TITLE:
Side-View Mirror Vibrations Induced Aerodynamically by Separating Vortices
AUTHORS:
Shigeru Ogawa, Taiki Kawate, Jumpei Takeda, Ittetsu Omori
KEYWORDS:
Side-View Mirror, Aerodynamic Force, Unsteady Flows, CFD Simulation, Resonance, Shedding Vortices
JOURNAL NAME:
Open Journal of Fluid Dynamics,
Vol.6 No.1,
March
17,
2016
ABSTRACT: 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.