Investigating impact of converging training walls of the ogee spillways on hydraulic performance

摘要

Due to their economical and structural aspects, ogee-crested spillways can be constructed and operated in a wide variety of situations. In this study, a three-dimensional physical model (1:50 scale) was planned and constructed to investigate the impact of converging training walls of the ogee spillways with a curve axis on flow characteristics such as the discharge coefficient, free surface profile, flow depth and pressure on the spillway. For this purpose, the spillway was constructed, tested and verified in both the symmetrical and asymmetrical convergence of training walls ranging from 0 degrees to 120 degrees. Observations from flow depth and piezometric pressure along the spillway in various convergence angles indicate that as theta increases, the flow depth and piezometric pressure increase at the bottom and the toe of the spillway model. Also, in convergence angles of 60 degrees and 90 degrees tested in both symmetric and asymmetric states, the angles with lower L-ch/L have larger flow depth and piezometric pressure. The results of the experiments indicated that in the converging ogee spillway, by increasing the total upstream head, the discharge coefficient shows increment for each of the convergence angles (theta's) and eventually the downstream flow changes its condition to either supercritical or critical stages. It must be considered that the discharge coefficient is independent of symmetric or asymmetric situations. On the other hand, at the submergence stage for the spillway, the difference in the discharge coefficient can be due to tailwater submergence occurring in some convergence angles. Also, the flow depth and the piezometric pressure on the bottom and the toe of the spillway increased more in the symmetrical convergence angles in comparison with the asymmetrical convergence angles. Also, by decreasing the downstream channel width to the crest length (L-ch/L), the flow depth and piezometric pressure increase subsequently. Results approved that the 60 degrees convergence is the most economic convergence angle due to its capability in passing the largest flow discharge in the maximum head. The reason for this choice is that the crest length of this angle is 33% lesser than that of 120 degrees convergence.