## 2 Modeling
### Grid Generation and Boundary Conditions  
The 3D simplified model of the slurry blast device is shown in Figure 2a. It is established in the preprocessing software and simplified to remove details such as the gap that does not affect the simulation analysis. The fluid domain consists of three parts: inlet domain, rotation domain, and outlet domain. The CFD simulation starts from the inlet domain, goes through the boundary of the interface into the rotation domain, and ends after the outlet domain. Considering that the hexahedral grid can better meet the fluid flow direction and the grid accuracy and calculation speed are high, the hexahedral grid is used as the grid element. In addition, the local unstructured mesh refinement based on a high-speed gradient is explored to solve the very large flow gradient in the turning region. After encrypting and refining the local area, especially the interface area, the mesh model is shown in Figure 2(b). Here taking the simulation variation of erosion rate and particle velocity as a reference, grid independence is tested to select the appropriate grid accuracy (as shown in Table 1). It can be seen from Table 1 the comparison of the calculation results of five different numbers of cells. The results show that with the increase in the number of cells, erosion rate and particle velocity increase first and then stabilize. The variation rate from Case 1 to Case 3 is relatively large, whereas that between Case 3 and Case 5 is less than 1%, which is almost unchanged. Therefore, considering the accuracy of the simulation results and the calculation speed, the grid generation method in Case 3 is adopted in the subsequent simulation. Here a multiphase model in the CFD simulation consisting of a two-phase flow (water and particles) is established; water is the first phase, and particles are the second phase. The inlet of the model is set as the pressure inlet. Sliding grids are used in the rotation domain. The liquid phase satisfies the nonslip condition of the wall, and the particle phase satisfies the wall slip condition. The outlet is set as atmospheric pressure. The simulation parameters are shown in Table 2. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**No.** & **Number of Cells** & **Erosion Rate (kg/m\({}^{2}\))** & **Particle Velocity(m/s)** \\ \hline Case 1 & 874,050 & 2.33 \(\times\) 10\({}^{-8}\) & 48.51 \\ Case 2 & 1,206,482 & 2.55 \(\times\) 10\({}^{-8}\) & 53.25 \\ Case 3 & 1,543,365 & 2.61 \(\times\) 10\({}^{-8}\) & 55.12 \\ Case 4 & 1,769,118 & 2.62 \(\times\) 10\({}^{-8}\) & 55.40 \\ Case 5 & 2,685,220 & 2.626 \(\times\) 10\({}^{-8}\) & 55.52 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Grid independence test of different number of cells. Figure 3: Models of the slurry blast device. Figure 4a reveals that erosion locations are concentrated on the downwind area of the rotating blades on the steel plate. The erosion rate can reach 5.405 \(\times\) 10\({}^{-8}\) kg/m\({}^{2}\), and the erosion range is relatively uniform, whereas the upwind area of the rotating blades on the steel plate has a relatively minimal erosion effect. The effective area of the rotating blade centrifugal jet is closely related to the relative position of the steel plate. Figure 4b shows the erosion rates and distribution areas of all particles. The erosion locations are concentrated on the downwind area of the rotating blades on the steel plate, and the erosion rate values are mostly concentrated in the range of 0-3.0 \(\times\) 10\({}^{-8}\) kg/m\({}^{2}\). The average erosion rate of 15,340 dispersed particles is 2.345 \(\times\) 10\({}^{-8}\) kg/m\({}^{2}\). According to Equation (7), the mass of the rust layer removed is 6.21 g/s. Studying particle velocity distribution is necessary because particle velocity is a key parameter affecting erosion rate [26].