Development of aerated chute flow

To protect hydraulic structures like spillways, chutes and bottom outlets against cavitation damage, air is normally added close to the bottom 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. Until today, the detrainment process has not been investigated in detail because of limited laboratory instrumentation. However, there is a fundamental requirement to understand this process for flow conditions where cavitation damage may occur. If too many aerators are arranged, uneconomically high sidewalls and increasing flow velocity result due to reduced friction. An improved and physically confirmed design guideline for aerator spacing is thus required. This research presents new model investigations for a hydraulic chute of variable bottom slope. For the present model investigations an advanced remote-controlled, fiber-optical instrumentation ofRBI, Grenoble, was employed. It enabled to investigate the air concentration contours, the velocity contours, and the air bubble size along the 14 m model chute. This project continues with previous research on aerators and thus adds to the research carried out more than ten years ago. It accounts not only for the aeration process at the aerator but considers also the detrainment and the free-surface air entrainment further downstream. The present project focuses on the streamwise development of average and bottom air concentrations. The main hydraulic parameter such as the bottom slope, the inflow Froude number, the inflow depth, and two distinctly different air supply devices for air-water flow generation were employed: (1) A deflector similar to a prototype aerator, and (2) Pre-aerated flow by adding air to the supply pipe. Both set-ups were investigated separately to determine the effect of the remaining hydraulic parameters and the influences caused by the aerator. Results enable to predict the reduction of average air concentration, depending only on the inflow air concentration and the chute slope. It is also proven that the minimum air concentration is a function of the streamwise Froude number and the flow depth only. This location collapses with the point of air inception, downstream of this point an increasing air concentration from the free surface was observed depending on the chute slope. It was shown that the air entrainment process downstream of the point of air inception follows a tangent hyperbolic function with the maximum air concentration in the uniform mixture region. Furthermore, this work focuses the streamwise development of bottom air concentration because this is the most significant factor for cavitation protection of chutes. It is also demonstrated that the bottom air concentration behaves similar to the average air concentration. Development of Aerated Chute Flow The air detrainment process was described with respect to the air concentration applying a new technique. It was shown that the air detrainment gradient depends mainly on a streamwise Froude number, that accounts for the inflow conditions and those at the point of air inception. It was further proven that the aeration device influences the air detrainment gradient significantly. The bubble rise velocity was derived from the air detrainment gradient downstream of the aeration device. The bubble rise velocity in high-turbulent flow and stagnant water differ significantly due to fracturing processes and turbulence, and additionally depends on the ambient air concentration.