# Effect of Baffle Board on Aerodynamic and Stealth Performance of Double S-Duct Caret Intake
## 3 Results and Analysis  
### Characteristic Analysis of S-Duct Caret Intake  
Figure 7 shows the RCS values of the two intakes at different radar incidence angles, and the stealth performance parameters are presented in Table 3. In addition to the three common statistical values of RCS, a new term named low scattering angle (LSA) is proposed, which corresponds to a RCS value of the intake that is less than 0 dBsm. Therefore, the higher the value of the LSA, the more conditions the intake can be safely applied to. According to Figure 7a, under the horizontal polarization condition, the RCS values of each intake were all below 0 dBsm with radar incidence angles over 20\({}^{\circ}\), indicating that both intakes had excellent lateral stealth capability. However, the RCS values of Model A were almost all significantly greater than 0 dBsm with incidence angles less than 10\({}^{\circ}\), resulting in the maximum value and the average value being as high as 12.71 dBsm and 2.49 dBsm, respectively. Therefore, the traditional S-duct intake had an obvious fatal defect in stealth performance. In contrast, the RCS values of Model B were generally smaller than those of Model A at most angles, with the average value reduced to \(-\)0.87 dBsm and the LSA number increasing to 36. A similar conclusion could also be proposed for the other polarization condition, except that the RCS values of Model B were higher than those of Model A, with angles between 13\({}^{\circ}\) and 23\({}^{\circ}\). Therefore, it can be concluded that the double \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multicolumn{6}{c}{**Aerodynamic Performance**} & \multicolumn{6}{c}{**Electromagnetic Performance**} \\ \hline \multirow{2}{*}{**Mesh Type**} & \multicolumn{2}{c}{**Mesh Number (million)**} & \multicolumn{2}{c}{**Flow**} & \multicolumn{2}{c}{**Average Total**} & \multirow{2}{*}{**Mesh Type**} & \multirow{2}{*}{**Mesh Size**} & \multicolumn{2}{c}{**Mesh Number**} & \multicolumn{1}{c}{**RCS**} \\  & **Inner** & **Whole** & **(kg/s)** & & & & & **(million)** & **(dBsm)** \\ \hline mesh A-1 & 0.54 & 0.83 & 80.133 & 76,811.5 & mesh E-1 & \(\lambda/4\) & 0.26 & 3.95 \\ mesh A-2 & 1.02 & 1.33 & 80.138 & 77,088.2 & mesh E-2 & \(\lambda/6\) & 0.71 & 3.37 \\ mesh A-3 & 1.98 & 2.41 & 80.145 & 77,203.9 & mesh E-3 & \(\lambda/8\) & 1.02 & 3.13 \\ mesh A-4 & 3.95 & 4.69 & 80.146 & 77,228.8 & mesh E-4 & \(\lambda/10\) & 1.59 & 3.11 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Mesh type and performance parameters. Figure 6: Y-plus of inner wall and parameter distributions of the outlet. S-duct diffuser could significantly improve the stealth performance of the caret intake, but the RCS values at some angles still need to be reduced. The flow field calculation results of the two intakes are shown in Figure 8, with the aerodynamic parameters calculated in Table 4. According to Figure 8c, for each intake, an oblique shock wave was formed at the inlet and a positive shock wave was formed at the throat, indicating that both intakes were at the critical state with an outlet pressure of 60,000 Pa. The Mach number distribution in the diffuser revealed a boundary layer separation phenomenon in both S-duct intakes. Taking Model A as an example, two obvious separations occurred near the upper and lower walls on one side of the front diffuser, resulting in two corresponding low-pressure zones at the outlet. As the diffuser wall compressed, the airflow near the other side accelerated to supersonic speed, forming a local shock wave at the S-bend. Due to the interaction between this local shock wave and the boundary layer, a strong separation also occurred in the rear diffuser, leading to an obvious low-pressure zone at the outlet. Therefore, the DC60 values of Model A exceeded 0.53, indicating that the traditional S-duct intake also had a significant aerodynamic performance defect.