## Materials and Methods
### Numerical Methods  
The strategy of the numerical modeling applied here is similar to [2; 13] and [14]. According to our previous study [11], the flow was computed based on the incompressible Navier-Stokes equations with the cell-center Finite-Volume-Method (FVM) [16] within the CFD software STAR-CCM+ (Siemens PLM Software, Plano, TX, USA). The turbulence flow was simulated based on the Reynolds-Averaged Navier-Stokes (RANS) and Unsteady Reynolds-Averaged Navier-Stokes (URANS) equations in combination with the SST (shear stress transport) \(k-\) two-equation eddy-viscosity model [17; 18; 19]. The model combines the good  
Figure 9: (**a**) Wake refinement of the fins shown at a mesh plane surface at the bottom wall. (**b**) Drag coefficient as a function of the number of control cells. Six different meshes with an increasing number of cells and a decreasing target cell volume were included in the mesh independence study. performance of the \(k-\varepsilon\) model in free separated flow regions and the \(k-\) model that showed high accuracy in turbulent boundary layers [42]. As shown in [11], RANS simulation results are appropriate for AoAs that range between \(0^{\circ}\) and \(15^{\circ}\), and for AoAs \(\geq 20^{\circ}\) URANS simulations are necessary to resolve the unsteady flow separation from the fins and the resulting force fluctuation. For a sufficient modulation of the near-wall flow, STAR-CCM+ provides an additional high-y+ model that can handle coarse meshes with a \(y^{+}>30\)[43]. The high-y+ model is based on the standard logarithmic law of the wall model. Depending on the AoA, the y\({}^{+}\) values for the first grid layer at the surfaces of the fins were specified to be in the range of 30 to 90. The target value for y\({}^{+}\) is 60. Therein, the first cell centers next to the wall are located in the log-layer of the turbulent boundary, which describes y\({}^{+}\) as logarithmic function of u\({}^{+}\) within the standard log-law [44]. For the transient URANS simulations, the time step size was adapted for each AoA to satisfy a Courant-Friedrichs-Lewy (CFL) condition \(\text{CFL}=1.0\ \pm 0.5\) at the fins surfaces where the mesh is finest [45]. As a time discretization scheme, a 2 nd order implicit unsteady method was applied, which works for CFL \(>1\)[46]. Figure 3a shows the lift and drag forces acting on the rear IF at an AoA (\(\alpha\)) of \(10^{\circ}\). In this study, the absolute lift and drag forces (\(\text{F}_{\text{L}}\) and \(\text{F}_{\text{D}}\)) itself, and the forces as non-dimensional lift and drag coefficients, as defined by [16], were determined:  
\[c_{\text{L}}=\frac{\text{F}_{\text{L}}}{\text{A}_{\text{Fin}}\cdot\frac{ \rho}{2}\cdot\text{u}_{\text{surf}}^{2}} \tag{5}\]  
\[c_{\text{D}}=\frac{\text{F}_{\text{D}}}{\text{A}_{\text{Fin}}\cdot\frac{ \rho}{2}\cdot\text{u}_{\text{surf}}^{2}}. \tag{6}\]  
\(\text{A}_{\text{Fin}}\) is the reference area (the planform area of the fins), and \(\rho\) is the fluid density. The absolute values of the lift and drag forces are calculated by summing up the pressure and shear forces for each element (index k) at the surfaces of the fins [43]:  
\[\text{F}_{\text{L/D}}\ =\ \sum_{\mathbf{k}=1}^{\mathbf{N}}\left[\left(\mathbf{f}_{ \mathbf{k}}^{pressure}+\mathbf{f}_{\mathbf{k}}^{shear}\right)\mathbf{n}_{ \mathbf{k}}{}^{\text{L/D}}\right]\text{, with} \tag{7}\]  
\[\text{Pressure force vector}:\ \mathbf{f}_{\mathbf{k}}^{pressure}=\left(\text{p}_{ \text{k}}-\text{p}_{\text{ref}}\right)\mathbf{a}_{\mathbf{k}} \tag{8}\]  
\[\text{Shear force vector}:\ \mathbf{f}_{\mathbf{k}}^{shear}=\ -\mathbf{T}_{ \mathbf{k}}\cdot\mathbf{a}_{\mathbf{k}} \tag{9}\]  
Hereby, \(\text{F}_{\text{L}}\) is the absolute lift force, \(\text{F}_{\text{D}}\) is the absolute drag force of the fin, \(\mathbf{n}_{\mathbf{k}}{}^{\text{L}}\) is the associated vector in flow direction, and \(\mathbf{n}_{\mathbf{k}}{}^{\text{D}}\) is the associated vector perpendicular to the flow direction; see Figure 3a.