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Automotive: Greenhouse Wind Noise

Get the Inside Story on Wind Noise

Reducing vehicle interior noise levels to achieve driver and passenger comfort is at the forefront of concerns for acoustic engineers in the automotive industry. Great strides have been made in reducing powertrain and road/tire noise, such that wind noise has become the dominant noise source during highway driving, directly impacting consumer brand loyalty and sales. A significant amount of engineering time and product cost is therefore being invested in achieving interior noise targets to remain competitive in quality. Typically, however, automotive OEM’s are not able to assess wind-related interior noise until late in the development process when high quality verification prototypes are built. At this point it may be too late to efficiently resolve noise problems, thereby requiring design re-work, additional sound package, or acoustic glass attenuation—all expensive and time consuming solutions. To reduce the associated costs as well as development times, there is a strong need for a reliable numerical prediction capability for use early in the development process and throughout the program life cycle.

PowerACOUSTIC Acoustic Simulation
Measured cabin SPL dB (red) compared to PowerACOUSTICS prediction (black) greenhouse interior noise graph
PowerACOUSTIC Acoustic Simulation
Excellent correlation cross-plot of predicted interior wind noise from PowerACOUSTICS versus experimental results for the 500 to 4000Hz octave bands from 30 test cases.

             

TECHNICAL CHALLENGES

PowerACOUSTIC Acoustic Simulation

For a vehicle traveling at highway speeds, turbulent flow provides a distributed force excitation on the greenhouse (e.g. side windows, windshield) panels, generating an acoustic field also acting on the panels. The greenhouse noise sources are complex transient flow structures produced by flow separations and vortices resulting from various exterior geometry features such as the cowl, A-pillar, or mirror assembly. Accurate prediction of greenhouse noise sources requires predicting the time varying flow structures and resulting wall pressure fluctuations on the greenhouse panels, including effects of small geometric details. The turbulent excitation, panel vibration and acoustics are of widely varying length scales (i.e. wavenumber spectrum), providing a significant technical challenge to predict accurately over the important frequency range. Sound transmission of the exterior acoustic field to the interior is particularly important near the acoustic/structure coincidence frequency and appears to be significant even though the turbulent wall pressure amplitudes far exceed those of the acoustic pressures.

EXA SOLUTION

Exa’s PowerFLOW® solver coupled with PowerACOUSTICS® offers a complete solution for simulating all important wind noise sources, and predicting their contribution to the full vehicle interior noise. PowerFLOW predicts transient flow around the vehicle, including highly accurate prediction of key noise-producing flow structures (such as the A-pillar vortex and mirror/pedestal/sail wake which directly impact the front side glass). PowerACOUSTICS converts the resulting time-domain pressure signals everywhere on the panels into structural power inputs, used as input to a fast-running Statistical Energy Analysis (SEA) model that predicts the noise inside the cabin. This approach quantifies the wind noise contribution coming from different panels at driver/passenger head space locations inside the vehicle. The predicted spectra may be converted to audio files for comparative listening to the effects of various design options. Exa’s complete solution goes beyond what is practical in physical test by enabling engineers to gain insight into behavior of key turbulent flow structures and corresponding greenhouse noise sources for targeted shape design improvement. This new, industry-first capability enables analysis of potential noise reduction solutions including: exterior shape changes, glass properties, and interior acoustic package modifications on the wind noise characteristics of a vehicle design early in the development process—before costly physical models or prototypes are built.

PowerACOUSTIC Acoustic Simulation
Interior noise level at the driver's head space (red for experiment, black for PowerACOUSTICS)

EXA SOFTWARE USED FOR THIS APPLICATION

Simulation Preparation: 
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TECHNICAL REFERENCES