Guide to protecting spillway structures from cavitation

Abstract

The "Recommendations" are a systematic and brief generalization of the abundant research data on cavitation accumulated in recent years and are a valuable guide for design organizations. The "Recommendations" cover practically all aspects of the problem: prediction of the start of cavitation, possibilities of preventing it, and measures to prevent the dangerous erosive action of developed stages of cavitation. The "Recommendations" are concerned with practically all elements of spillway structures now in use: pipe inlets, heads of spillways, hydraulic energy dissipators (deflectors and splitters), curvilinear elements of spillways, grooves, half-grooves, gate chambers, heads of dividing piers, gate slots. Problems of =he cavitation resistance of concrete and its protective coatings are examined as a supplement to the previously published standards P II-73/VNIIG and P 58-72. Examples of a calculation for predicting the start of cavitation and a method of dynamic calculations of linings are presented.On the whole the work deserves a high evaluation. Its creation required the examination and coordination of the results of investigations made by various organizations on different experimental devices and by different methods. The "Recommendations" fix the attained level of knowledge in the field and formulate problems which still require additional investigations and refinements. As any standard document, the "Recommendations'~ will be improved in the future as new data are accumulated and as new practical problems arise. It is desirable that the following considerations which come to mind while reading the document be taken into account. i. The cavitation number is determined by the classical formula Po--P~ K= p(vt/2) ,where Po and vo are the pressure and velocity at the same point 0 in the undisturbed flow in front of the investigated object exposed to the flow; Pd is the vapor pressure.The definition of the cavitation number given on p. 8 of the "Recommendations" does not quite correspond to the classical one. It is not specified that pressure Po and velocity vo (Pchar and vchar according =o the "Recommendations") are determined at the same point in the flow. It is stated that these parameters are determined "without consideration of disturbances caused ...by the body or element," i.e., in an undisturbed flow, but later this requirement is actually violated in many specific cases. As a result the cavitation number loses its sense of a parameter of the boundary conditions and becomes some random variable which depends on the object and method of investigation. This is explained by the complexity of flows in conduits, where =here are no sections with an undisturbed flow in general, or if they are present, it is technically inconvenient to use them. Apparently in the next edition of the "Recommendations" it is necessary for greater clarity and continuity to first give the classical definition of cavitation number and then to show in each specific case of its application why it cannot be used directly but must be modi...