## 3 Case Study
### Assessment of the POCS Pressure Drop  
#### 3.2.1 Three-Dimensional CFD Configurations  
The investigation into the fluid pressure difference established across the POCS structure was carried out for both the configurations reported in the previous subsection, namely straight and curved geometries. Six different flow velocities are imposed at the inlet of the considered ducts (0.5, 1.0, 5.0, 10.0, 25.0, and 50.0 m/s), while three different meshes are employed for each configuration. Table 2 reports the details related to the global number of cells for each considered mesh. The applied refinements on the geometrical walls, in terms of increments with respect to the base cell dimension, are preserved between the different mesh versions. A reference computational grid was created (namely B), in addition to coarse and fine configurations (A and C, respectively).  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Mesh** & **Cells Number** & **Base Cell Length (mm)** \\ \hline A-Straight & \(1.15\cdot 10^{6}\) & 2.00 \\ B-Straight & \(4.96\cdot 10^{6}\) & 1.00 \\ C-Straight & \(9.06\cdot 10^{6}\) & 0.75 \\ A-Curved & \(1.15\cdot 10^{6}\) & 2.00 \\ B-Curved & \(5.18\cdot 10^{6}\) & 1.00 \\ C-Curved & \(9.94\cdot 10^{6}\) & 0.75 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Fluid computational domains details.  
Figure 4: POCS geometrical characterization: visualization of the POCS module section for an increasing axial coordinate.  
Figure 5: POCS geometrical characterization: Curved configuration—fitting inside an exhaust manifold.  
A total of thirty-six simulations were performed to deeply characterize the fluid-dynamic performances of the structure under investigation. The fluid region is handled through a multi-component mixture, whose thermal properties are computed on the basis of the JANAF tables. Similarly, the transport properties are determined according to the Sutherland model to account for the dependency of the fluid viscosity on the temperature. The analysis is conducted considering an inlet fluid temperature equal to 1273 K, while adiabatic boundary conditions are applied to all the walls of the geometry. The concentration of each fluid species is given in Table 3, according to a given representative exhaust gases composition: the 1D simulation of a reference engine operating under low load conditions is carried out and the exhaust gases composition is extracted. Such a modeling approach is selected in order to simulate the interaction between the POCS and the exhaust gases for realistic operating conditions, neglecting the contributions related to heat transfer.  
The numerical methodology relies on the simulation framework described in the previous section, by solving a single region (fluid domain) for steady-state conditions and laminar flow. The turbulence modeling, given the shape of the structure and its subsequent interaction with the fluid flow, is expected to provide a negligible contribution for the pressure difference characterization. The POCS-induced flow mixing is strongly associated with the designed structure's shape and geometrical features, and hence, the laminar simulation is supposed to consistently represent the pressure losses without the need to model the turbulence. As such, the required computational power can be reduced. An additional simulation has been run to verify such an aspect, considering the operating condition corresponding to the highest inlet velocity.