The Active Control Study Of Vehicle Buffeting Noise Of Rear Window

Buffeting noise was generated in the vehicle cabin when opening the sunroof or side window.It would seriously affect driving comfort and driving safety.So far,automotive buffeting noise research was mostly focused on sunroof control.It was less concerned at the side window where buffeting noise pollution was more serious.However,the existing buffeting noise control technology was mostly limited to passive control which cannot provide well noise reduction and also affected the vehicle geometry.Therefore,in view of the problem that rear window buffeting noise was prominent and passive control measures were difficult to apply,the thesis would adapt active jet technology to reduce the rear window buffeting noise.This thesis started with a simple cavity model simulation which was similar to the vehicle cabin cavity.Combining with wind tunnel experiment data and numerical calculations,mechanism of cavity self-oscillation induced by turbulence flow at nonzero attack angle was revealed.Besides,the relationship between the angle of attack,cavity opening position,neck thickness,turbulent boundary layer thickness and selfoscillation was analyzed too.The analysis provided a theoretical foundation for the research on control of rear windows buffeting noise.Results showed that the cavity self-oscillation at non-zero angle of attack which was the result of the coupling effect of vortex shedding,feedback pressure wave and resonance lock-up effect.The intensity of self-oscillation was very sensitive to the change of attack angle.When the attack angle increased to a certain critical value,the stability of the shear layer increased duo to the increase of its thickness.At this moment,the resonance lock-in effect disappeared and the self-oscillation in the cavity was completely suppressed.Based on a real SUV vehicle,the pressure fluctuation in the vehicle cabin was calculated when left rear window opened.With the change of the flow field near the opening of the rear window,the mechanism of rear window buffeting noise could be expressed as follow.Geometry shape mutation when flow passes through the A-pillar,the triangular region of the rearview mirror and the B-pillar.Unstable shear layer would periodically shed the vortex structure,accompanying with the flow backward to the rear of the car.The two vortexes hitd the C-pillar to generate a pressure wave feedback and some of broken vortex flows into the vehicle,causing a periodic pressure pulsation in the vehicle.The vortex shedding from the B-pillar was dominated between these two vortexes.When the pressure wave reflected to the B-pillar,it would induce further vortex shedding.Helmholtz resonance occured when the vortex shedding frequency matched the natural resonance frequency in the vehicle cabin.Active jet technology could change the shear layer property and vortex shedding characteristics of rear window,achieving to destroy the rear window acoustic resonance feedback loop.This thesis was to use constant jet flow velocity on the Bpillar and C-pillar to avoid vortex shedding or prevent vortex to hit the C-pillar.The simulation results showed that the C pillar oblique jet could reduce the noise amplitude up to 12 dB,and the B pillar oblique jet could reduce the noise amplitude by 10 dB,both cases have significant noise reduction effect.In addition,this thesis also explored the relationship of jet velocity,jet angle and buffeting characteristics.The buffeting noise frequency was insensitive to the jet velocity and angle,but the peak value was greatly influenced by the jet velocity and angle.Therefore,proper jet velocity and angle could have good noise reduction effect.