## 4 Results and Discussion
### Effect of the Shape of SSC  
Another major difference in No.9 SCS of Xiluodu HPP is the shape of SSC. In this subsection, the effect of the shape of SSC under temporary IWP is investigated in multiple aspects. A hypothetical case is considered in which the cross sections between section 2 and section 5 are replaced by circular sections. The temporary IWP of both cases is chosen as 1.60 MPa, according to Chinese design code [26]. Hereafter, the terms non-circle case and circle case will be used to refer to the two cases for brevity. Figure 17 shows the displacement of the SSC in the longitudinal axis of the powerhouse. The peak value in the non-circle case is 22.46 mm, while it is 6.98 mm in the circle case, which is only 30% of that in the non-circle case. Besides, the peak value appears at different locations in these two cases. It appears at the straight tube area in the non-circle case, while it appears around section 5 in the circle case. Actually, the displacement value around section 5 in the non-circle case is close to that of the circle case. That is to say, the displacement values of most areas are similar in these two cases, and non-circle sections may attribute to the larger displacement of the SSC in the longitudinal axis of the powerhouse. Figure 16: Mises stress of the SSC (unwatered case) (MPa): (**a**) top view; (**b**) bottom view. A marked observation to emerge from Figure 17 is the large displacement at the inlet of the SSC in the non-circle case. In the operating period, the inlet of the SSC will be connected to an expansion joint or a penstock, which requires the inlet of the SSC to maintain its designed position. With such large displacement in non-circle case, it is actually impossible to connect the conduits smoothly. Therefore, an enhanced displacement control at the inlet of the SSC is highly suggested. Figure 18 shows the displacement of the SSC in flow direction. The peak value in the non-circle case is 6.62 mm, and it spans around section 7. In the circle case, the maximum displacement is 1.73 mm, which is only 26% of that in the non-circle case. However, the distributions of the displacement in the two cases are similar. Figures 17 and 18 show the overall distribution of structural displacement. However, since the difference between these two cases only exists between section 2 and section 5, a more detailed comparison is necessary. Figure 17: Displacement of the SSC in the longitudinal axis of the powerhouse (m): (**a**) non-circle case; (**b**) circle case. Figure 18: Displacement of the SSC in flow direction (m): (**a**) non-circle case; (**b**) circle case. Figure 19 shows the radial deformation of section 4. The maximum radial deformation in the non-circle case is 15.62 mm, and it appears at the left waist of the section. As for the circle case, the peak value is 6.12 mm, which is 39% of that in the non-circle case. What is more, a marked observation to emerge from the data comparison in Figure 19 is the difference in the distribution of radial deformation. In the circle case, the radial deformation in the right half of the section is small, especially in the lower half. Overall, the section tends to maintain its original shape with slight lateral displacement under the effect of the temporary IWP. In contrast, relatively severe structural distortion happens in the non-circle case. Negative radial deformation is observed at both the upper and lower half of the section, and the peak value reaches 14.05 mm at the upper half. In general, the shape of the section in the non-circle case tends to get rounded under the effect of the temporary IWP. In order to evaluate the stress condition of these two cases, Figure 20 shows the Mises stress of section 4. The value and distribution of the Mises stress are similar, with the stress value of the non-circle cases slightly larger. This happens because the loads of these two cases are actually the same, and the differences should be attributed to the shape of the sections.