# Aerodynamic Analysis and Noise-Reducing Design of an Outside Rear View Mirror
## 2 Design and Analysis
### Boundary Conditions and Analysis  
In general, the extruding components in the vehicle exterior such as the OSRVM, a rear wing, a radio antenna and the exterior accessories, cause the aerodynamic noise radiation while driving. In this study, we analyze the aerodynamic flow to generate aerodynamic noise. In the CFD simulation, the aerodynamic performances of the three mirror models are compared by quantifying the flow velocity, the sound pressure levels and the vorticity at three points (the gutter, the edge and the center of the mirror surface) around the OSRVM, as shown in Figure 2a. A cuboid computational domain was used to obtain the CFD solution in this approach, as illustrated in Figure 2b. It considers airflow rate and field, and the size of the computational domain is 580 mm \(\times\) 800 mm \(\times\) 2000 mm. The gutter, which is a small part of the A-pillar, is included in the computational domain to minimize the aerodynamic effect from the surrounding features during driving, since the vehicle body is placed in front of and to the rear of the OSRVM. The gaps on the gutter between the side mirror and A-pillar often provide flow separations. In addition, the mirror was mounted about 1000 mm downstream of the elliptically-shaped leading edge of the side plate, as illustrated in Figure 2b. In general, the main source of aerodynamic noise was the vortex and turbulent flow of the rear side, rather than the front side, of the mirror. Therefore, the computational domain  
Figure 1: Three-dimensional (3D) scanning process to gather a 3D computer-aided design (CAD) model and three types of OSRVM models: (**a**) 3D scanning apparatus; (**b**) Extraction of the mirror shape from the scanned data: (**c**) Reference model; (**d**) Outer duct model: (**e**) Inner duct model. was also set to the rear side of the OSRVM. This computational domain was based on the geometry considered by [PERSON] [16] and [PERSON] et al. [17], which have studied the aerodynamic experiments of the side mirror and the feasibility of the aerodynamic simulation. In the simulation, homogeneous velocity was applied at the inlet in the vehicle-driving direction. In addition, homogeneous pressure was applied at the outlet as a boundary condition considering the flow field. A stationary wall (no-slip) condition was applied to the surfaces of the objects and the computational domain. The computational fluid dynamic analysis using FLUENT was conducted using the environmental conditions of the vehicle speed, relative humidity and temperature of the inlet. The environmental conditions summarized in Table 1 are based on [PERSON] et al. [10], where the free stream velocity of 38.9 m/s is equivalent to the vehicle speed of 140 km/h. The speed regulation of city is 60 km per hour (kph), and the highway's regulation is from 80 kph to 100 kph. The freeway's regulation is about 110 kph. Therefore, the vehicle's speed was grouped by road environment and speed regulation. The reference of temperature was decided by an ordinary temperature between 18 and 25 degrees Celsius. Relative humidity was grouped by weather conditions (clear, wet, and rainy) between 30% and 70%. In order to identify the aerodynamic noise sources, the steady-state flow analysis was performed to guarantee an appropriate resolution of the vortex structure of the flow field. In general, the Reynolds number is an important parameter for describing the aerodynamic flow. If the Reynolds number increases as the velocity increases, then the transition position moves forward. With an adverse pressure gradient, the turbulent boundary layer keeps the flow attached longer before it separates. Moreover, the standard k-epsilon turbulence model was applied for the steady-state analysis. The k-epsilon model is the simple complete model of turbulence: its computational costs are lower than that of other methods, such as large eddy simulation. The k-epsilon model is a semi-empirical model based on model transport equations for the turbulence kinetic energy and its dissipation rate.