Study on the Fluctuating Pressure and Aerodynamic Noise at Car Rearview Mirror Zone

Abstract: In order to study on the flow characteristics and the formation of aerodynamic noise quantitatively at car rearview mirror zone, the numerical simulation method is applied in the paper. Firstly, a wind tunnel computational model that includes review mirror, body characteristics and detailed wheel structures based on an actual car is established. The effects of rotating wheels and moving ground on air flow are also taken into account in the model. Then, a large eddy simulation (LES) technology and transient numerical method are used to solve the computational domain and study the flow field characteristics and pulsating pressure at mirror and side window zone. Finally, the FW-H method is applied to calculate the aerodynamic noise at rearview mirror zone. The results show that a strong swirling motion in the wake of the A-pillar and rearview mirror is the reason caused fluctuating pressure that is the main dipole sound sources at side window zone. Additionally, it also shows that reducing flow separation movement at rearview mirror zone will ameliorate the drag eddy scale in the wake area which is the key to control aerodynamic noise source at side window zone.


Introduction
Recently, with the gradual improvement of road infrastructure, vehicle speed becomes faster and faster.Car aerodynamic noise is proportional to sextic vehicle speed and it will increase about 18 dB when vehicle speed is double [1,2].So car aerodynamic noise has become a dominant factor affecting the ride comfort and the life quality for people living near roads.The theoretical analysis, numerical simulation, wind tunnel test and vehicle road test are major research methods on car aerodynamic noise.With the rapid development of CFD software and computer technology, the numerical simulation has become an important research tool for automotive flow field aerodynamic noise, which is a convenient and reliable method to reveal the car flow field characteristics.Numerical simulation is not only with low cost, but also it can explore the complex flow phenomena of car flow field.It has become one of the important ways on car aerodynamic analysis in the automotive development process.Because car shape has a lot of surface corner, sectional change and some protruding objects, airflow will lose adhesion and induce complex unsteady vortex motion in these areas.So a strong pressure fluctuation will be produced in car surface layer and thus generate aerodynamic noise and radiation to the surrounding [3,4].In addition, improving the accuracy of numerical model for analyzing car external flow field is also important.By using a large eddy simulation of transient and the FW-H method to study the mechanism and characteristics of car aerodynamic noise at the side window zone, it has great significance to predict and control car aerodynamic noise in car design stage.

Geometric model
A 3D geometric model based on a real car was established in this paper, the model contains the main features of a real car such as body, mirrors, complex wheel structure, A-pillar and body waist.A numerical model for automotive flow field to simulate wind tunnel experiments is also established in software platform.The geometric size of the car is 4500 mm in length (L), 2200 mm in width (w) and 1350 mm in height (h).The front computational domain (L1) is three times of car length.The rear (L2) is seven times of car length.The height (C) is five times of car height and each side width is two times of car width.It is shown in Figure 1 and Figure 2.

Mesh generation
Considering the complexity of car body surface and the fully expressed body shape features, this paper applies a hybrid grid scheme, which can take advantages of tetrahedral grid has good adaptability and hexahedral grid has high accuracy [5].The method of mesh control to set the mesh size of the computational domain is about 3mm-8mm.The grid surrounding car body wall is refined as marked in dotted line shown in Figure 2. The element length is controlled about 5mm.
The domain near the wall of body, tires, review mirror and wheel cover is meshed with an inflation grid to improve the accuracy.The mesh settings are presented in Table 1.The total number of grid is about 12 million and the result of mesh generation is shown in Figure 3.

Boundary conditions
In order to match the actual situation and ensure the accuracy of calculation result, the effect of wheel rotation on flow field must be considered [6].So rotating wall is used to simulate rotating wheels and moving wall is applied to simulate the relative movement between vehicle and ground.To avoid solving the whole computational domain and reduce the computational cost, the amount of calculation is reduced by using a symmetric boundary condition.Wall function is applied with standard wall function.The parameter settings of boundary conditions can be found in Table 2.
In addition, six signal points are set to monitor the fluctuating pressure in the longitudinal of symmetry within the rearview mirror zone and another eight signal points are set to monitor the aerodynamic noise at the side windows zone.These monitoring points are shown in Figure 4.

Simulation and analysis
The flow field at the side windows zone of the vehicle combined with the effect of A-pillar and rearview mirror are calculated by numerical simulation.The total equivalent pressure contour of the flow field at side window zone at t = 0.15s is shown in Figure 5.There is a large pressure gradient at the surface of A-pillar, rearview mirrors and side windows as well as the longitudinal center plane of the rearview mirror and several independent vacuum central regions appears in those areas.There is high pressure around the wake zone of the rearview mirror and low pressure zone in the center, which demonstrates that the pressure is very unstable and there is a strong complex vortex motion in the rearview mirror wake region, thus making the car aerodynamic drag and the pressure pulsation increased.The velocity vector within the longitudinal center plane of rearview mirror wake at different times is shown in Figure 6.When the front airflow reaches the windward surface of the rearview mirror, the speed of airflow is greatly reduced since the flow is blocked.The dynamic pressure becomes static pressure and forms a positive pressure zone in front of rearview mirror.This part of airflow generates flow separation at the surface of rearview mirror.The velocity of upper airflow is accelerated significantly, but due to the impact of the body surface, the lower airflow separation is weaker.The small airflow through the gap between A-pillar and rearview mirror to the side window and it falls off in the edge of the A-pillar.In the early flow field, when the flowing time is 0.005s, the airflow is slightly twisted in the rearview mirror rear zone, but it is still mainly laminar flow.Due to the impact of air flow separation at the edge of rearview mirror, the rear airflow is outward flow gradually and causes the central pressure dropped in the wake zone.The core of the vortex is forming gradually when the flowing time is 0.03s.The vortex motion in wake region is completely developed when the flowing time is 0.06s, a clockwise drag vortex is formed in the upper region and the location of a counterclockwise drag vortex is lower.The swirling motion is gradually developed to downstream until crushing, dissipated and it is accompanied by energy dissipation.The drag vortex is disappearing when the flowing time is 0.09s and a larger negative pressure region appears.The surrounded airflow is sucked into the wake region because pressure difference and the swirling motion are developing again when the flowing time is 0.12s.It is given in Figure 6e and 6f.The swirling motion is a reciprocating process contains formation, development and shedding in rearview mirror wake vortex motion region.The Hussain and Zaman pointed out that the vortex motion and fragmentation is the main reason for the development of aerodynamic noise on subsonic jet terms.Thus mirror wake has significant unsteady transient characteristics and will induce fluctuating pressure at side window zone.
The total pressure curves of each monitoring points in the wake area of rearview mirror are shown in Figure 7.The values of monitoring point 1 and 2 remains negative.The pressure value of point1 is fluctuating within a certain range and the pressure value of point 2 is relatively stable which stays at about -500 Pa.The reason is that there are two symmetrical vortex cores located near the measuring point 1 and 2 and the strength and instability at the top vortex is greater than the downward vortex.Similarly, the fluctuating pressure strength of monitoring point 3 to 6 is significantly enhanced and presents a periodic fluctuation.It is also found that the pressure values at top pressure monitoring points are always greater than the below pressure monitoring points resulting in that visible swirling motion causes pressure fluctuation directly.The Sound Pressure Level (SPL) is a key evaluative indicator of car aerodynamic noise performance.Therefore, there are eight aerodynamic noise monitoring points set in the vicinity of the front and rear side window corresponding to the positions of car occupants' ears.By using FW-H method and fast Fourier transform, the spectrum SPL of each monitoring point is obtained as shown in Figure 7.It is shown from the results that car aerodynamic noise belongs to a wide spectrum of noise.In the low frequency band of 0 to 600 Hz, the magnitude of the sound pressure level is the maximum.The SPL peak reaches about 100dB and it presents a sharply declining.In the medium frequency band of 600 to 3000 Hz, the SPL values present a slow downward trend.In the high frequency band of 3000 to 5000 Hz, the SPL amplitude keeps stable.
In addition, located in the upper part of the mirror monitoring points I, III, V, VII, the amplitudes of sound pressure level are significantly greater than the lower part of the measuring point II, IV,VI VIII.The spectrum curve trends of the SPL monitoring point I and II as well as point III and IV are in consistence as shown in Figure 8a and 8b.It results in that the front of side windows near the rearview mirror is mainly impacted by the symmetrical drag vortex motion and it presents the same spectral characteristics.Because the upper vortex motion is intense, the SPL amplitude of upper location is bigger than the lower location.There is a difference for the SPL amplitude in the high frequency of the monitoring point V and VI as well as point VII and VIII.The SPL magnitudes of upper measuring point V and VII are higher than the lower measuring point 20dB, respectively.Moreover, the SPL amplitude of medium frequency range has smaller change than the monitoring point I and III.After the airflow separation occurs at the edge of A pillar and rearview mirror, the airflow is attached again in the lower region of rear side window and it causes that the SPL presents some weakening.But the upper airflow near the side windows is separated again at the edge of C pillar and the airflow motion is very unstable.

Conclusion
The following conclusion can be drawn from the study by numerical simulation: (1) The investigation illustrates the mechanism of fluctuating pressure that there is a strong swirling motion in the wake of A-pillar and rearview mirror and the causes that the main dipole sound sources is at side windows zone.(2) Aerodynamic noise belongs to a wide spectrum of noise.In low frequency band, it is with high amplitude and declined sharply.In high frequency band, it is with low amplitude and stabilized.From the height difference of monitoring points, it is shown that its impact on the amount of the high frequency aerodynamic noise is also very obvious.(3) Reducing flow separation movement at rearview mirror edges and the drag eddy scale in wake zone is the key to control car aerodynamic noise source at side window.The shape design optimization of rearview mirror will be able to ameliorate aerodynamic noise effectively.

Figure 1 .
Figure 1.Side view of computational domain

Figure 2 .
Figure 2. Top view of the computational domain

Figure 3 .
Figure 3. Mesh of computational domain

Figure 4 .
Figure 4. Monitoring points of fluctuating pressure and aerodynamic noise

Figure 5 .
Figure 5.The total equivalent pressure contour at the side windows zone

Figure 6 .
Figure 6.Velocity vector at different times

Figure 7 .
Figure 7.The total pressure curve of different monitoring points

Figure 8 .
Figure 8.The SPL spectrum of monitoring points at side window zone

Table 1 . Mesh settings Mesh Size Inflation
Max Face Size 60mm Proximity Min Size 6mm Growth Rate 1.15 1.05 1.15 Growth Rate 1.2 Minimum Edge Length 0.9mm Inflation Option First Layer Thickness

Table 2 .
Settings of boundary conditions Stationary Wall, Shear Condition: No Slip Ground Moving Wall; Speed: U=28 m/s Wheel Moving Wall, Speed: 100 rad/s Acoustics Model Ffowcs-williams & Hawkings Viscous Model Large Eddy Simulation(LES) Solution Settings Time Step Size: 0.0001s Number of Time Steps: 1500