The Numerical Investigation of hydraulic condition on flip bucket spillways and effect of inlet flow and shape on it

Introduction
Since beginning of the dam construction industry, one of the challenges that engineers have always been involved with, is how to reduce the enormous energy of water when it overflows from dams for which, many plans and ideas have been presented so far. One of these ideas is to utilize the energy dissipater structures among which, the flip buckets are well-known. Since the flip bucket is one of the most important components of a dam, whose destruction disrupts the normal performance of the dam, the engineers have always tried to enhance the efficiency of this component by investigating into the hydraulic conditions such as pressure, depth and velocity. Accordingly, it is of paramount significance to comprehensively study the hydraulic performance and condition of the flip buckets
Methodology
In this study, behavior of the fluid in the flip bucket of a dam has been modeled using Flow-3D software. The obtained results in Froude number, were compared with the data derived from an existing physical model. After determining the error percentage and selecting the turbulence model etc., using the continuity and motion equations in the fluid dynamics and finite volume method, a model was built and through a trial-and-error process, Froude numbers ranging from, 2 to 7 were chosen. The results pertained to the velocity, static pressure and depth were captured and compared with geometric characteristics of the bucket and incoming flow condition. To efficiently employ the results, the produced graphs were normalized using a similitude analysis. The comparisons have been made with the non-dimensional ratios of Froude number and x/r (x: distance from the bucket and r: bucket radius) in the middle as well as right and left sides
Results and Discussion
In order to choose the turbulence model (K-ε, K-ω and RNG turbulence models were used to compare the results of various models according to previous studies), after analyzing and optimizing the computational error between the numerical and the physical models, the k-ε model with the minimum rate of error was selected as the best practice. It needs to be noted that the error rate of pressure, velocity and depth are 1.52%, 1.5% and 1.58%, respectively. The results indicated that the velocity changes along the length of the bucket, generally have a slight increasing trend and reach their maximum value at the end of the bucket. In addition, velocity changes have an inverse trend as the Froude number increase. Moreover, it was observed that the depth changes are almost constant along length of the bucket and reach their minimum value at the end and decrease with the depth as the Froude number increases. Pressure changes also have a decreasing trend along the length of the bucket and also, decrease with increase in the Froude number. The situation of the above parameters in the sides is generally similar to the middle axis, but with a greater intensity
Conclusion
It can be generally stated that the most vulnerable zone of the flip bucket is where 0.1<x/r<0.3 (i.e., the region where flow runs backs to the top) in great Froude numbers. This zone is the critical area of the bucket structure due to the decrease in pressure, both in terms of the possibility of cavitations and increasing velocities. Furthermore, it was found that rate of vulnerability in the sides is greater than the middle. Moreover, range of the pre-final Froude numbers of the flow passing through to the bucket, is the turning point of the flow hydraulic condition that needs to be considered while designing this structure.