# Effect of Baffle Board on Aerodynamic and Stealth Performance of Double S-Duct Caret Intake
## 2 Computational Methodology
### Intake Models and Calculation Conditions  
In order to study the effects of different S-duct diffusers, two intake models were designed: a single S-duct caret intake, named Model A, and a double S-duct caret intake, named Model B, as shown in Figure 2(a). Both intakes had an ideal capture flow rate of 80.55 kg/s at an altitude of 15 km, and a Mach number of 2. The configuration parameters of the two intakes were the same, except for the Z-coordinate of each diffuser section center. The inlet shape of the intake was a parallelogram to match the aircraft's RCS characteristic. As shown in Figure 2(b), the section shape of the S-duct diffuser changed from a parallelogram to an equal-height rectangle in the front diffuser, and then to a circle in the rear diffuser, with the area increasing from 0.502 m\({}^{2}\) to 0.636 m\({}^{2}\). Figure 2(c) presents the center coordinate of each double S-duct diffuser section, with the area shown in Figure 2(d). To further reduce the radar echo intensity, various baffle boards were installed in the diffuser, and the performances of these intakes were numerically investigated. Initially, four curved baffle boards with nearly the same surface area were installed at different locations in the diffuser, as shown in Figure 3(a). The detailed dimensions of these boards are provided in Figure 3(b). After determining the optimal installation location, seven corrugated baffle boards were designed and installed at locations similar to model C-2 to optimize the RCS characteristics of the intake. The design approach for the corrugated baffle board is depicted in Figure 3(c). Taking a cross-section as an example, the coordinate system was established with the center point of the lower wall set as the origin. The mapping curve of the corrugated baffle board in this section could be expressed by a functional relationship. The functional parameters of each board are shown in the table, and the models of these boards are depicted in Figure 3(d). The optimal corrugation number can be determined by comparing the performances of the intakes with the first four boards. Subsequently, the optimal corrugation shape can be determined by comparing the performances of the intakes with the remaining boards. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Cavity** & \multicolumn{2}{c}{**Cylinder**} \\ \hline
**Diameter/m** & **Length/m** & **Diameter/m** & **Length/m** \\ \hline
0.286 & 0.3 & 0.16 & 0.16 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Model parameters of the complex cylindrical cavity. Figure 2: Model and RCS results at two polarization conditions. Figure 4: _Cont._  
Figure 3: Models and parameters of two S-duct intakes. Low-scattering material is generally coated on the surfaces of intakes to reduce the RCS. To minimize the errors caused by the direct radiation of radar waves on the outer wall, each intake was enclosed in a uniform cavity during the RCS calculation, as shown in Figure 5a. Additionally, the surfaces of each calculation model were coated by radar stealth material with relative magnetic permeability \(\mu\) = 1.29-0.57] and relative dielectric constant \(\varepsilon_{\text{r}}\) = 9.72-1.08], except for the ideal electrical conductor outlet. The radar wave used was the 3 GHz plane wave, and the frequency of the incident radar wave was the median of the S band, which is often used as the radar band of early warning aircraft. As illustrated in Figure 5b, the X\({}^{\prime\prime}\)O\({}^{\prime\prime}\)Y\({}^{\prime\prime}\) plane was defined as the horizontal plane, and the interval between the two incident angles was 1\({}^{\circ}\), parallel to the horizontal plane. Point O\({}^{\prime\prime}\) represented the inlet apex of the intake. The calculation conditions of the flow field are presented in Figure 5c, where the direction of the incoming flow was parallel to the \(X\)-axis. The flow coefficient, the TPR coefficient, and the DC60 were used to measure the aerodynamic performance of the intake.