FLOW CHARACTERISTICS AND SCOUR PATTERN AT ZERO DEGREE CONFLUENCE OF CHANNELS

Abstract

Confluences are common occurrences in natural and artificial networks of open channels. The merging of flow at the confluence creates complex flow pattern and bed morphology. Formation of the mixing layer, fluctuation in water surface level, presence of secondary flow and separation zone are some common phenomena associated with the confluence of the channels. Understanding of the complex flow pattern and bed morphology of channel confluences is essential for effective management of the channels. Numerous studies have been carried out by various researchers to understand the mixing layer formation, flow dynamics, flow separation, flow field and bed morphology at the confluence of channels. As per the knowledge of authors, there are no studies available at the confluence of rigid and mobile bed channels, such a confluence occurs when the discharge of power channel and spillway channel merge downstream of the dam. The present thesis aims to study the zero-degree confluence of the channels experimentally and numerically. The confluence of rigid (primary channel) and mobile (secondary channel) bed channels has been studied. Excessive scouring at the confluence of the channels leads to the failure of the common wall and floor of the rigid channel. Experimental studies show that primarily two scour holes develop, one at the confluence point of the channels and other downstream of the primary channel. The effect of various parameters such as discharge ratio, tailwater depth, length of the rigid apron downstream of the primary channel and sediment size on the maximum scour depth in scour hole has been investigated. The equations are proposed for calculating the maximum scour depth in the scour hole and extension of scour hole in the secondary channel. Also, in the present study loose boulders were used as scour protection measures near the confluence of the channels. Flow structures near the confluence of the channels are very complex and need investigation. Three dimensional velocities were collected with the help of Acoustic Doppler Velocimeter (ADV) on the equilibrium scoured condition. Flow characteristics such as flow field, turbulent kinetic energy and Reynolds shear stress were investigated experimentally. Flow field shows the downwelling movement of the flow and the vortex formation near the confluence of the channels. The maximum value of turbulent kinetic energy (k) was found downstream of the primary channel. The value of turbulent kinetic energy (k) increases from bed to surface because bed experiences less turbulence compared to the surface.The present study also assessed the scope of Computational Fluid Dynamics (CFD) based numerical modelling for investigating the flow field near the confluence of the channels. The Detached Eddy Simulation (DES) model was used for flow simulation, and the numerical results were validated with the experimental results. Experimental program for the present investigation was carried out in a rectangular flume 10 m long, 0.60 m wide and 0.54 m deep. The width of the flume was divided into two channels over a length of 5.6 m starting from the upstream end. A sediment recess of 0.25 m depth and 1.6 m length was provided to act as the mobile bed and a tail gate was provided at the downstream end of the flume to regulate the flow. Measurement of tailwater level and water surface level was taken with the help of a pointer gauge having the least count of 0.1 mm. The scour depth was measured with an accuracy of 0.1 mm. To study the velocity and turbulence characteristics three dimensional velocities were measured with the help of Acoustic Doppler Velocimeter (ADV). Experiments were performed for two sediment sizes (d50 = 1.92 mm, and 3.12 mm), length of rigid apron downstream of the primary channel (L = 0, 0.125 m, and 0.25 m), and tailwater levels (dt = 0.07 m, 0.1 m, and 0.13 m), radius of scour protection work (R = 0.08 m, 0.11 m, and 0.14 m). The flow was observed with the help of floating thread and fluorescent dye, which showed that the flow near to the interface of the primary and secondary channels return in the secondary channel and forms vortices. Such return of the flow at the interface clearly indicates transfer of the momentum from high velocity flow in the primary channel to low velocity or stagnant water in the secondary flow. The return flow led to vortex formation in the secondary channel. The thread follows the path of the flow and returns after traversing some distance downstream in the channel. Primary channel discharge, after traversing rigid apron enter in the confluence which has mobile bed. High flow velocity in the primary channel scours the mobile bed downstream of the primary channel. Moreover, it exchanges the momentum with the low discharge carrying secondary channel at the interface of the two channels. This phenomenon of the momentum exchange gives rise to vortex formation near the confluence. This vortex formation led to scouring in the secondary channel as well as downstream of it. As the flow traverses over the mobile bed, it gradually loses energy, which results in the reduction of the scouring capacity. Therefore, there exists a maximum scour depth which shows the end of the scouring capacity of the flowing flow. Another prominent observation was noticed when the rigid apron length downstream of the primary channel was increased. It was observed that the extension of scour hole in the secondary channel reduced with the increase in the rigid apron length downstream of the primary channel.Dimensional analysis was performed to obtain a functional relationship for maximum scour depth in the scour holes and extension of scour hole in the secondary channel. Maximum scour depth and extension of scour hole in the secondary channel are the function of median particle size, acceleration due to gravity, width of the channel, thickness of divide wall, rigid apron length downstream of the primary channel, downstream velocity, discharge ratio, tail water depth, density of flowing fluid, density of the bed material, dynamic viscosity of the flowing fluid and confluence angle. Analysis of data showed that the maximum scour depth and extension of scour hole in the secondary channel increased with the increase in densimetric Froude number and discharge ratio. However, decrease with the increase in rigid apron length downstream of the primary channel. An empirical relationship was proposed to compute the maximum scour depth and extension of scour hole in the secondary channel. The relationship was developed using 80% of the observed and remaining data was used to validate the proposed relationship. Sensitivity analysis was performed to check the sensitivity of maximum scour depth and extension of scour hole in the secondary channel with all the parameters affecting them. Densimetric Froude number (FD) was found to be the most sensitive parameter to the maximum scour depth and extension of scour hole in the secondary channel. The use of loose boulders near the confluence showed a maximum reduction of 67.2% in the scour hole downstream of the primary channel, and a reduction of 96.8% was observed in the confluence scour hole. A CFD based numerical study was performed to simulate the flow near the confluence of the channels. DES with one equation Spalart-Allmaras model was adopted for the simulations, which is a hybrid technique that uses Reynolds-Averaged Navier-Stokes (RANS) modeling near the wall and Large Eddy Simulation (LES) away from the wall. The DES model was able to reproduce the velocity profile and flow field in the different flow conditions. Both transverse and longitudinal profile showed the high velocity downstream of the primary channel. However, low, or negative velocity was observed downstream of the secondary channel. The velocity field obtained through DES model agreed well with observed values obtained by ADV measurement. Velocity field showed the vortex formation near the confluence, and the vortex formation moves downstream of the confluence with the decrease in the discharge ratio. Vorticity field revealed that the vorticity of flow moves downstream of the confluence with the decrease in the discharge ratio. The CFD study was also performed for the equilibrium scoured condition obtained in the experimental work. Near the bed velocity field showed that the weak vortex is formed near the bed compared to near the surface. Moreover, the presence of the downwelling movement of the flow near the confluence helps in the scouring. Vorticity contour showed the local rotation of the fluid is more near the bed compared to the near the surface.