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Air-Water Surface Roughness in Self-Aerated Stepped Spillway Flows

Author(s): Daniel B. Bung

Linked Author(s): Daniel B. Bung

Keywords: Stepped spillway; Air-water flow; Physical modeling; High-speed camera; Ultrasonic sensors

Abstract: Stepped spillways are known to be effective structures in regard to energy dissipation. Due to the step-induced macro-roughness, the flow resistance increases and self-aeration sets in earlier when compared to smooth-invert chutes. The totally conveyed air volume is composed by entrained air (i. e. single air bubbles which are transported within the water body) and entrapped air (i. e. air pockets between the surface waves of the air-water mixture). The surface waves are referred to as air-water surface roughness in the literature. In the present research, a comprehensive study on the surface wave characteristics is conducted by means of physical model tests. For this purpose, stepped spillway models were set up and air-water flow properties were measured with an intrusive double-tip conductivity probe. High-speed camera movies were captured in uniform flow region and the air-water surface levels were extracted from the single frames. A comparison of resulting water surface time series with characteristic water depths obtained by the conductivity probe is presented. The water depth h90 with an air concentration of 90 % , for instance, is often interpreted as idealized air-water surface as only a negligible part of the totally conveyed water discharge is transported above this level. However, the paper will show that the surface wave heights may even exceed the level 2 × h90. Particularly for low discharges, an increased turbulence level is found being manifested in a more chaotic mixture surface elevation. This flow feature is of particular importance for the design of this type of structure. Other wave characteristics, such as the maximum wave trough extension and the surface wave frequencies will also be presented. Results from the surface level extraction will be compared to ultrasonic sensor data. It will be shown that these probes may give some reasonable information. However, the sample rate of these probes is too low to identify some more significant flow features.

DOI:

Year: 2013

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