## 4 Results and Discussion  
At first, the wind force acting on the longitudinal finned tubes in the wind direction was determined using CFD analysis and experimentally for each of the analyzed cases. The aerodynamic drag coefficients of the examined profiles were then calculated according to Equation (3) and summarized in Table 1.  
The other results of CFD analysis are shown in Figures 9-14 as pressure distributions and velocity magnitude distributions. The results are presented in a plane parallel to the zx plane and located at the middle of the finned tube height. Comparing the local maximum pressure values that occurred on the finned tube profiles on the wind action side, it can be noticed that the highest value was obtained for the case c2 (profile with two fins), where the pressure was 133 Pa, followed by case c3 (profile with four fins), where the pressure was 124 Pa. This value was about 7% lower than the reported maximum. In the other cases, the  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Case**} & **Velocity** & **Aref\_exp** & **Aref\_CD** & **F\_w\_exp** & **F\_w\_CFD** & **cf\_exp** & **cf\_CD** & **(cf\_CD\textless{}cf\_exp)** & **cf\_error** \\  & **[m/s]** & **[m\({}^{2}\)]** & **[m\({}^{2}\)]** & **[N]** & **[N]** & **[s]** & **[–]** & **[–]** & **[–]** & **[\%]** \\ \hline c1 & 11.74 & 0.004 & 0.010 & 0.241 & 0.035 & 0.65 & 0.16 & – & –  
For each of the analyzed cases, except for the case c1, the aerodynamic drag coefficients obtained by the CFD method were, respectively, higher than the values obtained from the experiment. The differences ranged from 0.06 to 0.14, which is from 4% to 10%. These differences result from the fact that in the case of the experimental tests, the finned tube profiles were mounted on a cylindrical support of the force balance. Additionally, a short length of cylindrical pipe protruded at the top of the finned tube profiles. The reference areas of those elements were included in the calculations. In turn, in the CFD analysis, the cylindrical support of the force balance and the short tube section at the top were not modeled, which is reflected in the results. Figure 14: (**a**) Pressure distribution; (**b**) Velocity distribution for the tube with 8 fins (case c6). Figure 12: (**a**) Pressure distribution; (**b**) Velocity distribution for the tube with 4 fins (case c4). Figure 13: (**a**) Pressure distribution; (**b**) Velocity distribution for the tube with 8 fins (case c5). Consistently considering the case c1, one notices a significant difference in the CFD and experimental results. This result was also affected by the cylindrical support of the force balance. The arrangement of the two fins along the direction of the airflow separated the stream and resulted in a low value of the drag coefficient of 0.016 for CFD analysis. In contrast, the experimental result was 0.65, which was practically a coefficient typical for a cylindrical shape [29], or more specifically for a cylindrical support of the force balance. However, despite some slight discrepancies between the numerical and experimental results, it can be seen that for cases c2-c6, both the CFD and the experimental results showed an analogous trend, which made it possible to compare the results for these cases. The highest values of the aerodynamic drag coefficients that were obtained for case c2 referred to the profile with two fins. The second highest result was obtained for the case c3, which concerned the profile with four fins. In both cases finned tube profiles were oriented so that the two fins in the profile section were a surface perpendicular to the direction of the airflow. The smallest value of the drag coefficient was obtained for case c5, which concerned a profile with eight fins rotated by 22.5\({}^{\circ}\) to the direction of airflow.