Cavitation in Chutes and Spillways. Front Cover. Henry T. Falvey. U.S. Department of the Interior, Bureau of Reclamation, – Cavitation – pages. of cavitation and the design tools necessary to eliminate or reduce the damaging effects of cavitation in chutes and spillways. The monograph discusses basic. The course is intended to give the participants an understanding of cavitation on chutes and spillways. The course will be introduced with a video of a case study.
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“Cavitation in Chutes and Spillways” by H T. Falvey
The damaging effects of cavitation erosion may be reduced or stopped by decreasing the critical cavitation number e. For these reasons, it becomes usual to protect the spillway surface from cavitation erosion cavitattion introducing air next to the spillway surface Figure 2 using aeration devices located on the spillway bottom and sometimes on the sidewalls Figure 1 CHANSONChap.
Cavitation and air entrainment Cavitation is defined as the explosive growth of vapour bubbles. It involves the entire sequence of events: After formation, cavitation bubbles may be carried away into regions of higher local pressures, before disappearing by collapse.
Cavity collapses generate extremely high pressures in their immediate vicinity. In presence of gas content, flows may cavitate at higher static pressures and substantial quantities of air produce a large reduction in damage rate.
The presence of air bubbles within the flow might also affect the collapse mechanisms, re-directing the water hammer jets away from the solid boundary. Operation of a spillway aeration device Aeration devices 2 are designed to introduce artificially air within the flow upstream of the first location where cavitation damage might occur Figures 3 and 4.
Prevention of Cavitation on Chutes and Spillways
Aerators are designed to deflect high velocity flow away from the chute surface Figure 4. The waters taking off from the deflector behave as a free jet with a large amount of interfacial aeration. The basic shapes of aerators are a ramp, an offset and a groove. The ramp and the offset tend to deflect the spillway flow away from the chute surface. In the cavity formed below the nappe, a local subpressure is produced by which air is sucked into the flow e.
Usually a combination of the three basic shapes provides the best design: The main flow regions above a bottom aeration device are: CHANSON a,b presented detailed measurements of free-surface aeration along a spillway aerator model and in the downstream flow.
Air bubbles are redistributed downstream of an aeration device as in self-aerated flows and there is a complete analogy between the flow downstream of an aerator and self-aerated flows.
Discussion The quantity of air supplied by the air ducts is not always an important design parameter in term of aerator efficiency. In fact the total quantity of air entrained above an aerator is related to the interfacial aeration at both upper and lower nappes, rather the air supply. The practice to design a large number of air inlets is completely empirical and un-economical. In one instance i. Nurek tunnel spillwayaerators had to be shut down to reduce the excessive aeration in the tunnel.
In another case, calculations suggest that two aerators instead of three would have protected the entire spillway length from cavitation damage and at a cheaper cost.
The contribution of the downstream free-surface aeration is an uppermost important parameter, often neglected by design engineers. The optimum location of the first aerator and the required aerator spacing depend essentially upon the free-surface aeration potential. In the downstream flow region the air content tends to the uniform equilibrium air concentration for the channel slope see Self-aeration studies. These results are important and they suggest the following design recommendations: A For steep spillways, the first aeration device must be located near the upstream end of the channel to ‘trigger’ the free-surface aeration process and to use self-aeration in the downstream flow region to maximise air entrainment.
All the spillway length downstream of the first aerator is protected from cavitation damage and no additional aerator is required as long as the channel slope is larger than 20 degrees.
B On small-slope chutes, the first aerator must be located immediately upstream of the potential cavitation inception location in absence of aerators. Practical considerations The designers of aeration devices must: FALVEY suggested that the cavity subpressure should be less than one tenth of the critical pressure ratio for sonic velocity to prevent excessive noise.
Cavitation in chutes and spillways 
Altogether these considerations may be more important when designing an aeration device than the maximisation of the quantity of air supplied by the air ducts. His research interests include design of hydraulic structures, experimental investigations of two-phase flows, applied hydrodynamics, hydraulic engineering, water quality modelling, environmental fluid mechanics, estuarine processes and natural resources. He has been an active consultant for both governmental agencies and private organisations.
His publication record includes over international refereed papers and his work was cited over 4, times WoS to 16, times Google Scholar since Student Edition” IPC Hubert Chanson edited further several books: Useful Water Harvesting Systems or Relics?
Water in a Changing World ” Valentine et al. His Internet home page is http: More pictures of self-aeration are here More about timber crib weirs More about steel dams More about engineering failures More about rubber dams More about a tidal bore More about the Formal Water Garden More about rapid reservoir sedimentation in Australia More about Minimum Energy Loss culverts.
More about Minimum Energy Loss weirs This page was visited 10, times between and June Back to Prof Chanson’s Home Page.