## 3 Results and Discussion
### Non-Uniform Pressure Field  
The swirling field at the inlet of the draft tube forms a vortex and non-uniform pressure field, and the curved structure of MES blocks the flow of water, resulting in significant pressure changes. The pressure coefficient is introduced to represent the distribution of cross-sectional pressure, and its expression is:  
\[\mathrm{C_{P}}=\frac{P-P_{\mathrm{out}}}{\frac{1}{2}\rho U_{1}^{2}} \tag{11}\]  
where \(P\) is the total pressure, \(P_{\mathrm{out}}\) is the draft tube outlet total pressure, \(\rho\) is the fluid density, and \(U_{1}\) is the runner inlet circumferential velocity. In the pressure distribution of the symmetrical cross-section of the draft tube (Figure 13), the low-pressure zone before the RSS of the draft tube always exists near the axis. In RSS, for NLN and SLN, the low-pressure zone moves from one wall to the other wall. For ELN, the low-pressure region remains near the same side wall. The average pressure at the outlet of the draft tube is basically the same, but the ELN pressure gradient is relatively large. In the pressure distribution of the draft tube along the flow direction (Figure 14), there is a low-pressure zone in the middle area of the draft tube, and the range of the low-pressure zone gradually decreases along the flow direction. The periphery of the low-pressure zone is the high-pressure zone, which extends to the wall. For NLN, the low-pressure zone of the FSS inlet is elliptical near the axis, the high-pressure region is close to the wall, and the high-pressure region on one side of the wall is significantly higher than that on the other side. At the MES inlet, both the high-pressure and low-pressure zones decrease and rotate clockwise by about 45\({}^{\circ}\). At the halfway point of the MES, the range of the low-pressure zone further decreases and rotates clockwise by about 45\({}^{\circ}\), while the high-pressure zone disappears. At the RSS inlet, the range of the low-pressure zone continues to shrink and continues to rotate clockwise by a certain angle. For SLN, the low-pressure zone of the FSS inlet surrounds the locking nut, the pressure in the low-pressure zone slightly increases compared to that of NLN, and the high-pressure area is close to one side of the wall. The zone of high and low-pressure zones is reduced by more than half compared to that of NLN. Toward the MES inlet, the low-pressure zone fills the nut position and the range expands, while the high-pressure area basically disappears. In the halfway points of the MES and the RSS inlets, the range of the low-pressure zones is further reduced, and it continues to rotate clockwise at a certain angle. For ELN, the low-pressure zone in the inlet of the FSS is dispersed into two locations, one near the nut and the other near the wall, both of which are elliptical, and the zone is more than half that of SLN. One side of the high-pressure zone is close to the wall and  
Figure 13: Pressure variations in the draft pipe: (**a**) average pressure variation along flow direction; (**b**) pressure distribution at symmetry cross-section. the other side is close to the locking nut, with an area about twice that of SLN, almost consistent with NLN. At the MES inlet, the low-pressure zone is concentrated near the locking nut, and the range is expanded. The low-pressure zone near the wall disappears, and the range of the high-pressure zone slightly decreases, rotating clockwise by about 45\({}^{\circ}\). At the halfway point of the MES and RSS inlets, the low-pressure range slightly increases, while the high-pressure range continues to shrink and continues to rotate clockwise at a certain angle. It can be seen from the above that the existence of the locking nut will slightly increase the pressure in the surrounding area; however, it still belongs to the low-pressure range. The extended nut will greatly increase the pressure in the surrounding area, making it exceed the low-pressure range and belong to the high-pressure range.