## 4 Results and Discussion
### Effect of the WSM on the Flow Distribution
#### 4.1.1 Flow Distribution When the Rake Is Not Operational  
The velocity distribution due to the WSM for the fully and partially lowered configurations of the sub-screen is considered, whilst the inlet velocity is maintained at 0.2 m/s. Figure 6a and b show the normalized velocity magnitude contours over the longitudinal vertical plane \(y=0.1\) m for the fully and partially lowered configurations of the sub-screen, respectively. The arrows in these plots represent the velocity magnitude: the shorter arrows indicate slower flow, and the longer arrows indicate faster flow. Fluid enters the channel in the positive x-direction and passes through the screening mechanism. An intense flow separation is observed between the upstream and the downstream regions of the WSM in either case. This flow separation is due to the drag in the direction opposite to the flow provided by the WSM. In the papers by [PERSON] and by [PERSON] and [PERSON], an obstruction in front of the flow results in head loss, thereby decreasing the area of the flow [37, 38]. However, in the present study, we note a head increase as indicated by an increase in the z-component of the velocity field arrows in the downstream region in Figure 6a,b. In the present study, the cross-sectional area of the channel in the downstream region is lower than the cross-sectional  
Figure 6: Normalized velocity contour plot around the WSM at \(y=0.1\) m for the inlet velocity of 0.2 m/s when the sub-screen is (**a**) fully lowered and (**b**) partially lowered; the arrows represent the velocity magnitude at the trailing points. area in the upstream region of the channel. This condition accommodates the possibility of head increase. We conclude that that the head increase caused by this reduction in the width offsets the head loss caused by the screening mechanism because mass conservation must be preserved. Comparing Figure 6a,b, we note that the flow separation for the partially lowered configuration is significantly larger than the fully lowered configuration of the sub-screen. The sub-screen creates a high drag because it is in front of the main screen. Consequently, a large amount of fluid is directed below the lifted sub-screen, inducing an intense flow separation between the upper and lower regions of the sewage channel. The flow separation in the partially lowered sub-screen traverses until the end of the channel compared with the fully lowered sub-screen, in which the flow is uniform over the channel cross-section at the end of the channel. The effect of this flow separation on the components at the inlet of the WTP can be clarified in Figure 7a,b, which depict the arrows for the magnitude of the x-component of the velocity for the fully and partially lowered configurations of the sub-screen, respectively. The shorter arrows indicate slower flow, and the longer arrows indicate faster flow. These arrows are plotted over the longitudinal vertical plane \(y=0.1\) m. The velocity along the x-direction is fairly uniform over the channel cross-section at the end of the channel when the sub-screen is fully lowered. However, the x-component of the velocity assumes separation over the channel height towards the end of the channel. This phenomenon suggests an uneven stress distribution on the components at the inlet of the WTP, which should be established to account for this uneven stress distribution. Figure 7: Velocity profile along the x-direction for the inlet velocity of 0.2 m/s when the sub-screen is (**a**) fully lowered and (**b**) partially lowered; the arrows represent the horizontal component of the velocity at the trailing points. The flow treatment near the surface for the fully lowered configuration of the WSM is highlighted in Figure 6a. The water is slowed down in the immediate upstream vicinity of the sub-screen, followed by a rapid increase in velocity for the flow between the bars.