in) |
(in) |
| Crest |
|
|
|
| A |
|
|
|
| B |
|
|
|
| C |
|
|
|
| D |
|
|
|
| E |
|
|
|
| F |
|
|
|
Observe shapes for flows over the two weirs at a high flow rate. Sketch the patterns. Note the
conditions near the rectangular weir for both vented and non-vented flows.
FLOW PROFILE WITH SHARP-CRESTED WEIR: INPUT (OBSERVED) DATA
CONSTANT DATA
Profile Run Flow Rate: ________ gpm Rectangular Flume Width: ________ ft
Rectangular Weir Height: ________ ft
VARIABLE DATA
Flow Profile Along Rectangular Channel:
Zeroing Datums Reading With Flow
Tap Distance to Point Gage (ft) Manometer Point Gage Manometer
No. Tap (ft) Bottom Slight Flow Head (in) at Surface (ft) Heads (in)
------ ----------- ---------- ------------- ------------- -------------- -------------
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