Energy dissipation structures: Influence of aereation in supecritical flows

Adequate design of energy dissipation structures is essential for effective flood control. Theoretical analysis of these structures is complex due to the turbulent nature of supercritical flow and is further complicated by air entrainment. The effect of aeration on water flow has been one of most analysed phenomena during the last decades due to its influence on the behaviour of hydraulic structures. The objective of this study is to characterize the influence of aeration on the boundary friction in supercritical conditions and fully turbulent flows. Our analysis is based on a physical model designed to reproduce these phenomena in the Hydraulics Laboratory of CEDEX (Spain), funded by a research project of the Spanish Ministry of Economy.

The structure consists of a spillway chute 6.5 m high, 0.5 m wide and slope of 75%, followed by a 10 m horizontal channel where the hydraulic jump is confined. Water and air are supplied by a pump and compressors and controlled at the entrance by several valves and flowmeters. This device has a pressurized intake, with 10 m head of additional pressure, which generates the mixture to reproduce different scenarios of emulsified flow, with rates up to 300 l/s of water and 3000 l/min of air. Under these conditions, velocity ranges from 6 to 12 m/s with Froude number between 4 and 11.

Currently, the channel is monitored to measure the velocity profile and air concentration in the flow. The aim of our research is to analyse the relation between air concentration and energy dissipation by friction, in order to link the air concentration rate to the Manning roughness number and other methods of friction quantification. This characterization is based on the velocity and concentration profiles along the vertical axis. We collected data of 12 different scenarios to analyse the relationship between aeration rate and roughness coefficient. We found that greater air entrainment implies acceleration of the flow. Since friction is the main energy dissipation mechanism in the chute, hydraulic structures should be designed to minimize air entrainment and thus enhance energy dissipation by friction.