## 3 Results and Discussion
### Aerodynamic Noise Characteristics
This is expected to be caused by the separation in the leading edge of the airfoil consisting of the fan with the increasing radial velocity that resulted in the increasing angles of attack. According to [PERSON] et al. [32], at moderate angles of attack of the low Reynolds numbers, the flow separates from the trailing edge of the airfoil, thereby producing a trailing edge noise owing to shed turbulent vorticity. However, at very high angles of attack of the low Reynolds numbers, the separated flow near the trailing edge of the airfoil, generates a large-scale separation, thereby causing the entire airfoil to emit a low-frequency noise [33]. Therefore, the blade component is more dominant compared to the ring and the hub as dipole sources causing the tonal noise. The separation noise is considered to be of more major importance for the automotive axial fan when an improper fan design is present. Figure 13 shows the predicted overall sound pressure level (OSPL) history of the dimensionless distance plotted in the blue line according to distance doubling downstream from the fan and the experimental OSPL values plotted in red dots. The OSPL is the equivalent SPL taken by integrating the SPLs across all frequencies. In the present study, frequencies up to 4000 Hz were considered. The results of the FW-H model showed good tendency with those of the wave theory equation in the far-field region after Z/D = 5.4, where the sound pressure produced by a point source decreased to 6 dB each time the distance from the point was doubled. The simulation results confirmed the 6 dB decay characteristic from Z/D = 5.4 to Z/D = 350 with distance doubling. The OSPL values at Z/D = 1.35 and 2.7 were 74.4 dB and 69.3 dB, respectively, and were a little underpredicted compared  
Figure 12: SPL visualization of the fan noise source on the suction (**left**) and pressure (**right**) sides between 242.7 Hz and 246.6 Hz band range. to the experimental data (i.e., 75.2 dB and 70.2 dB, respectively). The discrepancy came from the low resolution in high frequencies up to 4000 Hz. Unlike the far-field region, the near-field regions around the fan decreased approximately 5 dB to Z/D = 5.4 because the sound fields were complex, and the noise propagation did not develop well due to the reflecting wave by objects, such as equipment installation and walls [34]. Therefore, the sound pressure did not decrease as rapidly as it did in the free field. In other words, it decreased by less than 6 dB in the near-field region each time the distance from the sound source doubled. Table 2 shows the predicted OSPL ratio of each component (blades, rig, and hub) to the total dipole source of the fan at Z/D = 1.35 and 2.7 to analyze the contribution of each component. Each OSPL ratio was calculated using the SPL formula. The contribution of the dipole source to the tonal noise in the blades, rig, and hub was 69.3%, 10.6%, and 20.1%, respectively, at Z/D = 1.35. Furthermore, the blade's contribution to the noise increased to 71.6% at Z/D = 2.7 when the distance doubled. In the present study, the quadruple source due to the unsteady momentum transport in turbulent flow was disregarded because of the subsonic flow caused by the fan blades. According to [PERSON] and [PERSON] [35], the dipole source in the rotating wind turbine blades was more dominant when the incoming velocity was low, but became less predominant for a higher wind speed. In conclusion, the dipole source caused by the fan blades in normal conditions plays an important role in tonal noise and is the main sound source at a low frequency. \begin{table}
\begin{tabular}{c c c c c} \hline
**Z/D** & **Fan (Blades + Rig + Hub)** & **Blades** & **Rig** & **Hub** \\ \hline \multirow{2}{*}{1.35} & 74.39 & 71.21 & 55.92 & 60.44 \\  & (100\%) & (69.3\%) & (10.6\%) & (20.1\%) \\ \hline \multirow{2}{*}{2.70} & 69.25 & 66.39 & 51.25 & 53.28 \\  & (100\%) & (71.6\%) & (12.5\%) & (15.8\%) \\ \hline \end{tabular}
\end{table}