# Effect of Baffle Board on Aerodynamic and Stealth Performance of Double S-Duct Caret Intake
## 2 Computational Methodology
### Method Verification  
To test for precision, the SST k-\(\upomega\) model was used to simulate the flow field of a mixed-compression intake, as shown in Figure 1[20]. To ensure that all y+ values were less than 1, a structured mesh with the first layer height of 0.0001 mm was used for the calculation. The surfaces of the intake model were set as the adiabatic standard no-slip wall. Using the commercial software Ansys Fluent 2020 R1 [21], the Reynolds-Averaged Navier-Stokes (RANS) equations were solved using the steady density-based formulation. The results of the experiment and calculation of the compression wall are presented in Figure 1b, with the ratio of static pressure to total pressure shown on the \(Y\)-axis. The results for the lip wall are presented in Figure 1c. It can be concluded that the SST model provides sufficient precision in calculating this type of supersonic intake flow field. Therefore, this turbulence model was selected to numerically simulate the flow field of the S-duct caret intake.  
The RCS of an object can be defined by the projected area of an equivalent reflector with uniform radiation in all directions, which has the same echo power as the object in the unit solid angle of the receiving direction. In the vacuum environment, the RCS value can be solved for using Equation (1).  
\[\text{RCS}=\lim_{\text{R}\rightarrow\infty}4\pi\text{R}^{2}\frac{\text{S}_{ \text{s}}}{\text{S}_{\text{i}}} \tag{1}\]  
\(\text{S}_{\text{s}}\) and \(\text{S}_{\text{i}}\) are the power densities of the scattered wave and the incident wave, respectively, and R is the distance between the object and the receiving system. The unit of RCS is m\({}^{2}\), but the unit commonly used in practical application is dBsm. The conversion between the values with the two units can be defined as Equation (2).  
\[\text{RCS}_{\text{dBsm}}=10\text{lg}(\text{RCS}_{\text{m}^{2}}) \tag{2}\]  
Because the radar waves could be reflected several times inside a S-duct intake, which has a complex cavity structure, using the traditional cylinder or a dihedral angle to verify the accuracy of a CEM could be inaccurate for this phenomenon. Figure 2 depicts a complex cylindrical cavity [22] used to test the accuracies of four different methods. The model parameters are shown in Table 1. In addition to the two numerical solutions mentioned above, the physics optics (PO) method and the ray launching-geometrical optics (RL-GO) method are both approximate solutions. With the radar wave defined as a 5 GHz plane wave, the mesh for calculation was an unstructured mesh with the maximum size of 0.75 mm. Using the commercial software Altair Feko 2020.0.0 [23], the RCS values of this cylindrical cavity were calculated with the above four methods. Figure 2,c present the results under the vertical polarization condition and the horizontal polarization condition, respectively. It could be concluded that the MOM and MLFMM  
Figure 1: Verification for CFD method.  
are more suitable for calculating the cavity RCS value compared to the other two methods. Due to the inefficiency of the MOM in solving large objects, the MLFMM was chosen as the most suitable method to investigate the intake stealth performance.