# Air Entrainment and Air Demand in the Spillway Tunnel at the Jinping-I Dam
## 3 Results
### Effect of Air Entrainment on Cavitation Damage Reduction  
In order to evaluate the effect of air entrainment on cavitation damage reduction, the cavitation noise of water flow and damage depth of concrete surface were measured.  
Cavitation noise results from sudden collapse of cavitation bubbles. Previous studies have shown that cavitation level and its damage are significantly related to cavitation noise. Measurement of cavitation noise has been proven to be an effective approach in investigation of the cavitation of hydraulic structures [33; 34]. In the present observation, six measurement points (as shown in Figure 3) were installed in the spillway tunnel for cavitation noise measurement.  
In the process of determining the occurrence of incipient cavitation, the SPL of cavitation noise measured under closed-gate condition is used as the background noise. The SPL amplitude spectrum of  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Series**} & \multirow{2}{*}{**Qw (m\({}^{3}\)/s)**} & \multicolumn{2}{c}{**Oblique Line Segment (C1 to C2)**} & \multicolumn{2}{c}{**Ogee Segment (C7 to C8)**} \\ \cline{3-5}  & & **Distance (m)** & **Attenuation Rate (\%/m)** & **Distance (m)** & **Attenuation Rate (\%/m)** \\ \hline
3 & 1462 & 30.74 & 0.94 & 65.34 & 0.23 \\
4 & 2130 & 30.74 & 0.89 & 65.34 & 0.21 \\
5 & 3200 & 30.74 & 0.81 & 65.34 & 0.19 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Attenuation rate of air concentration in the oblique line segment and ogee segment.  
cavitation noise measured by typical measurement point is shown in Figure 10 and the SPL increments of all measurement points are shown in Table 4. As can be seen, the cavitation noise possesses a wide frequency domain, but amplitudes of cavitation noise are relatively concentrated. The SPL increments of measurement points K2, K3, K4 and K5 are small, all less than 2 dB. The SPL increments of point K1 and K6 are relatively large, the maximum values are 18 dB and 9.31 dB, respectively. The measurement point K1 is near the No. 1 aerator, where the flow just begins to be aerated artificially and air concentration of water is small, so its SPL increment is relatively large. Measurement point K6 is located between the points C10 and C11. According to the air concentration results, when the flow rate reaches 2130 m\({}^{3}\)/s, it should be in the vicinity of the air-water interface where the flow is not stable, and the fluctuation of flow causes the SPL to be a bit large. Other measurement points (K2, K3, K4 and K5) are located at the location where the water has been fully aerated, and SPL increments at these measurement points are small. Taking the SPL increment of 6 to 10 dB as the criterion for judging cavitation [35], perhaps the cavitation phenomenon occurs at positions of the measurement points K1 and K6.  
Although the cavitation phenomenon maybe occurs at some positions, but because of the artificial aeration, the concrete surface is effectively protected from cavitation damage. The fact that low air concentration at the end of protective ranges of No. 1 and No. 3 aerator is also able to effectively prevent cavitation damage, confirms the small bubble theory [8]. As long as the number of air bubble is great enough, low air concentration can also play a protective role.