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Fluid-mechanic-broad-crested-weir1

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University of Duhok
College of Engineering
Water Resources Department
Group B
Fluid mechanic
Experiment name: Discharge Over Broad Crested weir
Name: Amed Hakeem Abdulrahman
Class: second stage
Date of experiment: 4/3/2019
Date of submitting: 1/4/2019
Lecturer :Bshkoj
Experimental: No .(2)
1
1-2 Introduction :
Weirs are a small overflow-type dams commonly used
to raise the level of a river or stream and cause a large
change of water level behind them. The use of portable
instrument like kinds of weirs, flumes, floats, and
volumetric tank are common. Discharges measured range
from a trickle in ditch to a flood on the Amazon. Many
researchers have studied the head discharge relations for
flows over sharp-crested weirs and broad-crested weirs with a simple
cross section shape, such as rectangular, triangular, trapezoidal,
truncated triangular. The laboratory flume, or flow channel, is one of
the most important tools available to the hydraulics engineer whether
engaged in teaching or research. Frequently, so-called indirect
methods of discharge, measurement are the only practicable means
of obtaining the magnitude of a peak flood flow past a given site.
These determinations are based on the water surface profile,
usually defined from high-water marks, and upon the geometry and
hydraulic .figure(2-1):
2
2-2 Objectives :-
The objectives of Experiment are:
1-To show the measurement of flow rate.
2-To determine the relationship between upstream head
and flow rate For water flowing over a Broad crested weir.
3-To calculate the discharge coefficient Cd.
3-2 Equipment and Materials:-
1- Rota meter
2-Broad weir
3-Stop watch
4-Gage reading
5-Motor
6-Flow mete
5-2 Procedures and Readings:3
1-Ensure the flume is level, with no stop logs installed at the
discharge end of the channel.
2- Measure and record the actual breath (b) and the height (P)
of the broad crested weir. Then, the weir plate was placed
and fixed carefully perpendicular to the sides and bottom of
the flume and leveled on all axes by a carpenter’s level
Ensure that the weir is secured using a mounting hook through
the bed of the flume. For accurate results the gaps between
the weir and the channel should be sealed on the upstream
side using Plasticine.
3- A series of different flow rates were overtopped over the
weir and the corresponding heads above the weir crest were
recorded after the zero on the point gauge must correspond
to the level of the weir crest or the apex of the weir. Take
enough care not damage the weir and the point gauge.
4- For each flow rate, wait until steady condition is attained
then measure and
record the head (H) some way upstream from the weir point
gauge, see Fig.(2-3).
4
Figure (2-3) :Cross section of Broad Crested Weir.
5- While the flow rates passing over the weir were calculated
from the flow equation of broad crest weir.
𝑄the. = h*b*(2*g(H-h))^1/2
Where:
𝑄the. = Theoritical flow rate,
H= upstrem head of water over the weir crest, and
6= For each flow rate (actual discharge), measure and record
the volumes in the collecting tank and the time required to
collect that volume by stopwatch.
Qact= Volume/time
Where:
𝑄act. = actual flow rate
V = volume of the collecting tank,
t = time taken to rise volume (liter).
5
The coefficient of discharge Cd is defined as the ratio of
actual discharge obtained experimentally to the theoretical
discharge. i.e
cd= Qact / Qth
Where: Cd = Coefficient of discharge is a dimensionless
discharge coefficient which takes account the effects
of neglecting losses and the contraction of the jet as it
passes over the notch in Qthe.
7- Repeat the above procedure for different flow rates by
adjusting the inlet valve opening and tabulate the readings.
For each step measure the flow rate Q, the upstream depth
of flow H and the depth of flow over the weir h (where
flow becomes parallel to the weir). The flow rate Q can be
determined using the direct reading flow meter or the
volumetric tank with a stopwatch.
8- Complete the tabulation and find the mean value of Cd.
9- Draw the necessary graphs and calibrate the notch.
6
6-2 Results and Calculations:No. Volume
(Lt)
Time
(s)
H
(mm)
h
(mm)
Qact
Qth
cd
1
10
21.7
25.2 15.8
0.4608
0.0495
9.30
2
10
17.8
28.2 18.1
0.5617
0.0588
9.55
3
10
13.8
33.8 21
0.7246
0.0768
9.43
4
10
11.2
41.4 24.9
0.8928
0.1065
8.38
5
10
8.7
50.1 30.5
1.1494
0.13807
8.32
Tabulate your readings and calculations as follows:
Breadth of weir (b) = 7.5 cm
Height of weir (P) = 10 cm
Theory: Qthe. = area ∗ velocity
Specific gravity=9.81*100= 9810
Qth(1) = 15.8*7.3*(2*9810(25.2-15.8))^1/2 =0.0495
Qac = V/t
= 10/21.7
=0.4608
Cd= 0.4608/0.0495 = 9.30
7
7-2 Sketch :-
Qact
10
1
1
0,1
10
100
H
Sketch(2-2) ) Relation between log H and Q log
8
Qact
8-2 Discussion :the discharge coefficient (cd) of rectangular broad-crested weir
can be written as a function of the width of the
,)channel (b), total energy head upstream of the weir (h)
mean flow velocity in the main channel (v), length of broad-crested
weir (l), and acceleration due to gravity (g), dimensional analysis
based on buckingham‟s theorem
was used to find non-dimensional variables in the present
study).
There is no doubt that the theoretical results are different from
actual results and this is because in theory we neglect many
factors which affect on the results and we always take an ideal
case, but in actual we saw the influence of all these factors on
our results. In our results the Qtheoretical and Qactual were not close
to each other in value, maybe because we didn’t place the
calibrated scale on the center of the weir during taking the value
of (h), maybe we read the values of calibrated scale in a wrong
way because the scale was accurate to (mm), or maybe student
careless when taking the time during the Weight-Time method.
in order to estimate the outflow over a
rectangular broad-crested weir, the discharge coefficient in
the weir equation needs to be known.
9
9 -2 Conclusion:The experiment was so useful to us because by it we can find the
discharge of any stream, channel, and also it useful when we want
to rise the level of the water to make branch from dam or stream
at specific discharge, and thus the importance of this experiment
had been clarified to us, and specially in large hydraulic
constructions.
In this study, laboratory measurements were carried out
on rectangular broad-crested weir with different geometries
located on a straight rectangular main channel to investigate
the new equation for discharge coefficient. As a result of
dimensional analysis, the results indicate that the dimensionless
parameter of h 1/B should not be ignored in equations
determining the discharge coefficient of the rectangular broadcrested weir. Multiple regression analysis equations based on the
dimensional analysis concept were developed for computing
the discharge coefficient of a rectangular broad-crested weir; and
discharge coefficient equation was used for computing the
discharge over rectangular broad-crested weir.
10
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