Spillway with nappe aeration

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 ...

Induced aeration flow over stepped spillways: mean pressures, air entrainment and flow behavior

“…Peterka (1953) demonstrated that 2% of air concentration reduced significantly and 8% eliminated the damage caused by cavitation. Semenkov & Lentyaev (1973) suggested that the stronger the concrete, the less need for air concentration. According to the authors, considering a 40 MPa concrete, 3% of air concentration is needed to eliminate cavitation damage.…”

Experimental Investigation of Artificial Aeration on a Smooth Spillway with a Crest Pier

“…Note that for this investigation, the hydraulic and geometric characteristics of the standing and shock waves were not evaluated in any further detail, as these factors did not form part of the study objective. From the above results, it is observed that the bottom air concentrations downstream of chute aerator in the spillway model with no pier can be considered sufficient to protect the chute surface against cavitation damage, since the measured air concentrations during the present investigation are into the range of the amount of air needed for cavitation protection, proposed by previous researchers [10][11][12][13][14]. This is further reinforced by Kramer [71] statement, who corroborated that a small percentage of air distributed as small bubbles next to the spillway invert is sufficient to protect a concrete surface.…”

“…Rasmussen [11] developed a series of experiments and demonstrated that no cavitation damage occurred by adding a 0.8% to 1% of air by volume distributed as small bubbles. Semenkov and Lentiaev [12] investigated the cavitation damage experienced by concrete surfaces and found that an air concentration of at least 3% is needed to avoid damaging effects. Pylaev [13] corroborated that an air concentration of 3% is required for avoiding cavitation erosion.…”

Development of Bubble Characteristics on Chute Spillway Bottom

“…Up to now, there is no reliable design guideline for the distance required between two aerators to produce sufficient bottom air concentration [5]. Based on prototype observations on Russian dams, Semenkovand Lantyaev [20] provided an average bottom air concentration decay of 0.40% to 0.80% per chute meter.…”