Numerical Study of Spillway Sidewall Convergence Effects on the Hydraulic Characteristics of Flow and Probability of occurrence of cavitation phenomenon

Spillways are one of the most important and most sensitive parts of the dams' structures, which are responsible for the drainage of the dam in emergencies and floods. Although many studies have been conducted to identify flow characteristics over the spillways, but little information is available on the effect of the convergence of spillway walls on the hydraulic characteristics of the supercritical flow downstream of the chute and the potential for cavitation occurrence. To Reduce the Cost of Long Spillways and also With Considering Topography, Spillways Side Walls Especially in Chutes is Execute Convergent. Due to the convergence of the walls and the interaction of the flow of spillway with these walls, the supercritical stream is formed, which the result of this interaction is the formation of waves at the downstream of the spillway and on the walls of the chute. This phenomenon affects the downstream flow, causes uneven hydraulic conditions on the spillway, and therefore the height of the waves on the wall affects the design of the walls of the chute, and in this regard, the profile of the waves on the wall is also important. In this Study, using Flow-3D Software, Turbulence and Finite Volume Method, Flow Over Convergent Spillway of Gavshan Dam Simulated and Then, the effect of convergence of the spillway walls of Gavshan dam under different angles of convergence including 0, 1, 2, 2.5 and 3 degrees of convergence on hydraulic characteristics of the flow, such as the average velocity distribution, medium pressure and mean depth of flow, as well as the probability of occurrence of cavitation were investigated.. To verify the Numerical Result, Experimental Data of Hydraulic Model was used. The results showed that with Increasing in Convergence of Spillway side walls, the Average Speed and Depth of Flow Increased. Location of Minimum Depth of Flow on Spillways with Increasing Convergence on the side walls becomes closer to the Spillway Crest. Convergence of the side walls Creates Two Sets of Secondary wave in axes and corner of Spillways, which leads to an increase in the height of the Side walls. With the Increasing Convergence Angle, Froude number Decreases along the Flow path. Also, through exploring the pressure values, it was relieved that due to the rise of the flow depth along the walls, the pressure increases near the chute walls accordingly. Furthermore, by calculating the cavitation number at different sections of each numerical model it is realized that as the convergence angle of the model narrows, the cavitation index increases and consequently, the highest cavitation risk will be spotted for the least convergent model.