## 4 Results and Discussion
### Effect of Flow Distribution on the WSM
#### 4.2.3 Von Mises Stress on the Sub-Screen  
The wastewater treatment system experiences significant stresses when the sewage fluid flows through the screens. Hence, it is imperative to retain the strength of the WSM in order to guarantee its ability to endure the fluid load conditions to which it is subjected. The present study employs the von Mises yield criterion to assess the potential yielding of the sub-screen and rake models under varying fluid forces. If the von Mises stress value of the sub-screen and the rake approaches the yield stress of the structural steel material (250 MPa) utilized for the components, it may be concluded that the WSM would experience failure when subjected to the specified load conditions [21]. The main contributing factors to the von Mises stress are abrupt changes or irregularities in the geometric model (fillets, holes, grooves, keyways, splines, tool marks, or manufacturing defects) and are known as stress concentrators; the model is likely to be fractured near these stress concentrators because of high stress [39; 40; 41].  
The von Mises stresses on the FSI boundaries of the sub-screen were computed with the effects of the different inlet flow velocities and positions of the sub-screen. The orientation for the selected bars in Figure 17 is from the bottom to the top. The legend in the graph shows the selected bars of the sub-screen. B1, B6, B12, and B18 represent the 1 st, 6 th, 12 th, and 18 th bar of the sub-screen, respectively. h represents the displacement on the FSI boundary of the sub-screen with an initial point at the bottom. Furthermore, hss represents the total length of the FSI boundary on the sub-screen. A high von Mises stress was observed at the base of the sub-screen because of the corner acting as a stress concentrator. Similarly, high von Mises stress values also exist due to the basic structural support installed in the sub-screen. These phenomena are visible in the first and second peaks in Figure 17a,b for the fully lowered sub-screen and in Figure 17c,d for the partially lowered sub-screen. This contrasts with the 2D study conducted by [PERSON] et al. [7], in which the von Mises stress on the sub-screen consistently decreased along a sub-screen bar from bottom to top. This observation stemmed from the fact that their sub-screen was mounted at the floor of the water stream and a horizontal supporting rod for the sub-screen was not considered in their geometry.  
The presence of the two peaks in the present study can be attributed to specific features that are included in the present model. The first peak corresponds to the presence of corners aligning with the horizontal support bar at the bottom, while the second peak corresponds to the opening for a supporting rod which passes the through the sub-screen. The highest peaks are for 0.3 m/s for the fully lowered sub-screen at 5.5  
Figure 17: von Mises stress profile on the sub-screen bars when the sub-screen is fully lowered at (a) U = 0.1 m/s and (b) U = 0.3 m/s and for partially lowered at (c) U = 0.1 m/s and (d) U = 0.3 m/s.  
and second peaks, respectively, which are lower than the material yield stress (250 MPa). Therefore, the sub-screen and the rake models are safe under applied fluid loading in the screening process.