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LABORATORY EVALUATION OF THE EFFECT OF ANTI-VORTEX BLADES' LENGTH ON
DISCHARGE COEFFICIENT OF SHAFT SPILLWAY
Ebrahim Nohani*
Department of hydraulic Structures, Dezful Branch, Islamic Azad University, Dezful, Iran
*Corresponding author’s Email: nohani_e@yahoo.com
ABSTRACT
Where there is not adequate space to build other types of spillways, shaft spillways are used to pass the
excess water from the headwater to the dam downstream. In this paper, the effect of increasing the length of antivortex blade as a percentage of the spillway diameter was examined using two types of anti-vortex blades on the
discharge coefficient of the shaft spillway and the results indicate that the effects of the length of anti-vortex
blade is independent of the shape of anti-vortex blade and it has an increasing trend up to 20 percent of spillway
diameter, and more than this, it has a declining trend.
KEYWORDS: discharge coefficient, length of anti-vortex blade, shaft spillway.
INTRODUCTION
Shaft spillway is one of the spillways used in dams which is formed from a circular crest which directs the flow into a
vertical or inclined axis. The mentioned axis is connected to a tunnel with a low slope. Connectivity of the axis to the
tunnel is done by the bend with a proper radius that eventually transfers the water to the downstream
(Samani,2009). The main problem that these spillways are facing is creating whirlpools in their span that leads to loss
in productivity of the reservoir discharge system. The whirlpool prolongs the flow path and thereby reduces the
discharge and discharge coefficient in spillway. Whirlpool flows occur as a result of change in the direction of flow,
viscosity and surface tension. The presence of such flows has a negative impact on the performance of shaft spillway.
One of the effective methods in controlling the whirlpool is to use anti-vortex blades, which are used to increase the
discharge coefficient of the shaft spillway (Nohani and mousavi,2010).
The relationship of the discharge of shaft spillway is as follows:
Q = CLH 3/2
(1)
Q = C (2ΠR) H 3/2
(2)
In the above equations, Q is the discharge passing through the spillway, C spillway discharge coefficient, L the length
of spillway crest, H the water level on the spillway and R the radius of spillway crest. Fattor and Bacchiega (2003)
concluded that if in the shaft spillways, the spillway is submerged, the discharge rate is 1.34 times more than the flow
discharge in free mode and we will have turbulence in the case of non-aeration to the pressure tunnel in the
spillway. Ellesty et al. (2005), by making the laboratory model of shaft spillway have conducted the laboratory studies
on the shaft spillways with anti-vortex blades and without anti-vortex blades, and concluded that using these types of
vortex control structures is highly effective in the discharge coefficient. Increasing the number of blades, its
performance is lower than the fewer number of this type of structures (Ellesty, 2006).
In a study, by making the physical model of shaft spillway and performing the experimental studies, Nohani et
al. (2013) have examined the impact of the number, thickness and the angle of placing the vortex breaker blades on the
discharge coefficient of the shaft spillway and the results showed that the position of blades with 30 and 60 degrees has
more greater impact on increasing the rate to discharge to 90 degrees angle, and also increasing the thickness of the
blades reduces the discharge and the discharge coefficient. In this study, the height of blades were studied and it was
shown that the increased height of blades to 1.5 times does not increase the rate of discharge and only in depth that the
weir is submerged, it will have little effect on the discharge coefficient. Also, by studying the number of blades, it was
found that by placing 3 blades, discharge and discharge coefficient increased compared to the 6 blades (Nohani and
Naghshine, 2013 ). Bagheri (2009), by making the laboratory model of the shaft spillway as well as making the
spillway crest multifaceted has studied the changes in the discharge coefficient of shaft spillway and concluded that
making the spillway crest multifaceted will increase the discharge coefficient of shaft spillway and the maximum
increase will occur in the discharge coefficient in a state where the spillway crest is made as trihedral, but making the
spillway crest seven-sided has the minimum impact on increasing the discharge coefficient because the shape of
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ISSN: 2319–4731 (p); 2319–5037 (e)
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spillway crest is close to the base case (Bagheri et al. 2010). In this study, we examine the effect of anti-vortex blade
length to the outside of the spillway opening.
MATERIALS AND METHODS
To study and achieve the objectives of the study, the physical model was made according to Figure (1) in the hydraulic
laboratory of Water and Power Authority of Khuzestan and was tested. To supply the water required for the tests, a
pond with a volume of 2000 liters with steel sheet with a thickness of 3 mm is used. Connecting the basin reservoir to
the spillway reservoir is done by a field-tee on which two valves were used to adjust the inlet discharge. Conveyance of
water from the basin to the spillway reservoir is done by a pump with a discharge coefficient 250-1000 liters per
minute. After pumping from the basin reservoir, water is entered into the triangular spillway size 0.2 × 0.2 × 0.2 meters
and the internal angle of 60 ° for the accurate measurement of the input discharge. The exact value of the bulk
discharge is calculated using the scale in the reservoir of the triangular spillway and a graduated glass in
the downstream end of the tunnel. To control the volatility and turbulence inside the reservoir, two buffer systems were
used, a buffer is considered when the water enters the reservoir of the triangular spillway and the second one is
considered as chamber. In this chamber, which is like a metal mesh screens, a series of straw was used to slow the flow
that the water passes through the second buffer and entered the reservoir of the shaft spillway. The reservoir of the shaft
spillway is sizes 1.2 × 1.1 × 1.2 meter which is completely enclosed from three sides and on the other hand, to observe
the phenomenon inside the reservoir, a glass sheet with a thickness of 10 mm and a scale connected to reservoir glass
was used to measure the height of the water on the shaft spillway. Shaft spillway in the reservoir according to
Figure (1) is made of Teflon with a crest diameter of 35 cm, crest length of 1.1 m, guttural diameter 7 cm, bend
diameter 10.16 cm, height 28.2 cm and diameter of downstream tunnel 7.62 cm do the reservoir discharge. At the end
of the downstream tunnel, two gate valves were sued: one to enter the graduated glass for the calculation of discharge
and the other to return the water to the basing reservoir. To investigate the effect of increasing the length of anti-vortex
blade to the outside of the spillway span on the discharge coefficient of shaft spillway, as in Fig. (3) and (4), two types
of anti-vortex blades in 4 different lengths of 10, 15, 20 and 25% of the spillway diameter were used that increasing the
length of anti-vortex blade to the outside of the spillway opening is respectively 0, 0.05, 0.1, 0.15 percent of the
diameter of spillway. For all Figures, 6 anti-vortex blades with a thickness of 2 cm and a height of 4 cm were used. To
study, 99 tests were done in 9 steps and the specifications of experiments are presented in Table 1.
Figure 1: Plan of the physical model
Volume- 4 Issue- 2 (2015)
ISSN: 2319–4731 (p); 2319–5037 (e)
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Figure 2: View of the shaft spillway
Figure 3: View of the anti-vortex blade type A in various lengths
Figure 4: View of the anti-vortex blade type D in various lengths
Number
10
10
12
12
12
10
10
11
12
Table 1: Profile of the tests
Ratio of increasing the Ratio of anti-vortex
anti-vortex blade length blade length to diameter
to the outside of the of spillway (L/d) in %
spillway span (L/d) in %
0
5
10
15
0
5
10
15
10
15
20
25
10
15
20
25
Number
of tests
S
SA1
SA2
SA3
SA4
SD1
SD2
SD3
SD4
In Table 1, S is the spillway without anti-vortex blade or control spillway, L length of anti-vortex blade and D is the
diameter of the spillway.
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DISCUSSION AND CONCLUSION
Using the experimental data obtained from experiments, spillway discharge coefficient was calculated for each test. To
determine the shaft spillway discharge coefficient, formula (2) was used, and the results were shown as a discharge
coefficient curve to the submergence in Fig. (5) and (6).
Figure 5: curve of discharge coefficient to the submergence of weir without anti-vortex blade and the weir with
anti-vortex blade type A in various lengths
Figure 6: View of the spillway with anti-vortex blade type A in length with a ratio (L/d = 0.25)
Volume- 4 Issue- 2 (2015)
ISSN: 2319–4731 (p); 2319–5037 (e) © 2015 DAMA International. All rights reserved.
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Figure 7: Discharge coefficient curve to the spillway submergence without anti-vortex blade and the spillway
with anti-vortex blade type D in various lengths
According to Figure 5, it can be seen that the anti-vortex spillway discharge coefficient when the anti-vortex blade is
used is more than the spillway with no anti-vortex blade. Also the effect of anti-vortex blade length is clear with
increasing the depth of submergence. The effect of anti-vortex blade length for rates (L/d = 0.1), (L/d = 0.15), (L/d =
0.2) and (L/d = 0.25) respectively for 21, 27, 35 and 39 percent increase the spillway discharge coefficient to the
spillway without anti-vortex blades that the anti-vortex blade with a length to ratio (L/d = 0.25) has the greatest impact
on increasing the discharge coefficient. Figure (6) shows a view of the position of anti-vortex blade A with a length to
ratio (L/d = 0.25).
With an overview of Figure (7), we see that as the anti-vortex blade type A, lengthening the anti-vortex blade to the
outside of the openings of the spillway increases the discharge coefficient of the spillway. The effect of the length of
anti-vortex blade for ratios (L/d = 0.1), (L/d = 0.15), (L/d = 0.2) and (L/d = 0.25) respectively 20, 24, 29 and 33 percent
increase the spillway discharge coefficient without anti-vortex blades that the anti-vortex blade with a length of
ratio (L/d = 0.25) has the most impact on increasing the discharge coefficient. A view of the position of anti-vortex
blade D with a length to ratio (L/d = 0.25) is shown in Figure 8.
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Figure 8: View of the spillway with anti-vortex blade type E in length with the ratio (L/d = 0.25)
CONCLUSION
In this study, the effect of anti-vortex blade length to the outside of the spillway opening was examined. The results
showed that the effect of the length of the anti-vortex blade is independent of the shape of the anti-vortex blade and
increasing the length of anti-vortex blade increases the spillway discharge coefficient. Also, the effect of anti-vortex
blade length is to the ratio (L/d = 0.2) and more than this ratio of the impact of anti-vortex blade has a declining trend.
REFERENCES
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Drainage Networks National Conference, Shahid Chamran University, Iran, Ahvaz. In Persian. pp: 8.
Fattor C. A. and Bacchiega J. D. (2003). Analysis of Instabilities In the Change Regime in Morning Glory Spillways,
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Nohani E. and mousavi H. (2010). The Effect of number and thickness Vortex Breakers on Discharge Coefficient for
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Samani H. (2009). Design of Hydraulic Structures, Second Edition, published by consulting engineers PP 200.
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