## 3 Results and discussion
### Discussion
To realize the mutual mixing of water and soil in the model, the multi-material coupling ALE_MULTI_MATERIAL_GROUP keyword was used in the model. After setting the keyword command, a cell was allowed to contain multiple ALE materials. To simulate the real environment of water and soil, the gravity condition was applied to the water-soil coupling model. The BOUNDARY_NON_REFLECTING keyword was used to constrain the surface of the soil and water, and the full constraint of SPC was applied to the bottom of the soil. ALE mesh smoothing was activated by the CONTROL_ALE keyword. The water-soil composite model is shown in Figure 11.  
Based on the working state of the optimized weeding wheel and the agronomic requirements for weeding, the forward speed was 0.64 m/s, the rotating speed of the weeding wheels was 140 r/min, and the weeding depth was 56.8 mm. The coupling keyword command of the fluid structure was CONSTRAINED_LAGRANGE_IN_SOLID. The ALE element (water-soil model) was defined as MASTER, the Lagrange element (weeding wheels model) was defined as SLAVE, and the penalty coupling algorithm was set to simulate fluid-solid coupling. The simulation process is shown in Figure 12.  
LS-Prepost can be used to analyze the density distribution at different times, and the density distribution is shown in Figure 13.  
Figure 11: Water soil composite model. Figure 12: Fluid–solid coupling simulation of the weeding wheels and water soil. Figure 13: Density distribution map: (**a–d**) represent density distribution map at different times. Figure 13 shows that the pressure generated by the contact between the weeding rake teeth and the topsoil destroys the topsoil, and the weeding wheels greatly disturb the mud layer after entering the soil. As the simulation time increases, the weeding wheels continue to roll in the soil model. The path density increases significantly after the disturbance of the weeding wheel, which is due to the effect of the weeding wheel plowing the soil layer under the water layer. The results show that the weeding wheels greatly disturbed the soil and could bury weeds and plow soil. The coupling stress is the average value of the coupling stress generated on the coupling surface during the interaction between the weeding components and the water-soil model. The smaller the coupling stress is, the smaller the resistance of the component and the smaller the operation energy consumption [32]. The stress distribution at different times was measured by LS-Prepost. The stress distribution is shown in Figure 14.  
Figure 14 shows that after the rake teeth of the weeding wheels were embedded in the soil, the back of the rake teeth compressed the soil in the direction of rotation while the weeding wheels rotated, so the stress on the soil on the backside of the weeding wheels with rake teeth was positive. The soil on the front of the rake teeth was subject to negative tensile stress as the rake teeth plowed backward and disturbed the soil. As the rake teeth were distributed in a uniform distribution and circumferential arrangement on the weeding wheels, the coupling stress of the weeding wheels and water-soil composite model was in a pulse state with the rake teeth entering the soil successively. In addition, when the weeding wheels moved in the simulated paddy field environment, the mud mixture did move, and the overall coupling stress was small, which indicated that the resistance of the weeding parts was small and the energy consumption of the weeding wheels was low. Based on the results of the field experiment, the design of the weeding wheels is reasonable. In the field experiment, a large soil disturbance would lead to seedling damage when the weeding wheel was activated because the soil in the paddy field was soft. In the follow-up research stage, modern testing technology was used to design and build a real-time test system for the force of seedling damage.