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Vehicle OEMs invest significant effort and cost into reducing noise sources in order to improve the level of passenger comfort. Recent J. D. Power surveys have identified wind noise as a strong influence on consumer perception of vehicle quality. Among the wind noise complaints, vehicle buffeting due to a side window was identified as a significant contributor. Buffeting (also known as wind throb) is an unpleasant, high-amplitude, low-frequency booming caused by flow-excited Helmholtz resonance of the interior cabin. Vortex shedding in the shear layer over the cavity opening (side window) couples with the cabin acoustics, leading to a self-sustained oscillation of shear layer and cabin pressure. Generally, buffeting is not evaluated by OEMs in the early phase of new vehicle development since prototypes are not of sufficient detail to provide realistic aerodynamic and acoustic behavior. A numerical prediction capability of the buffeting phenomena that provides design guidance in the early design phase is therefore highly desired.
Simulating Train Underbody Noise
Accurate prediction of the vehicle buffeting phenomenon requires capturing the bidirectional coupling between the transient shear layer aerodynamics (vortex shedding) and the acoustic response of the cabin. Quantitatively correct prediction of the flow excited resonance requires an accurate prediction of complex transient behavior including turbulent vortex formation, shedding, and convection through the shear layer, and direct capture of the acoustic interaction requires a compressible solver. The acoustic response of the vehicle cabin must also be correctly modeled. In addition, small geometric details can significantly impact the shear layer/acoustic coupling behavior, requiring a high degree of accuracy with fully detailed geometry. These considerations make simulation of vehicle buffeting a challenging problem, especially with regard to predicting the severity (peak levels) for a given vehicle design, and the effectiveness of various buffeting suppression methods. (Graph above: Cabin interior peak SPL (dB) vs. windspeed PowerFLOW predictions versus experiment.)
Exa®’s breakthrough software, PowerFLOW®, includes the coupling of external flow dynamics with vehicle internal cabin acoustics, thereby providing engineers the ability to reliably predict buffeting behavior from either sunroof or side window openings. This capability, which includes modeling the cabin acoustic response, allows assessment of buffeting severity, including onset/offset velocities and peak level, early in the development process when the potential design solution space is greatest. PowerFLOW’s unique technology, an inherently transient and compressible solution, provides accurate prediction of complex flow structures and allows engineers to visualize the flow vortex shedding behavior that occurs over the sunroof/window to provide insight for design improvement, including the details of sunroof deflector geometry and location. Using PowerFLOW, a virtual testing environment can be used to predict buffeting behavior and test potential design improvements in all phases of a vehicle development cycle, including the early phases.