Hydrodynamic Performances Of Free-flow Tunnel And Its Inlet Transition Of Water Conveyance Channel

Free-flow tunnel,a common form of open channel,is often connected to the upstream reach by a contraction transition.The hydrodynamic performances of the tunnel as well as the transition have crucial influences on the conveyance capacity,which guarantees the benefits of water diversion projects.Therefore,oriented towards the channel conveyance capacity,this dissertation studies the hydrodynamic performances of the tunnel as well as the contraction transition adjacent to its inlet.An improved Reynolds stress model has been proposed to model the uniform flow in free-flow tunnels characterized by a semicircle top.This model introduced a reduction factor associated with filling ratio into the surface reflection term in the Reynold stress transport equations,based on the proportion of turbulent kinetic energy in three directions near the surface.Then it suggested an appropriate value in the expression of turbulence dissipation rate at the water surface boundary.The model was validated against experimental results of uniform flow in circular open channel with different scales and wall roughness.Results showed that the model could predict the phenomenon that the maximum velocity is located below the surface,which is caused by the secondary currents.More importantly,the improved model could give more accurate results of velocity profiles near the water surface for high filling ratio cases,compared to the previous one used in rectangular open channel simulation.The uniform flow in horseshoe shaped tunnel with different filling ratios has been investigated by conducting laboratory experiments and using the improved Reynolds stress model.The pattern of secondary currents driven by turbulence anisotropy,as well as its influence on the distribution of longitudinal velocity,turbulence intensities and bed shear stress were studied.Results showed that when the filling ratio is no less than 50%,the maximum longitudinal velocity is located below the surface;in the region away from the bed,the longitudinal and transverse turbulence intensities firstly decrease,and then increase with the vertical position going up.When the filling ratio is less than 50%,the maximum longitudinal velocity is located at the surface;in the region away from the bed,the longitudinal turbulence intensity decreases monotonically with the vertical position going up.The larger filling ratio is,the lower the position of maximum longitudinal velocity is,and the positions where the longitudinal and transverse turbulence intensity reach their respective minimum value are lower as well.The vertical profiles of longitudinal and transverse turbulence intensities were suggested to be described using a quadratic polynomial.The bed shear stress distribution is nonuniform,with its maximum value appears at the corner.The results could provide theoretical basis for the determination of conveyance capacity and the protection against bed scour.Two tunnel inlet transition models characterized by a rapid contraction and a gradual contraction have been investigated respectively by conducting laboratory experiments and employing the VOF numerical model.The vertical profiles of the longitudinal and transverse turbulence intensities of flow along the centerline of the contraction transition were studied.The decrease in the longitudinal turbulence intensity within the contraction was explained by the vortex stretching mechanism and calculated by the rapid distortion theory.The characteristics of secondary currents adjacent to the tunnel inlet,and their influences on the longitudinal velocity distribution,especially the position of the maximum velocity,were discussed.Comparison was made between the performances of rapid contraction and gradual contraction concerning the water surface profiles within and local losses of the tunnel inlet transition,as well as the longitudinal velocity and turbulence intensity distribution in the tunnel.Results suggested that gradual contraction should be preferred in the transition design.The improvement of the conveyance capacity of Karamay water diversion project has been taken as an application.The rapid contraction before the tunnel inlet led to insufficiency of headroom at the tunnel inlet,thus inhibiting the conveyance capacity of the whole channel.By prolonging the transition length and making the transition changing gradually,the water surface became smooth in the transition,the headroom at the tunnel inlet increased and the water surface fluctuation in the tunnel was reduced.The improved channel can perform safely under design condition,and the conveyance capacity is increased by 14%.