| In recent years,lots of researches have been done on mechanical noise(such as engine noise,transmission noise,etc.),and effective noise control methods were proposed,wind buffeting noise is becoming more and more prominent with the continuous increase of the practical speed of vehicles.Wind buffeting noise is caused by the coupling of flow instability of shear layer and the air of the vehicle cabin while driving a vehicle with sunroof or window open,which is a kind of aerodynamic noise.Wind buffeting noise has the feature of high strength and low frequency.It directly affects the vehicle ride comfort,and then the driving safety.Therefore,to improve the quality of automobile products,it is very important to predict and analyze the buffeting noise efficiently for automotive manufacture in the designing stage.To provide a reliable theoretical basis and effective research method of decreasing the interior noise and improving ride comfort,a research on a computational method to efficiently obtain buffeting noise sources has been carried out,the characteristic and mechanism of wind buffeting noise was analyzed,and controlling methods of wind buffeting noise were developed.The main research contents are as follows:1.A new zonal scale-adaptive simulation k-ε turbulence model is presented.At low Mach numbers,SST-SAS turbulence model can’t completely inspire LES behavior.To avoid the disadvantage,the second derivative of velocity was added into the original Rotta’s transport equation,von-Karman length scale is defined as the ratio of the norm of the first divided by the norm of second derivative of the velocity vector(times the von Karman constant k = 0.41),k3/2/ε was introduced as turbulence integral length scale in the framework of two-equation turbulence modeling,a zonal scale-adaptive simulation k-ε turbulence model was proposed.This approach uses zones with different cu and c2ε.Incorporated with general non-reflecting boundary conditions at open boundaries of computational domain,a new method to compute the source of vehicle buffeting noise was constructed.Zonal SAS k-ε turbulence model and SST-SAS turbulence model were employed to simulate a rear mirror wind noise,respectively.Compared with SST-SAS turbulence model,zonal SAS k-ε turbulence model can accurately capture complex flow structures and wall pressure pulsation characteristics,and thus the accuracy of numerical simulation was improved.Zonal SAS k-￡ turbulence model was employed to simulate a simplified cabin buffeting noise.Compared with the available experimental data,the accuracy of the numerical calculation method based on the zonal SAS k-￡ turbulence model was further validated.The results show that zonal SAS k-ε turbulence model can accurately predict the frequency and amplitude of the self-oscillation at low Mach numbers.Compared with LES turbulence model,zonal SAS k-ε turbulence model can reduce dependence on grid numbers.Therefore,zonal SAS k-ε turbulence model can meet the demand of high efficiency in the process of vehicle design and development.2.Research on mechanism and characteristics of wind buffeting noise of a simplified cabin.So far,generation mechanism of vehicle wind buffeting noise is still not very clear.To reveal the mechanism and characteristics of self-oscillation induced by a simplified cabin flow at low Mach numbers,numerical simulation of different inflow velocity was conducted.The switching from high mode to low mode is accurately captured as the inflow velocity increases.The results indicate that vehicle wind buffeting noise is a joint result of aeroacoustic feedback and Helmholtz resonance.The frequency distribution reveals that vehicle wind buffeting behavior is a quasi-harmonic or harmonic self-oscillation.Locked-in flow tones due to shear flow over a simplified cabin were investigated by analyzing fluctuating pressure induced by different vortex shedding modes.The buffeting noise is the strongest when the mode frequency of the single vortex of the flow instability is "locked in" the Helmholtz resonant frequency of the simplified cabin.The perturbance in the cabin and convection speed of the vortices were analyzed,the characteristics of buffeting noise of a simplified cabin were revealed,which provide the basis for analysis of sunroof buffeting noise and window buffeting noise.3.Research on sunroof buffeting characteristics of a car and the instability of shear layer.The instability of the shear layer plays an important role on the generation of sunroof buffeting noise.Sunroof buffeting characteristics of a car were investigated numerically at a certain speed range.The results show that sunroof buffeting occurs in specific frequency range,and the shear layer exhibits at most two vortex modes.When the shear layer exhibits a single vortex mode,sunroof buffeting noise is stronger.The generation of sunroof buffeting noise can be attributed to combined action of acoustic feedback and Helmholtz resonance,while the resonance mechanism plays a leading role,this also verify the buffeting mechanism obtained from the study based on a simplified cabin.For the case of the strongest sunroof buffeting noise,the root mean square of the velocity fluctuations in the streamwise and transverse directions are all the biggest,respectively,which indicate that the disturbance of shear layer is amplified by Helmholtz resonance and thus generates the strongest buffeting energy.The instability and momentum growth of the shear layers over the sunroof opening were analyzed using a linear inviscid instability analysis.The result shows that streamwise mean velocity profile of the shear layer take on hyperbolic tangent distribution.Combined with the initial momentum thickness and the dimensionless streamwise fluctuation wavenumber,numerical results by a hyperbolic-tangent mean velocity profile of the shear layer were used to calculate the velocity fluctuation gain factors.When the gain factor exceeds a threshold value,it will cause the obvious sunroof buffeting noise,and thus sunroof buffeting characteristics was deeply analyzed.4.Research on buffeting noise reduction mechanism and optimize control of a grooved spoiler.A grooved and uniform spoiler test platform was built.Wind tunnel experiments were performed to investigate the surface pressure field downstream of uniform and grooved spoilers to observe the effects of the grooves on the surface pressure.The wind tunnel test results indicated that the presence of grooves on the spoiler can bring forward reattachment point,and give rise to a decrease in the coherence of the wall pressure,and induce a monotonic spanwise phase lag distribution at low frequencies typical of flow induced buffeting.Based on the analysis on the buffeting reduction mechanisms,a modern optimization algorithm was introduced to optimize the shape of the grooved spoiler.The grooved spoiler’s installation angle,groove width and depth were taken as design variables.Based on twenty sampling points with the method of design of experiment,a Kriging model was constructed.Then,the genetic algorithms were used to optimize the shape of the spoiler,and a set of optimal parameters was obtained.Simulation results showed that the monitoring point’s SPL of buffeting noise reduces 21.6%after the spoiler optimization,compared with that of the original spoiler.The noise reduction mechanism of sunroof spoiler and optimization method of spoiler can provide guidance for the design of sunroof spoiler.5.Analysis of window buffeting noise characteristics of a car.Road acoustic measurements in a car were conducted.Experiment results show that the characteristics of wind buffeting noise caused by different side window opening patterns are different.When a single rear window open or two rear windows open at the same time,window buffeting noise exhibits multi harmonic oscillation modes.In other cases,the sound pressure level spectrum shows a single peak value.The buffeting noise is the strongest when the two rear windows simultaneously open.Through the CFD numerical simulation,from the point of view of flow structure and window shape and energy dissipation,generating mechanism and characteristic difference of window buffeting noise are revealed.6.Passive control measures of a car side window buffeting noise.The effect of several measures to reduce front window buffeting noise was assessed by CFD numerical simulation.The analysis focused on the suppression effect of the front rain weatherstrip.The results show that the front rain weatherstrip changed A-pillar vortex shedding trajectory,and thus reduced the driver’s ear pressure fluctuation.On this basis,the optimization algorithm was employed to optimize the shape of the weatherstrip.For the more serious rear windows buffeting noise,three suppression measures were also numerically evaluated,respectively.The road test results showed that the grooved spoiler on the B pillar can eliminate the oscillation characteristics of multi harmonic.The results show that these measures can effectively reduce window buffeting noise.In conclusion,the method based on zonal scale adaptive simulation k-εturbulence model offers an effective numerical method for analysis of vehicle buffeting characteristics.This paper systematically conducts an intensive study on wind buffeting noise,and contributes valuable reference for thinking and approaches to the research on this subject for engineering application. |
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