Unsteady flow over automotive side-view mirrors may cause flow-induced vibrations of the mirror assembly which can result in blurred rear-view images, adversely affecting marketability through customer comfort and quality perception. Prior research has identified two mechanisms by which aerodynamically induced vibrations are introduced in the mirror. The first mechanism is unsteady pressure loading on the mirror face due to the unsteady wake, causing direct vibration of the mirror glass. The second mechanism, and the focus of this study, is a fluctuating loading on the mirror housing caused by an unsteady separation zone on the outer portion of the housing.
A time-dependent Computational Fluid Dynamics (CFD) methodology was developed to correctly model mirror wake behavior, and thereby predict flow-induced mirror vibration to improve performance estimations. The unsteady CFD methodology utilizes Detached Eddy Simulation (DES) to resolve the turbulent shear layer and wake regions around the mirror. To validate the methodology, qualitative flow characteristics and quantitative results observed in the CFD simulations were compared to unsteady PIV and surface pressure measurements obtained in wind tunnel tests.
An agreeable correlation was developed between CFD and PIV test results for the qualitative separation and wake behavior. The quantitative frequency component of the fluctuating flow was captured by CFD and showed a reasonable correlation in both frequency and magnitude to the measured surface pressure data. Additionally, a correlation was developed between mirror vibration frequencies obtained using accelerometers and frequencies observed in both surface pressure data and CFD simulation. Based on the correlated results, the CFD methodology provides a predictive foundation for evaluating mirror vibration design criteria and is an incremental step towards eliminating the need for prototype tooling and physical testing to guarantee performance target achievement as part of the development process.