Drag and Dirt Deposition Mechanisms of External Rear View Mirrors and Techniques Used for Optimisation

This paper gives details of the drag and dirt deposition mechanisms related to rear view mirrors. The major design parameters affecting mirror-generated drag and dirt deposition are described. A detailed analysis of the mirror noise properties is not covered for reasons of brevity.

A range of test methods is also described which can be successfully used in the mirror optimisation process.

The detailed drag breakdown of several rear view mirrors has been made by use of a combination of balance and pressure measurements. The drag breakdown gives an insight into the drag mechanisms and identifies the critical geometry parameters.

It is concluded that the relatively high level of drag experienced by some of today's mirrors is primarily the result of premature tip separation and/or an unnecessarily large mirror foot. A level of drag close to the minimum possible, for a given mirror glass area, can be achieved by optimisation of the tip and foot areas.

The main mirror-related dirt deposition mechanisms are found to be:

1. 1
   The local dirt/spray mixture in the vicinity of the side screen may be increased due to droplet impact, or ‘splash’ on the mirror surface, with resulting droplet break-up and rebound.
2. 2
   Dirt/spray that is deposited on the mirror housing by the droplet ‘splash’ or ‘spread’ regimes flows over the surface and collects at the rear of the housing. The dirt/water mixture may then be released as droplets within the mirror wake. The direction that released droplets take is influenced by the mirror wake direction.

The mirror wake structure and direction has been measured and visualised for several rear view mirror shapes and is correlated to measured mirror forces and geometry parameters.

A mirror design with the combined properties of low drag and low dirt deposition is possible and has been demonstrated by both wind tunnel and test track methods.