Investigations into the causes and methods of reducing airflow induced buffeting over vehicle rear windows.

摘要

Rear Window Buffeting (RWB) is the low-frequency, high amplitude, helicopter-type sound that results in many 4-door vehicles when driven 30-70 mph with one rear window lowered. This event is a cause for concern among the occupants of an affected vehicle, and may result in great discomfort to the occupants if actions are not taken to reduce the magnitude of the event. In this dissertation, a comparison is made between moon roof buffeting and rear window buffeting, and data are presented where Sound Pressure Levels approaching 130 dB have been measured. This level may actually be causing ear pain in some people.;Various explanations are presented in the literature for the root causes of rear window buffeting. This dissertation presents the results of an investigation to determine the root cause of rear window buffeting in a compact Sport Utility Vehicle. A brief synopsis of the types of flow-induced noises is provided, and three types of wind noises are described. Comparison between the classic spring-mass-damper system is made to the Helmholtz resonator, and the application of the Helmholtz resonator system to a vehicle cabin is discussed. A comprehensive explanation is provided to explain the mechanism resulting in the closely related moon roof buffeting problem. The presence of frequency lock-on is discussed, and data are provided demonstrating lock-on. Data are presented showing how rear window buffeting also exhibits lock-on, and three regions of differing air flow patterns along the side of the test vehicle are described. Data are presented supporting the three region concept and how the various attachments and geometries of the vehicle interact that make some vehicles more susceptible to the rear window buffeting problem. A comparison between the relative magnitudes of Moon Roof Buffeting and Rear Window Buffeting is made and an explanation is provided.;Previously published computational fluid dynamics (CFD) work advocating that the root cause of rear window buffeting is rear mirror vortex shedding is disputed, and an alternate theory for the cause of rear window buffeting is proposed. Evidence obtained from CFD modeling, wind tunnel smoke visualization utilizing still and high-speed photography, as well as on-track testing confirm that the mirror wake is not the cause of the response. Indeed, airflow interference from the mirror wake actually reduces the magnitude of the response.;Mechanical systems experiencing a vibration response to a forcing function can be passively addressed in only three fundamental ways: changing system resonance frequency ωn, changing the frequency of the forcing function ωf, and changing the system damping ratio ζ. Data are presented where each of these parameters are modified and the effect on rear window buffeting is quantified.;Based on the results of this work, conclusions are developed to recommend engineering design modifications that are both economically and practically viable and may result in the reduction of rear window buffeting. Finally, recommendations are made proposing further areas of research into addressing and gaining a more thorough understanding of the rear window buffeting problem.