# Aerodynamic Analysis and Noise-Reducing Design of an Outside Rear View Mirror
## 3 Simulation Results  
### Aerodynamic Simulation of the Reference OSRVM Model  
In general, the tonal noise occurs from the interaction of vortices with the gutter and edges of the side mirror. High levels of aerodynamic noises are generated in the gutter and edges during driving. Figure 3a-f show the CFD analysis results of the airflow velocity and the sound pressure levels (dB) around the reference OSRVM for three vehicle speed cases of 60 kph, 80 kph, and 110 kph at 23 \({}^{\circ}\)C temperatures and 50% humidity conditions. The value and region of aerodynamic flow rate and sound pressure level rapidly increase while the driving speed changes from 60 kph to 110 kph.  
Figure 3a-c show flow the velocity vector and contour in the cross-sectional plane of the reference mirror. The values of flow velocities at the gutter, mirror surface, and outside edge are displayed in the graphs. Vortexes are formed underneath the trailing edge of the mirror, and the gradual decrease of vortex length occurs for increasing speed. Strong vorticity (the curl of the flow velocity) appears at the gutter or behind the trailing edge of the mirror surface when the airflow rotates.  
Figure 3d-f shows the aerodynamic sound radiation around the reference side mirror. At higher driving speeds, the sound pressure level dramatically increases at the gutter and the mirror surface. The graphs indicate that the sound pressure level becomes the largest around the gutter at all of the speeds. The noise level at the gutter at the vehicle speed of 110 kph is 97 dB. The aerodynamic noise  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Vehicle Speed (kph)** & 60 (Urban or City) & 80 (Highway) & 110 (Freeway) \\
**Temperature (\({}^{\circ}\)C)** & 18 & 23 & 25 \\
**Relative Humidity (\%)** & 30 & 50 & 70 \\ \hline \hline \end{tabular}
\end{table}
Table 1: The inlet conditions of vehicle velocities, temperatures and relative humidity in computational fluid dynamics (CFD) simulation.  
Figure 3: Flow velocity contour with vector field (**a-c**) and maximum sound pressure level (**d–f**) around the reference outside rear view mirror (OSRVM) model at 23 \({}^{\circ}\)C and 50% humidity conditions for three vehicle speeds: (**a,d**) 60 kph, (**b,e**) 80 kph, and (**c,f**) 110 kph.  
radiation is mainly due to pressure fluctuations by the changes of the flow velocity and the air density, which are induced from the difference of vorticities around the gutter or the trailing edge of the mirror surface. This result agrees well on previous studies reporting that the intrinsic mirror noise sounds like a whistle, radiating from the pressure fluctuations near the trailing edges of the mirror housing [4] and considerable change in the A-pillar vortex [5]. A higher sound level at the mirror surface compared to that of the outside edge agreed with the previous result [3], which showed that the maximum magnitude of the fluctuating pressure appeared at the central bottom region on the mirror surface.  
In order to investigate the effects of driving speed, temperature, and relative humidity on the sound pressure levels around the mirror, a comparative analysis was performed for 27 cases using the conditions of Table 1. Figure 4 shows the graph charts of the sound pressure levels of the reference OSRVM model for various cases. Figure 4a-c are measured at the gutter, and Figure 4d-f are measured at the surface of the mirror. These results indicate that the sound pressure level of the reference OSRVM is mainly affected by vehicle velocity, but relatively less changed by temperature or relative humidity variation.  
Figure 4: The graph chart of the sound pressure level of the reference OSRVM model for various relative humidities and temperatures: the gutter from (**a–c**) and the surface of the mirror from (**d–f**) for three speed cases of 60 kph, 80 kph, and 110 kph.