Ski-jump energy dissipation : design of a ski-jump to maximise energy dissipation and aeration.

[摘要] ENGLISH ABSTRACT: One of the most effective and economical methods for the dissipation of hydraulic energy from flood waters is to project the flows into a free trajectory jet form to a location where the impact creates a plunge pool in the downstream river bed. This type of energy dissipation can be created by a ski-jump energy dissipator which has become an increasingly popular form of hydraulic energy dissipation for large dams in recent years due to its ability to safely convey high velocity flow in excess of 30m/s to the downstream river, however very limited definitive and comprehensive guidelines have been created. There is not only insufficient documentation for the conceptual and detailed design of ski-jump energy dissipators, there is also insufficient documentation for the dimensioning of the downstream plunge pools. Both are necessary to guarantee that the passage of major floods do not threaten the structural integrity of the permanent works.The origins of ski-jumps can be dated back as far as the mid-1930s where they were successfully introduced on the Dordogne hydraulic scheme in France. This revolutionary scheme designed a circular arc spillway over the power plant with the intention of conveying high velocity flow in the form of a trajectory jet over power plant and plunge down onto the riverbed at a substantially far distance away from any dam apparatuses as to mitigate potential structural damage. Due to the success of this design, it became very popular in France, Spain and Portugal. During the period 1930-1940 the first spillways of its kind were constructed under its new name, 'Saut de ski.This research includes hydraulic testing of different ski-jump buckets for a general design. The objective of this is to obtain a design that would maximise energy dissipation and enhance air entrainment as well as establishing a comprehensive guideline to the design of ski-jumps. Energy dissipation by a ski-jump may be assessed by evaluating a number of identified contributing parameters by means of a physical hydraulic model. The parameters of importance include; 1) the geometric profile of the water jet trajectory such as the trajectory distance, trajectory height, horizontal and transverse impact width; 2) dynamic impact pressure distribution; 3) maximum dynamic impact pressure head; 4) impact velocity head; and 5) air entrainment. These results demonstrate the significant effect of the Froude number, bucket angle and bucket shape. The results for the different ski-jump buckets of all mentioned parameters are presented, descripted and discussed, and concluding with the design that best dissipates energy for the ski-jump buckets tested. This design was able to improve the pressure distribution area significantly as well as decrease the maximum dynamic pressure head by up to 20m when compared to a standard 40o circular shaped flip bucket design. An increase in the aeration by up to 20% at the centreline was achieved when compared to a standard design.

[发布机构] Stellenbosch University