Descripción: Paper from Journal of Roman Military Equipment Studies (volume 1, 1990, 161-4) speculating on the form of the helmet plumes/crests used by legio V Alaudae
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1.0 TITLE : BROAD CRESTED WEIR 1.1 Introdu Introductio ction n
Level 2 laboratory activities refer to the condition where only the problem is guided and given. Students are required to find the ways & means and provide the answers to the given assignment using the group creativity and innovativeness. The activity will enable the students to appreciate indepen independen dentt learni learning ng and prepar preparee them them for a much much harder harder task of open ended labora laborator tory y activities. In this laboratory activity students will be eposed to the apparatus and appropriate methods to carry out the hydraulic parameters in uniform flow for open channels. 1.2 Objective
The ob!ective of the test is" •
To determine the coefficient of discharge #$d% through a broad crested weir.
1.3 Lernin! Outco"e#
t the end of the laboratory activities' students would be able to" (. Identify Identify and use the correct correct apparatus)t apparatus)tools ools to carry carry out test to to determine determine hydraulic hydraulic parameters in uniform flow for open channels. 2. nalyse nalyse the collected collected data data correctly correctly and present present in in proper technica technicall format. format. *. +ork +ork in group group to undertak undertakee the task task and produce produce the the technical technical report report 1.$ T%eoretic& Bc'!round
+eirs +e irs are typically installed in open channels such as streams to determine discharge #flow rate%. The basic principle is that discharge is directly related to the water depth #h% is known as the head. ,road- crested weirs' also called long-weirs' have crest lengths that are significantly longer than sharp-crested weirs. These weirs are usually constructed of concrete' have rounded edges' and are capable of handling much larger discharges than sharp-crested weirs. There are several different designs of broad-crested' of which the rectangular weir can be considered representative. weir is a device used for measurement of o f flow in open channels and rivers. It is
nothing but a partial obstruction placed across the flow in the channel causing the liquid to back up' upstream of the obstruction and then flow over it. +hen the liquid flows over the weir the depth of flow above the crest level of the weir bears a relationship with the discharge over it. Thus the discharge through an open channel can be obtained by the measurement of a single parameter like the head of liquid above the crest of the weir. nappe is the sheet of water which passes through the notch and falls over the weir crest. +hen the downstream water surface is far enough below the crest to allow air to circulate beneath the nappe' the flow or drop is to be free or critical. If air does not freely circulate benea th the nappe' then the flow is submerged or subcritical. +eirs are calibrated for free-flow conditions and' thus' submerged flow conditions are not desirable and can result in erroneous readings. igure *.(" /etails description of critical flow over a broad crested weir
igure *.(" /etails description of critical flow over a broad crested weir 3
Q theoretical =1.705 B H 2
Q actual =
Volume Time
Q act =C d .Qtheoretical
2.0 (rob&e" Stte"ent
weir is said to be broad crested if its crest spans all the way across the width of the channel and has substantial crest length along the direction of flow. The length of the crest should be greater than three times the maimum head under which the weir is to be operating' so as to ensure that the streamlines become parallel to the surface of the crest and the underside of the nappe adheres to the weir crest throughout its length. The upstream edge of the weir is well rounded to prevent the separation of flow and eddy formation so as to minimi0e the loss of energy. In this laboratory activity' the group is required to perform tests on the determination of coefficient discharge #$d% through a broad crested weir. 3.1
(. broad crested weir was installed to the height of 1 cm #0% in the channel 2. The pump was switched on and the flow rate was ad!usted so that the critical flow was obtained over the weir *. The actual discharge was measured at flow rate using the flow meter attached to the flume by taking a time of a certain volume of water. . The depth was measured in the channel upstream of the weir 1. The above procedures was repeated for two more different flow rates 3. The height of weir was increased to (4 cm in the channel and repeated all the above procedures.
3.3 Dt Ac*ui#ition
5eight of weir' 6 7 4.41m
+idth of flume' , 7 4.(4m QTheo
+o.
De)t% u)#tre", D -"
/D
Ti"e, t -#
-
3
m /s
3
m
(¿¿ s ) Q act ¿
C d
(
4.484
4.424
((1
−4 4.82 × 10
−4 8.70 × 10
2
4.4:4
4.4
2
−3 1.57 × 10
2.38 × 10
*
4.(44
4.414
*3
−3 1.91 × 10
2.78 × 10
5eight of weir' 6 7 4.(4m De)t% u)#tre", D -"
−3
−3
/D
Ti"e, t -#
-
3
m /s
3
(¿¿ s ) Q act ¿
C d
−4 6.74 × 10
−3 1.37 × 10
(
4.(21
4.421
8*
2
4.(*1
4.4*1
1*
−3 1.12 × 10
−3 1.89 × 10
*
4.(9
4.49
*3
−3 1.79 × 10
2.78 × 10
3
Q Theoretical =1.705 B H 2 Q actual =
Volume Time
Q act =C d .QTheoretical
(.3
m
$alculations
(.12
+idth of flume' , 7 4.(4m QTheo
+o.
(.94
2.4* (.3:
−3
(.11
C d=
Q act QTheoretical
or height of weir' 6 7 4.41m 3
(.
−4
Q Theoretical =1.705 × 0.05 × 0.020 2 = 4.82 × 10
Q actual =
C d=
0.1 m
3
m /s
3
=8.70 × 10−
4
115 s
−4
m /s
−4
m /s
8.70 × 10 4.82 × 10
3
m /s
3 3
=1.80
3
2.
−3
3
Q Theoretical =1.705 × 0.05 × 0.044 2 =1.57 × 10 m / s
Q actual =
C d=
0.1 m
3
=2.38 × 10−
3
42 s
−3
m /s
−3
m /s
2.38 × 10 1.57 × 10
3
m /s
3 3
= 1.52
3
*.
−3
Q Theoretical =1.705 × 0.05 × 0.050 2 =1.91 × 10
Q actual =
C d=
0.1 m 36 s
3
−3
=2.78 × 10
−3 3 2.78 × 10 m −3 3 1.91 × 10 m
/s =1.46 /s
3
m /s
3
m /s
or height of weir' 6 7 4.(4m
3
(.
−4
3
−3
m /s
−3
3
Q Theoretical =1.705 × 0.10 × 0.025 2 =6.74 × 10 m / s
Q actual =
C d=
0.1 m
3
=1.37 × 10−
3
73 s
−3 3 1.37 × 10 m −4 3 6.74 × 10 m
3
m /s
/s = 2.03 /s
3
2.
Q Theoretical =1.705 × 0.10 × 0.035 2 =1.12 × 10
Q actual =
C d=
0.1 m
3
3
−3
=1.89 × 10
53 s
−3 3 1.89 × 10 m −3 3 1.12 × 10 m
3
m /s
/ s =1.69 /s
3
*.
Q Theoretical =1.705 × 0.10 × 0.048 2 =1.79 × 10 m / s
Q actual =
C d=
0.1 m 36 s
3
−3
=2.78 × 10
−3 3 2.78 × 10 m −3 3 1.79 × 10 m
3
m /s
/s =1.55 /s
$.0 Di#cu#ion
,road crested weirs are robust structures that are generally constructed from reinforced concrete and which usually span the full width of the channel. They are used to measure the discharge of rivers' and are much more suited for this purpose than the relatively flimsy sharp crested weirs. dditionally' by virtue of being a critical depth meter' the broad crested weir has the advantage that operates effectively with higher downstream water levels than a sharp crested weir.
rom the eperiment' we get the coefficient of discharge #$d% by using formula
C d=
Q act QTheoretical
'
where
Q act
7
volume and time
3/ 2
QTheoretical =0.1705 BH
.
The
discharge#$d% were decrease when the depth upstream #/% were increase. or eample' / 7 4.48' $d 7 (.94 and / 7 4.4:' $d 7 (.12. +e take the $d with two different height of weir and we repeat two more different flow rats for each height of weir. /uring eperiment' the error we had found is paralla error for height / when we take the reading' our eyes is not :4 degree to water level. To overcome this error the eyes must be :4 degree to the water level to get the accurate value of depth upstream #/%.
.0 Conc&u#ion
,ased on this eperiment' the main ob!ective is to determine the coefficient of discharge #$d% through a broad crested weir. In conclusion' it was noted that a large discrepancy between the flow rate of theoretical and flow rate of eperimental values occurred. This was because of the different velocity of the water while conduct in the eperiment compared to the theoretical 3
value which was using the formula given
2
1.705 BH
. There are some error take place during
the eperiment was carried out which could give effect to the data obtained to get the coefficient of discharge #$d%. irstly' error due to the o bserver of paralla error. This is most likely due to errors in measurement or the depth of upstream #/%. Thu s' to overcome this error' the eye of the
observer must be :4o perpendicular to the measurement in order to get an accurate result. To get Qactual
¿ the value coefficient of discharge #$d% the equation was Qtheoretical . ;et' the disturbance due to the water and air also can affect the measurement taken. Thus' to o vercome this error' the eperiment should being done in ideal condition so that an accurate measurement will be obtained. Re4erence
