## 2 Materials and Methods
### CFD Calculations and Mesh Independency  
In the 3D design studies determined by the Taguchi method, SIEMENS NX 11 was used for 3D design, HYPERMESH 14.0 for creating surface mesh structures and geometry surface preparation, and the STAR CCM++ 11.02.010 engineering software for generating volume meshes and obtaining simulation solutions. After completing the 3D designs, a 2D surface meshing process was performed to prepare the model for simulation. The surface mesh structure of the model was created using the HYPERWORKS 14.0 software. Well-known continuity, momentum, turbulence, and energy equations are used. To avoid compromising surface quality and eliminating details, a mesh independence study was conducted to determine the optimal mesh criteria. The study was completed using tria and quad mesh types. In Figure 4, pressure drop, uniformity index, and velocity results in respect to mesh size are presented. The tetrahedral mesh was preferred over others because it provided more accurate results for the irregular geometric shape of the model under investigation. A total of 32,841,430 elements were used. Details of the mesh are presented in Table 2.  
This study focuses specifically on investigating the impact of the porosity medium property of a fuel cell structure, particularly on the homogeneity of flow characteristics due to pressure loss. Initially, the internal volume is modeled, and boundary conditions are established. Subsequently, pressure loss and uniformity index values are computed for each geometry. The 3D geometries obtained through the Taguchi method were modeled, and CFD setups were established. An important parameter that can significantly affect the performance of fuel cells is the gas diffusion layer (GDL) porosity, which can vary between 0.3 and 0.8. The highest current density values correspond to situations with the highest GDL porosity, attributed to the increased oxygen concentration in the catalyst layer and subsequently increased reaction rate [42]. The GDL geometry is depicted in Figure 5. A typical porosity medium consists of two regions: the solid matrix, which is the material of the medium (in the case of the gas diffusion layer (GDL), it is a solid matrix of carbon fibers),  
Figure 3: Designed geometries. and the void space between the solid matrices, referred to as the pore body. The GDL is represented with a regular pore network. The X-Y planes are parallel to the interface between the GDL and the catalyst layer, and the Z direction corresponds to the thickness of the GDL. Liquid water is injected into the GDL from the Z = 0 interface. After the network is created, it undergoes a process to become partially hydrophobic. This is achieved by assigning a high contact angle to a series of pores. Subsequently, a displacement algorithm is employed to move the liquid between pores and simulate the flow of water within the porosity medium. \begin{table}
\begin{tabular}{l c c} \hline \hline \multicolumn{3}{c}{**Surface Mesh Structure Parameters for the Pressurized Cabin**} \\ \hline Regions and Boundaries & Min. Surface Size & Target Surface Size \\ \hline Geometry & 2 mm & 2 mm \\ \hline Mesh Growth Rate & 1.3 [-] & - \\ \hline \multicolumn{3}{c}{**Surface Mesh Structure Parameters for the Pressurized Cabin**} \\ \hline Tet/Poly Density & 1.1–0.9 [-] \\ \hline Prism Layer Number & 5 [-] \\ \hline Prism Layer Thickness & 3 mm \\ \hline Initial Prism Layer & 0.3 mm \\ Thickness & \\ \hline Prism Layer Target & \\ Thickness & None \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Mesh properties. Figure 4: Mesh independency results for pressure drop, uniformity index, and velocity. To investigate the solution of these structures in the CFD model, a porosity media method has been developed to calculate inertial and viscous coefficients from the [pressure drop/thickness] = f [velocity] curve function.