At the present time there are experimentally justified and reliable methods wMch permit indicating, with assurance, under which hydraulic regimens cavitation may develop in any element of a spillway structure, or on any rough surface of such a structure. However, the available data indicate also that, frequently, for structures operating under heads of 40-50 m or greater, the complete elimination of cavitation is practically impossible or requires the application of complex engineering measures. For this reason it would be desirable to allow a certain degree of cavitation, provided the resulting destruction is not excessive and is acceptable from the standpoint of reliable operation of the structure over a sufficiently long period. The possibility of such a solution is confirmed by the operating experience with many structures which have operated reliably despite cavitation phenomena. However, the cases in which inadmissible and catastrophic destruction of isolated elements of hydraulic structures has taken place under cavitation [1] were in the past always used as specific warnings against such an approach.The present state of research on the problem of the mechanism of cavitation erosion of materials and on the processes which determine its development is not sufficient for making quantitative predictions of the extent of cavitation destruction, especially for the cavitation stages corresponding to a very intense erosion.The tests performed indicate that it is possible to establish the form and stages of the cavitation phenomenon for which the erosive action is either very small or practically does not take place over a sufficiently long period of operation of the structure. For hydraulic structures the most characteristic feature is the development of the so-called "separation" forms of cavitation, which take place when the water flows around different projections,
Many works describing cavitation erosion in high-head spillway works, including in gate chambers, are known. However, in the majority of cases the analysis and interpretation of the observation results are approximate. The main reason for this is the complexity of the hydrodynamic picture of flow in the gate chamber and mutual effect of various sources of occurrence of cavitation. Methods of laying out and designing gate chambers of high-head structures have been improved in the past decade. Sources of occurrence and effect of cavitation on surfaces, including on metal-lined surfaces, are divided into "external" and "intrinsic." It is assumed that sources of occurrence and effect of cavitation that are external" with respect to the lining are not allowed at all and their absence is provided by design and regime measures. Only "intrinsic" sources related to the design of the lining itself (irregularities, joints, roughness, etc.) are allowed. Moreover, an analysis of the conditions of occurrence and development of cavitation in hydraulic structures carried out under laboratory conditions and on the prototype led the majority of investigators and engineers to the opinion that in high-head (high-speed) spillways, owing to the absence of full assurance against the occurrence of cavitation on "intrinsic" sources of linings, some allowance of it within safe limits is not only acceptable but is also inevitable.As a consequence of such an opinion it was suggested to introduce cavitation parameters K =; determining its admissibility from the viewpoint of erosion of concrete. For particu~r=~ases of dam spillway surfaces the values of Ker.e d were determined in the prototype and were 30% less than the critical parameters of the occurrence of cavitation Kcr. Whereas for concretes, as a general rule, the start of visible erosion is that "reasonable limit" beyond which the condition of the structure should not deteriorate (owing to the comparatively weak erosion resistance of concrete), for steel linings the need for a similar strict limitation is not obvious.The basic principles of the layout of modern chambers of hlgh-head gates with head up to 200 m, as is know, are the diversion of the jet issuing from under the gate from the loadbearing downstream chamber walls and mandatory aeration of the separation zones and wall (bottom) regions of the flow. Under these conditions it turns out that a considerable part of the surfaces of the gate chambers is either n6t in contact with the high-head jet at all or is exposed to an aerated flow. On all these surfaces there is no object for analyzing the problem of the cavitation safety of linings.An analysis of the problem of cavitation safety is necessary only for limited areas of the linings at places where the through-going high-speed flow encounters the wall (bottom) of the outlet part of the spillway~ As a rule, these are zones located at a distance of 2-6 heights of the sill or half-groove from the opening closed by the main gate and with a maximum length of 2-3 thicknesses of the t...
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