Abstract

In the past few decades, spillways have attraction significant as flow measuring devices as well as for release of flood discharges for dams. Severe cavitation damage has occurred on many high spillways around the world, including Russia, Pakistan, Venezuela, and Iran. In Iran, this phenomena caused serious damage to the Karun I dam spillway. Many studies and investigations related to cavitation effects were performed on different scale models and prototypes. Presently, standard shape spillways are designed based on previous experiences. Every case is subjected to a physical model study. It seemed necessary to systematize the evaluation of the flow field and the pressure distribution on the crests of spillways. The investigation of the subatmospheric pressure which is the main cause of cavitation was primary objective of the thesis. Consequently, a functional numerical model for design purposes was developed. Since viscous effects can be negligible, a two-dimensional irrotational flow was assumed. Comparable results between numerical and experimental spillway models validated this assumption. The developed numerical model was based on Navier-Stokes equations and the Bernoulli equation for Boundary-Fitted Curvilinear Coordinates (BFCC). The discretizations and computations were executed on a fixed, square grid regardless of the shape of the physical boundary. Equations were discretized using the finite difference method. The results show that the BFCC system is capable of treating Navier-Stokes equations for a wide range of Reynolds numbers with free surface boundary. In order to validate the numerical results of the problem, they were compared with WES 1 experimental data from the U.S. Army (1952). Also a small lab model was built to determine the free surface and pressure over an existing spillway. The investigation demonstrated agreement between the numerical, lab-scale and other researchers results. WES spillways are shaped knowing the design head. New shapes and pressures can easily be predicted by executing the BFCC program and introducing a optimal head for reiterated designs. The developed model can predict the formation of subatmospheric pressure at the early stage of spillway design. This model can also be applicable to flow in partially filled tunnels and in the design of supercritical slopes. While the BFCC model is a significant step forward in the development of numerical methods for predicting flow behavior on spillways, the flow patterns can often defy present analytical techniques. The thesis also contains several recommendations about spillway design in order to avoid damage due to the cavitation. 1 The Waterway Engineering Station