Numerical investigation of the river confluence hydrodynamics. [Doctoral dissertation] | Хидрауличка анализа речних ушћа применом нумеричких модела. Докторска дисертација

This dissertation deals with the hydrodynamics of discordant beds confluences. Although some twenty five years ago extensive field investigations revealed that the bed of one or both tributaries was usually elevated above the bed of the post-confluence channel, hydrodynamics of such confluences has not yet been completely and adequately investigated and understood. Very little experimental and field data are available to describe complex flow patterns at these confluences and to answer what parameters control three-dimensional flow and mixing and transport processes. Thus the dissertation attempts at gaining better insight into hydrodynamics of the discordant beds confluences. To do this, a 3D finite-volume-based numerical model with the multiblock grid (SSIIM2) is used. In addition to the previously investigated confluences where the width-to-depth ratio of channel was less than 3, confluences of wide-channels are also considered in this dissertation. It is shown that the correction coefficient for the average flow angle delta at the tributary entrance to the confluence in the analytical model of Hager is not consistent, i.e. it depends on the alpha and M sub(R). Additionally, it is shown that the component of the tributary force of inertia that acts in the main-channel direction is overestimated by 30% if the average flow angle delta is used to calculate this component. Further, lowering of the high velocity core towards the bottom at the confluence and deviation of the shear layer from the vertical with the increase in the junction angle are direct consequences of the pressure reduction at the downstream junction corner and no-uniform momentum-flux distribution throughout the flow depth at the tributary entrance to the confluence. For the given M sub(R)-value, transfer of momentum (from the tributary to the main-river) per unit width of the tributary channel increases as the width of the tributary channel is reduced. However, the increase is not accompanied with the deeper penetration of the tributary flow into the main-river. The tributary flow stays on the tributary-side of the post-confluence channel, i.e. close to the river bank on the junction-side. In this case the stability of opposite bank is not endangered. However, stability of the river bank that lies on the junction-side might be endangered. Numerical simulations of the laboratory experiment with B/h = 1.0 have shown significant influence of the upstream planform curvature on the confluence hydrodynamics. However, the influence of upstream planform curvature on the flow pattern at the junctions of wide channels is negligible.