# Hydraulic Performance of Wave-Type Flow at a Sill-Controlled Stilling Basin
## 3 Results and Discussion
### Air Entrainment  
The jump formation in the stilling basin would result in extreme water turbulence and pressure fluctuations on the bottom or the sidewall (e.g., with a restricted width) [23]. Due to the structural vibration caused by these pressure fluctuations and high velocity near the bottom of the basin, the risk of cavitation increases.  
Figure 9: The relative first and second pressure peak: (**a**) first peak; (**b**) second peak.  
The cavitation damage can be greatly reduced by introducing enough air, and more attention should be focused upon the air concentration of the flow (i.e., the ratio of air volume to the sum of the air and water volumes) on the bottom and the sidewall. Air concentrations on the bottom (\(C_{\text{b}}\)) and the sidewall (\(C_{\text{s}}\)) in the vicinity of the sill at different inflow unit discharges \(q_{\text{w}}\)s for both Case M32 and M31 are highlighted in Figure 10. In this figure, \(X=(x-l_{\text{s}})/l_{\text{s}}\) represents the location of the measuring point relative to the sill position along the silling basin floor.  
As shown in Figure 10a, air concentrations (\(C_{\text{b}}\) and \(C_{\text{s}}\)) decreased with an increasing \(q_{\text{w}}\) for M32. At each \(q_{\text{w}}\) value, a decreasing trend was observed in the upstream flow zone of the sill, and an opposite trend was found downstream of the sill. The data for M31 also exhibited the same trend at most of the flow zone in Figure 10b, but most bottom air concentrations upstream of the sill measured as zero due to the flow pattern of wave-type flows. For instance, with the increase in \(q_{\text{w}}\) for Case M32, the jump type changed from submerged to non-submerged, and the amount of entrained air was gradually transported downstream of the sill. At a small \(q_{\text{w}}\) value, entrained air resulting from the submerged jump at the toe of the weir reached the stilling basin bottom (e.g., \(q_{\text{w}}=0.102\) m\({}^{2}\)/s and \(0.154\) m\({}^{2}\)/s in Figure 5a). The blackwater zone close to the silling basin bottom gradually enlarged with an increase in the unit discharge. In contrast, for Case M31, the flow zone upstream of the sill was always characterized by the blackwater in this study (Figure 5b).  
Figure 10: Air concentrations on the bottom (\(C_{\text{b}}\)) and the sidewall (\(C_{\text{s}}\)) for both Case M32 and M31 at different \(q_{\text{w}}\) values: (**a**) Case M32; (**b**) Case M31.  
Table 2 shows the air concentrations on the bottom and the sidewall along the stilling basin for all cases. Generally, air concentrations on the sidewall (\(C_{\rm s}\)) were higher than that on the bottom (\(C_{\rm b}\)). [PERSON] [24] proved that when the air concentration on the structure's surface was 1.0-2.0%, the cavitation damage could be considerably reduced. It was worth noting that the flow zone in the vicinity of the sill had a relatively small air concentration value (i.e., the value is below 1%) both on the bottom and the sidewall. Within these zones, the pressure profiles obviously fluctuated, especially for the location upstream of the sill, as observed in Figure 7. Thus, the resultant slight air entrainment and large pressure fluctuation should be focused upon in future work.