فهرست مطالب aa akhtari

Transitions are commonly used structures in both natural and artificial open channels. With increasing the transition dimensions along the flow, the flow decelerates. Under subcritical steady state flow conditions, reducing the flow velocity increases the water pressure and reverses pressure gradient. This phenomenon creates separation zone and turbulent eddy flow, and causes the flow energy losses. Due to the complexity of flow pattern and scale effects, physical models can solely provide a clear understanding of the physical principles governing the flow field, so it is necessary to study flow pattern numerically along with the field and experimental studies. In this study, the flow pattern in a rectangular to rectangular expansive transition, has been simulated under subcritical flow with RSM turbulence model using Fluent software. Water surface and flow velocity profiles obtained by the two methods at different sections of transition were compared with experimental results. The results showed a good agreement between the simulated and experimental data. After validation of the numerical model, the effects of inflow Froude numbers on strength of secondary current, hydraulic efficiency of the transition, turbulent kinetic energy and bed shear stress at different cross sections were simulated. The results showed that with increasing the inflow Froude number, the strength of secondary current along the transition increased, while the hydraulic efficiency decreased. Maximum efficiency (60.83%) occurred in Fr1=0.40. Also, with increasing the Froude number, turbulent kinetic energy and bed shear stress increased, so that, from transition inlet to the outlet, turbulent kinetic energy for total Froude numbers decreased with a 30.71% reduction in turbulent power.

In this study, the effect of flip bucket at the end of ogee spillway on the energy dissipation rate and jet length has been investigated experimentally. Experiments on standard ogee spillway for buckets with take off angles of 32 and 52 and for no bucket condition were conducted. The rate of energy dissipation in this type of spillway was investigated with conducting 21 tests with discharge range between 330 to 864 L min-1 .The results showed that at the spillway with flip buckets and take off angles of 32 and 52 , the rate of energy dissipation was increased from 10 to 20 percent compared to the spillway having no bucket. The highest energy dissipation was occurred in the flip bucket with take off angle of 52 . Also the relative energy dissipation was decreased in all three modes with increasing discharge. The length of jet for flip buckets with angles of 32 and 52 was measured and it was releaved that the length of jet for bucket of 32 was greater than that for bucket of 52 . By increasing the discharge, the difference between the jet lengths of the two buckets was decreased and at the discharge rate of 864 L min-1, the length of jet was equal for the two buckets.