Alistair Duff, for technical assistance Mr. Andrew Crockett, for strain gauge assistance
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- 16. OPTIMIZATION (II)
15.2 RESULTSThe information in Table 1 displays the drag results from both the original and the optimized design; it also details the percentage reduction in drag resulting from the design optimization.
Table 1 - Optimization (i) Results As shown in the table, it would appear that the optimization effort towards reducing the wing mirror drag were successful with a resulting average reduction in drag of just under 5% from a speed of 10mph and upwards. To check that the design alteration did not have a detrimental effect on the wing mirrors surface acoustic power levels, a comparison between the acoustic power level plots for the original and optimized design was performed. Upon reviewing these plots there were no clear signs that the design changes made had any negative impact on the wing mirrors aeroacoustic performance with regards to surface broadband noise sources; even around the area of the new channel. As mentioned previously, some motor vehicle manufacturers design their wing mirrors to be fork mounted in an effort to increase the wind speed aft of the wing mirror. This increased speed results in any water droplets or dirt shed from the wig mirror being deposited on the side of the car further downstream thus reducing the safety risks associated with water or dirt accumulation on the front side window. The altered wing mirror mount in the first optimization attempt can be assumed to be a crude approximation of a fork shape, and therefore should in theory result in a higher velocity flow aft of the wing mirror thus delivering the associated safety benefits. Through studying a velocity vector plot on the surface of the flat plate, a comparison between the original design and the optimized design of the downstream velocity magnitude and distribution could be made (Figure 16). Figure 16 - Velocity Vector Plot on Flat Plate at 60mph From these plots it can be identified that the alterations of the wing mirror mount design resulted in a narrowing of the low velocity wake on the flat plate (as seen in blue). This narrowing of the low velocity wake could result in improved water droplet and debris shedding performance. However this can only be taken as an indication of improved performance and further physical testing (or complex CFD analysis) would be required to validate this assumption. 16. OPTIMIZATION (II)16.1 MODELLINGThe second optimization effort was made with the aim of minimizing the sources of high acoustic power levels on the wing mirror surfaces. As mentioned previously, the primary source of high levels of acoustic power in aerodynamics arise from sharp changes in surface curvature. For this reason it was decided that any recesses or obtrusion on the wing mirror surfaces would be smoothed over. The first changes to the geometry were applied to the joint between the trim piece on the trailing edge of the wing mirror casing and the main wing mirror casing itself. This recess was smoothed over by deleting the recessed faces, creating a new face over the hole and then stitching the faces together to create the new volume. This process was then also applied to the indentation that ran along the front face of the wing mirror casing. Due to the underside of the wing mirror casing featuring several sharp changes in surface curvature, it was decide to create a new face to smooth over the entire underside as can be seen in Figure #. 
Figure 17 - Optimization (ii) As with the first design optimization, this new design was meshed with the same properties of the original geometry and the same boundary conditions were set before exporting for use in Fluent. The model was then run through the same six analyses as before.
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