Author(s)

Shigeru Ogawa, Taiki Kawate, Jumpei Takeda, Ittetsu Omori

Department of Mechanical Engineering, National Institute of Technology, Kure College, Hiroshima, Japan.

While driving a car at high speed cruising, the mirror surface of side-view mirrors happens to vibrate. The vibration often leads to image blurs of objects reflected in the mirror. Once the phenomena happen, drivers cannot clearly identify the approaching vehicles from the rear. The paper aims to clarify the vibration modes of side-view mirror experimentally and to capture forces on the mirror surface induced by separating vortices around the mirror numerically. Experimental study clarified two findings. One is that the mirror has the primary natural frequencies of 25, 30 and 33 Hz. The other is that vibrations of the mirror increase in proportion to flow velocity and their frequencies have peak values at 120 and 140 km/h. The frequencies of the mirror vibration coincide completely with the primary natural frequencies. In order to capture the external forces vibrating the mirror surface, numerical study was performed by unsteady air-flow analyses. Relationships between flow velocity fluctuations close to the mirror surface and pressure fluctuations on the mirror surface were investigated. It was found that the two power spectra have peak values at the same frequency of 24.4 Hz at 120 km/h. This shows that flow velocity fluctuations with the frequency of 24.4 Hz affect directly pressure fluctuations on the mirror surface. Numerical analyses clarify that the frequencies of shedding vortices are 24.4 Hz at 120 km/h and 28.3 Hz at 140 km/h. The frequencies of mirror vibration are very close to those of flow fluctuations. This shows that the frequencies of the mirror vibration have much to do with the frequencies of the forces induced aerodynamically by vortex shedding. Therefore it follows that image blurs at high speed cruising are caused by resonance phenomena that the mirror surface resonates with the frequencies of shedding vortices around the mirror.

Side-View Mirror, Aerodynamic Force, Unsteady Flows, CFD Simulation, Resonance, Shedding Vortices

Ogawa, S. , Kawate, T. , Takeda, J. and Omori, I. (2016) Side-View Mirror Vibrations Induced Aerodynamically by Separating Vortices. Open Journal of Fluid Dynamics, 6, 42-56. doi: 10.4236/ojfd.2016.61004.