Experimental Investigation of Roughness Effect on the Cavitation Index in Ogee-Spillway

Introduction:
Due to the height of the dams, water behind dams have high energy and velocity on the spillway is high. Downstream of the spillway as a result of high energy and velocity of water on the spillway, is at the risk of cavitation and damage. The study of cavitation is crucial in this respect, and indicates the amount of flow degradation in the structure. Various works have been conducted on how to eliminate cavitation. Given the nature of cavitation, caused by high speed and pressure reduction, the use of roughness in parts of the spillway can help reduce or eliminate this phenomenon. This study aimed at investigating the effect of roughness under different conditions on changes in cavitation index in the downstream peak body.
Methodology:
To achieve a better understanding of how roughness functions, this research was carried out by changing its arrangement, number and different heights on the cavitation index using a laboratory model. In order to achieve a more real-life simulation, a spillway was installed at a distance of 5.5 m from the opening of the canal to ensure the flow is expanded and that the turbulence of incoming current is minimized. Peak spillway was considered in the study, designed and constructed according to USBR standards. Eight holes were installed on the spillway which were connected to 8 piezometers located in the channel body through flexible pipes to read the amount of pressure static. Effective parameters were identified with dimensional analysis, and three parameters (roughness arrangement, number of roughnesses, effective height of roughness) were tested. The roughness used in this research is made of PVC. After installing the spillway in the canal, the roughness was installed on the spillway. The piezometers were ventilated before each experiment. The channel slope was adjusted. Then, the pump was turned on, and after adjusting the flow rate, the test began. The depth gauge was calibrated for each aperture to measure and correct the height on the apertures. The equivalent pressure height inside the piezometer tubes was read and adjusted for the piezometer base and channel slope. Finally, according to the data, the roughness performance was evaluated using the cavitation index.
Results and Discussion:
The experiments of the present study were performed with different Froude numbers and slopes. After the control experiments, the experiments were evaluated in three modes: the effect of arrangement, number and effective height.
• Evaluation of cavitation index changes in control experiments:
Observations show that cavitation index decreases as water falls on the spillway. The minimum cavitation index in the control experiment occurs at point 8. Moreover, the average cavitation index increases compared to the control mode by placing the roughness.
• Investigation of roughness arrangement on the cavitation index:
Observations show that the placement of roughness with convergent, divergent and row arrangement increase the cavitation index compared to the control. In the Froude number of 1.08, the changes in the cavitation index range from 1 to 8, 3.7, 4, 4.2, 4.4, 4, 3.7, 3.8 and 3.5%, respectively, compared to the case.
• The number of roughnesses on the cavitation index:
By placing 9, 12, 15 and 18 roughnesses in the row arrangement, the observations showed that in all graphs, the cavitation index increased compared to the control. Moreover, with the number of roughness, the cavitation index has a slight increasing trend. With 18 roughnesses in the flow rate of 1.08, the increase in the percentage of cavitation index (compared to the control) was piezometric points 1 to 8, 11.1, 4.03, 12.3, 17.6, 4.4, 4, 3.7 and 7.1.
• Investigation of effective roughness height on the cavitation index:
Considering the three effective heights of roughness, the observations showed that in all three cases, there is an increase in the cavitation index compared to the control mode, and the most evident change in the effective height has occurred at 0.017. With 1.08% increase in the flow rate of cavitation index compared to the control condition, 1 to 8, 7.5, 4, 12.7, 26.9, 4.1, 3.8, 7.3 and 6, respectively were obtained.

Conclusion:
In general, it can be concluded that the installation of roughness on the spillway, the effective height of roughness, the number of roughnesses and its various arrangements, increase the cavitation index compared to the control. The presence of roughness reduces the flow velocity and increases the pressure on the spillway. This issue has a key role to play in eliminating or mitigating the cavitation phenomenon on the spillway. Compared to the control, the average cavitation index increased by 15.17% in the convergent arrangement, 11.8% in the divergent arrangement and 16.11% in the row arrangement. The results show that the greatest change in row arrangement and effective height occurs at 0.017 and n = 18.