No abstract

An increase in the height of spiUways of high dams leads to an intense increase in the cavitation action of the flow. Experimental investigations by many authors have shown that cavitation erosion occurs at a stream velocity greater than 12-15 m/sec and its intensity increases proportionally as the 5-7 power of the velocity. With an increase in spillway height, for example, from 50 to 100 m, the rate of cavitation erosion increases by a factor of 6-8. and with an increase to 150 m by a factor of more than 40. If the concrete surface has irregularities and areas with an insufficiently gradual change of profile and also poorly streamlined structural elements intended for guiding the flow or for other purposes (stream deflectors, energy dissipation baffles, openings of discharge conduits, ventilation, drainage, and other pipes), this can lead to the occurrence of cavitation and to subsequent destruction of the spillway. Cavitation erosion in some cases can be so intense as to disrupt the normal operation of the structure. Our observations and the data of publications in the technical literature showed that within a reLatively short time cavitation cavities can reach a depth of 1.5-2 m and the volume of removed high-strength concrete, tens of cubic meters. Repair of cavitation damage requires considerable Labor and time.The greatest number of cavitation cavities occur below surface roughnesses. Special measures for controlling surface defects are usually taken when conslzucting spillways, which complicates and increases the cost of construction but does not rule out the appearance of some defects Later upon aging of the concrete under atmospheric and climatic effects. Expensive high-strength grades of concrete are usually used in regions with a rigorous climate and large temperature difference in order to increase the strength of the spillway surface.To control cavitation erosion below surface roughnesses and around spillway structural elements, it has proved to be effective and economic to supply air directly to the zoiae of expected cavitation or, if the number of possible cavitation zones is large, to saturate with air (aerate) the boundary layer of the flow depthwtse. The admixture of free air changes the physical properties of water: the water becomes a less elastic, compressible medium, absorbing well the cavitation impacts. Smoothing of the spillway or its anticavitation strengthening is not required in the case of discharging an aerated flow. With sufficient saturation of the flow with free air, individual irregularities of the spillway surface or an increase in its total roughness does not cause cavitation erosion. [2] showed that, on aerating water in the amount of 1.5-2.5~o, cavitation erosion of concrete specimens decreased considerably and stopped in the case of 7-8% air (Fig. i). Experiments conducted at the research department of the State PLanning, Surveying, and Scientific Research Institute (Gidroproekt) in cavitation tunnels with a cylindrical cavitation exciter on specimens of concrete and cement ...

Improvement of the design of a high-head spillway is impossible without consideration of the occurrence of cavitation, cavitation erosion of the spillway surface, and aeration of the flow. Knowledge gained about the regularities of air entrainment on spillways and experience in operating aerating devices enable the designer to control aeration of a high-velocity flow within certain limits. To prevent the development of cavitation erosion of the material of the spillway surface, on the entire length of the cavitation-hazardous section of the spillway passage it is necessary to form an aerated flow with a volume air concentration in the boundary layer with thickness 0.2-0.25 m of not less [1] than 7-8%. Here it is desirable, as far as possible, to avoid an extreme increase of the depth of the flow being discharged, which "swells" from the air being transported, in order to prevent overflowing the sides of the spillway or a decrease of the discharge capacity of a spillway tunnel as a result of its transition into a pressure operating regime.As is known [2], in a liquid flow a local decrease of pressure leading to cavitation and cavitation erosion of the material of the spillway surface occurs most often as a result of curving of streamlines during flow of a high-velocity stream past local roughnesses of the concrete spillway surface. As a rule, typical roughnesses of a technological nature become the source of cavitation erosion. For instance, in the case of concreting with the use of wood and metal formwork secured in place by reinforcing bars, typical roughnesses of the concrete surface in the form of a projection or ledge across the flow with a height up to 30-60 mm were recorded [3] at the Places of the joints of the forms on various spillways. Experience in flattening individual large roughnesses and imparting erosion-free outlines to them on the bucket of the spillway dam of the Krasnoyarsk hydroelectric station showed [3] that even with the aid of means of mechanizing the works it was not possible to obtain slopes of the faces more gentle than 1:12 for roughnesses with a height of 20-40 mm and more gentle than 1:20 for roughnesses with a height of 5-20 mm. Such slopes still provided an erosion-free operating regime of the spillway for heads from the upper pool level (UPL) at the site less than 50 and 70 m, respectively (Fig. 1). Intense cavitation erosion of the spillway surface was observed at a distance of more than 60 m from the spillway inlet with a slope of the spillway face of 0.79.Air entrainment into a turbulent water flow occurs as a result of breaking of waves and disturbances at the water-air interface due to mutual diffusion penetration of the two media. Therefore natural self-aeration of the flow is possible only below the point of emergence of the turbulent boundary layer onto the free surface (Fig. 2). The length of the initial section [4, 5] of development of the boundary layer xs=h ~ _~3z (1+ 1,3V,2o)1o 2]~+l.svz0 (1) is greater, the greater the depth of the flow, and, consequently, the spe...

When water conduits operate with velocities exceeding 20-25 m/sec, high-vacuum regions are created, which result in the development of cavitation, followed by heavy erosion of the concrete, and in many cases by damages to the hydraulic equipment and the steel facings of concrete surfaces.This article presents some results of work carried out at the S. u Zhuk Scientific-Research Department of the All-Union Design and Scientific-Research Institute for Hydraulic Structures (NIS Gidroproekta), which permit preparing recommendations for the prevention of hazardous cavitation erosion of elements of hydraulic structures.Cavitation on Surface Irregularities of Water Conduits. In order to prevent cavitation erosion, the allowable dimensions of irregularities on the surfaces of water conduits should be specified for each structure.The degree of cavitation under a flow is determined by the cavitation index [1?. ]')o~ --/OjWoo c% in which L,e --2g P~,is thecharacteristicpressureinkg/mZ, v~oisthevelocityinm/sec, Pv is the saturated vapor pressure of the fluid in kg/m 2, and y is the unit weight of water, in kg/m 3.

The 1960's saw the beginning of construction of several high-head hydroelectric schemes in the Central Asian republics of the Soviet Union, viz., Charvak, Nurek, and ToktognL For river diversion during the construction of these hydroelectric schemes, large cross section tunnels were used, which operated unt.il 1972 to 1974. The experience gained during the extended service of these tunnels under different conditions, extending over six to severr years, permitted their operational features to be defined and recommendations to be formulated for more accurately defining design decisions for similar high capacity diversion tunnels.Descriptions of the diversion tunnels at the above-mentioned hydropower schemes are given elsewhere [1,2,3]. Presented in Tables 1 and 2 are, respectively, their principal engineering features and operational factors.Stage I and II tunnels of the Nurek scheme and the Toktogul tunnel were designed to pass only the construction floods. The Charvak diversion tunnel, after remodeling, was placed in permanent service. The service spillway is connected to it 480 m from its upstream portal.All the tunnels were designed to operate under free-and pressure-flow conditions. The Nurek I and Toktogul tunnels are unregulated; they were designed to operate only in the first stage of construction-flood diversion, before reservoir filling. The leaf gates installed in the Toktogul and Nurek I tunnels were designed for single closure for their isolation. The Charvak and Nurek II tunnels were used to pass the construction floods during both stages. During the second stage, i.e., reservoir filling for the commissioning and operation of the Stage I turbogenerator units, the flow through the Charvak and Nurek II turmels was regulated by radial (sector) gates. These gates were desigr~ed for a maximum head of 80 and 110 m, respectively.The Toktognl and Nurek tunnels have a flat-bedded cross section, the Charvak has a circular profile. The reinforced-concrete linings of all the tunnels were designed to prevent cracking.In connection with the high velocities in the tunnels (Table 2), certain design standard criteria were developed for the lining surfaces. However, even during the first stage of construction-flood diversion, a nonuniform erosion of the linings occurred, which resulted in local irregularities with dimensions significantly exceeding the design limits. This indicates the necessity for a secondary treatment of the concrete lining surfaces, in order to prevent cavitation damage during subsequent stages of tunnel operation. It should be noted that it was the first