Development of Air Concentration on Chute Spillways

Abstract

To protect hydraulic structures such as spillways, chutes, and bottom outlets against cavitation damage, air is normally added by means of aerators in regions where the cavitation number falls below a critical value. Although aerators have been investigated for more than 30 years, the current design methods for aerator spacing are not reliable. The detrainment process was not previously investigated in detail because of limited laboratory instrumentation. The research presented in this paper provides new model data for hydraulic chutes of variable bottom slope. An advanced remote-controlled, fiber-optical instrumentation was employed to investigate the streamwise development of air concentration contours, velocity contours, and air bubble size along a 14-m model chute. The main hydraulic parameters such as bottom slope, inflow Froude number, inflow depth, and two distinctly different air supply devices for air-water flow generation were employed. Results enable prediction of the reduction of bottom and average air concentration, depending on the inflow air concentration and the chute slope. The minimum air concentration is proven to be a function of the streamwise Froude number. The point of minimum air concentration is constrained by the point of air inception. Downstream of this point the air concentration increases from the surface aeration, depending on the chute slope.