Analysis of spatial flow in the lower beach of tubular sloped structures

Purpose. Development of theoretical and methodological foundations and new design solutions to ensure stable and trouble-free operation of downstream structures of tubular culverts on sloping lands by studying the regularities of spatial (two-dimensional) turbulent flow formation, assessing the efficiency of existing dissipator types, and substantiating the principles of active dissipation of excess kinetic energy under conditions of significant variability in local runoff. Methodology. The study was based on a comprehensive analysis and generalization of the results of field surveys, theoretical principles of open-channel and pressure flow hydraulics, as well as an analysis of existing empirical and statistical-probabilistic methods of hydrological calculations. The methods used include the systematization of hydraulic regimes of tailwater connection (submerged jet, crossed/confluence flow, free spreading), as well as a critical analysis of existing approaches to dissipating excess kinetic energy of two-dimensional and three-dimensional turbulent flows in the tailwaters of structures. Findings. It was established that over 65 % of cases of unsatisfactory performance of anti-erosion systems and basin irrigation systems are caused by the destruction of tailwater structures induced by excess kinetic energy of the flow and the complex spatial (two-dimensional) nature of its motion. The features of spatial flow formation in the tailwaters of short tubular structures are characterized. The main forms of flow connection are identified: a submerged jet, a crossed flow (with impingement on the side walls), and free spreading (formation of oblique hydraulic jumps in the shape of a "petal"). It was proven that traditional reactive-type energy dissipators (stilling basins, walls) are inefficient under conditions of significant variability in local runoff and two-dimensional turbulent flow in a wide tailwater, as they do not provide a uniform distribution of specific discharges and are prone to siltation. Wave processes and the transition from semi-pressure flow to incomplete and perfect hydraulic jumps inside the culvert pipe were analyzed. Originality. The understanding of the regularities of the formation of a complex spatial (two-dimensional) turbulent flow at the outlet of short tubular structures into a wide tailwater under conditions of significant variability in the design discharge of local runoff has been deepened. The limitations and impossibility of the correct application of classical methodologies for calculating dissipation parameters due to the non-uniformity of velocity and depth distribution curves (diagrams) at the initial spreading section and the transition of the flow from a two-dimensional to a three-dimensional turbulent regime were proven. The feasibility of adapting the principles of active energy dissipation (by analogy with an "energy-dissipating diffuser") for the conditions of spatial flow spreading from circular tubular structures was substantiated. Practical value. The proposed provisions and conclusions make it possible to increase the reliability and durability of anti-erosion hydraulic structures and basin irrigation systems, ensuring their trouble-free operation over the standard service life (over 50 years). Key requirements for new designs of active-type energy dissipators were determined (high degree of energy dissipation over a wide range of discharges, minimization of dynamic loads, unobstructed passage of bedload sediment and debris), which will serve as a basis for designing improved spillway and culvert structures on sloping lands.