## 4 Conclusions  
LES of an automotive axial cooling fan with seven blades in the near-field and noise prediction to the far-field using FW-H method were carried out to provide a better understanding of unsteady flow and aerodynamic noise characteristics. The computational domain had a cylinder type consisting of rotating and stationary parts with a pressure condition with 0.2% turbulence intensity and the analysis was performed under a constant rotational speed of 2100 rpm. The reliability and validity of the simulations were verified with experimentally measured data. Comparatively good agreements were achieved for the velocity distributions in the upstream and downstream and the aerodynamic noise of SPLs at two different receivers. However, for the further reduction of these discrepancies in the unsteady flow and aerodynamic noise, a longer data acquisition duration and sufficient mesh resolution may be required. The quadruple source due to the unsteady momentum transport in turbulent flow was disregarded owing to the subsonic flow caused by the fan blades, and only the dipole source due to the fan surface was considered for efficient and less expensive computation. Important findings arising from this research are as follows:  
(1) The radial flow near the fan's ring was accelerated into the passages between the blades owing to the low pressure of the strongly pushing air from Z/D = 0.638 in the downstream to Z/D = 0.114 in the upstream; this caused an uneven velocity distribution in the upstream and downstream, and enabled the formation of a few dead zones around the ring, including the wake immediately after the hub. The simulation results for the velocity distributions were slightly overpredicted due to the misalignments of the measurement locations. Additionally, the instantaneous pressure pulsation was limited to a significantly small range of -365 Pa and 234 Pa owing to the vortices over the blades. (2) This study experimentally confirmed that the highest SPL was 57.8 dB at 245 Hz of the first BPF, and its first and second harmonics were 53.8 dB and 47.9 dB at the 2 nd and 3 rd of 409 Hz and 735 Hz, respectively. At the location of 1.0 m, the SPL decreased by 6.7 dB, 6.9 dB, and 7.5 dB at each BPF per distance doubling. Therefore, the dipole sources emitted from the fan made a major contribution to the aerodynamic noise generation. In contrast, the simulation results demonstrated that the highest SPL was overpredicted by 8.6 dB and the peaks at the 2 nd and 3 rd BPFs were underpredicted in comparison with the experimental results at the location of 0.5 m due to the shorter data acquisition duration, particularly at high frequencies, and the insufficient mesh resolution close to the fan blades. (3) A strong tonal noise numerically existed near the leading edge of the blades at the tip and amounted to 111 dB SPL. Furthermore, the pressure side of the blades was higher than the suction side in the SPL. This was a result of the large-scale separation near the leading edge of the airfoils caused by the increasing angles of attack with the increasing radial velocity near the ring, which caused the entire airfoil to emit a low-frequency noise. Therefore, the blade component was more dominant than the ring and the hub as a dipole source causing the tonal noise. The separation noise is considered to be of greater importance for the automotive axial fan when an improper fan design is present. (4) The numerical observation revealed that a 6 dB decay in the far-field region existed from Z/D = 5.4 to Z/D = 350 with distance doubling, whereas approximately 5 dB in the near-field region decreased less. The sound fields near the fan were complex, and the noise propagation did not develop well owing to the reflecting wave. Therefore, the sound pressure decreased by less than 6 dB in the near-field region compared to that in the far-region each time the distance from the sound source doubled.