## 3 Results and Discussion
### Non-Uniform Pressure Field
The locking nut has a suppressive effect on the generation of low pressure, and the extended nut has a more significant inhibitory effect. The pressure gradient is introduced to represent the pressure change per unit distance along the fluid flow direction, and its expression is:  
\[\
abla p=\frac{\partial p_{x}}{\partial x}i+\frac{\partial p_{y}}{\partial y}j +\frac{\partial p_{z}}{\partial z}k \tag{12}\]  
If the low-pressure zone is adjacent to the high-pressure zone, a large pressure gradient is prone to occur, resulting in a significant head loss. In the pressure gradient distribution along the flow direction of the draft tube (Figure 15), the pressure gradient gradually decreases along the flow direction, and there is a large pressure gradient before the MES. For NLN, the high-pressure gradient region of the FSS inlet is located near the wall, which is a circular region, while the low-pressure gradient region is located near the wall and axis. At the MES inlet, the high-pressure gradient zone is closely attached to the wall, with a reduced range. The low-pressure gradient zone is only near the axis, with a slightly larger range, but dispersed into two zones. Halfway through the MES, the high-pressure gradient zone disappears, and the low-pressure gradient zone occupies half of the cross-section range. At the RSS inlet, the low-pressure gradient zone almost completely occupies the cross-section. For SLN, the high-pressure gradient zone at the FSS inlet is located near the wall, forming a circular shape, which reduces the range compared to that of NLN. The low-pressure gradient zone is located around the wall and the locking nut. The circular  
Figure 14: Pressure of draft tube cross-section along the flow direction. low-pressure gradient zone around the wall is slightly thickened compared to that of NLN, and the range of the low-pressure gradient zone around the locking nut is slightly reduced compared to that near the axis of NLN. At the MES inlet, the high-pressure gradient zone disappears, and the axial low-pressure gradient zone slightly increases compared to that of NLN. Halfway through the MES, the high-pressure gradient zone disappears, and the low-pressure gradient zone occupies more than half of the cross-section range. At the RSS inlet, the low-pressure gradient zone almost completely occupies the cross-section. For ELN, the high-pressure gradient region of the FSS inlet occupies most of the area of the section, which is significantly larger than that of NLN. The low-pressure gradient region is located in the circular region of the wall less than half of the circumference, which is significantly smaller than that of NLN. At the MES inlet, the high-pressure gradient zone slightly decreases, located in the circular ring near the wall and nut, which is a significantly larger range compared to that of NLN. To the half of MES, the high-pressure gradient region disappears, and the low-pressure gradient region occupies more than half of the cross-section. At the RSS inlet, the low-pressure gradient region almost completely occupies the cross-section. As can be seen from the above, the existence of a locking nut significantly affects the pressure gradient distribution. The small size locking nut significantly reduces the pressure gradient, and the extended locking nut significantly increases the pressure gradient before MES; however, the decrease after entering the MES increases rapidly.