Start Date

2018 10:30 AM

Creative Commons License

Creative Commons License
This work is licensed under a Creative Commons Attribution 4.0 License.

Abstract

Recent advances in technology have permitted the construction of large dams and spillways. One type, the stepped spillway, is designed to spill floods over the chute with substantial regular energy losses. This article presents an experimental investigation of free-surface instabilities within the non-aerated and aerated region of a stepped spillway. Intense splashing and ejection of water droplets are characteristics of the transition flow regime and typically follow the primary breakup of the liquid phase. These formation processes might be of particular interest concerning the dimensioning of stepped chute sidewalls. The current study determines velocities of evolving liquid ligaments by means of high-speed video analysis. The video sequences are recorded within the upper transition flow sub-regime. To minimise the influence of sidewall effects, the camera is focussed on a vertical plane inside the channel. Simultaneously, unsteady water surface elevations are measured with a series of acoustic displacement meters mounted alongside the flume, perpendicular to the pseudo bottom formed by the step edges. A correlation analysis is performed on the output of the acoustic sensors in order to determine characteristic time scales of surface fluctuations as well as celerities of liquid structures next to the free-surface. The turbulent fluctuations of the free-surface increase rapidly with further distance from the inception point of self-aeration. This is likely to be associated with enhanced air entrainment and equally increased amount of air-water ejections above the aerated flow region, showing the occurrence of strong hydrodynamic fluctuations within the transition flow regime. The present investigation emphasises the feasibility of using high-speed video analysis to provide relevant flow information next to the sidewall of spillway models.

Share

COinS
 
May 16th, 10:30 AM

Free-Surface Instabilities in High-Velocity Air-Water Flows down Stepped Chutes

Recent advances in technology have permitted the construction of large dams and spillways. One type, the stepped spillway, is designed to spill floods over the chute with substantial regular energy losses. This article presents an experimental investigation of free-surface instabilities within the non-aerated and aerated region of a stepped spillway. Intense splashing and ejection of water droplets are characteristics of the transition flow regime and typically follow the primary breakup of the liquid phase. These formation processes might be of particular interest concerning the dimensioning of stepped chute sidewalls. The current study determines velocities of evolving liquid ligaments by means of high-speed video analysis. The video sequences are recorded within the upper transition flow sub-regime. To minimise the influence of sidewall effects, the camera is focussed on a vertical plane inside the channel. Simultaneously, unsteady water surface elevations are measured with a series of acoustic displacement meters mounted alongside the flume, perpendicular to the pseudo bottom formed by the step edges. A correlation analysis is performed on the output of the acoustic sensors in order to determine characteristic time scales of surface fluctuations as well as celerities of liquid structures next to the free-surface. The turbulent fluctuations of the free-surface increase rapidly with further distance from the inception point of self-aeration. This is likely to be associated with enhanced air entrainment and equally increased amount of air-water ejections above the aerated flow region, showing the occurrence of strong hydrodynamic fluctuations within the transition flow regime. The present investigation emphasises the feasibility of using high-speed video analysis to provide relevant flow information next to the sidewall of spillway models.