Alistair Duff, for technical assistance Mr. Andrew Crockett, for strain gauge assistance
Figure 22 - Velocity Vector Plot on A-Pillar, 60mph
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- 17.7AERO ACOUSTIC NOISE SOURCES
- 18. DISCUSSION AND FURTHER STUDY
- 18.2 OPTIMIZATION EFFORTS
- 18.3 A-PILLAR
- 18.4 AEROACOUSTICS
- 19. CONCLUSION
- REFERENCES
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Figure 22 - Velocity Vector Plot on A-Pillar, 60mph Similar to the flow on the Flat Plat in the previous analyses, the plot shows a region of low velocity flow on the side window aft of the mirror (as shown in blue). The plot also details the area of flow acceleration over the A-Pillar and the flow stagnation present at the bottom of the windshield. 17.7AERO ACOUSTIC NOISE SOURCESThe sources of broad band noise production on the car surfaces were plotted as shown in Figure 23. 
Figure 23 - Contours of Surface Acoustic Power Levels on whole car, 60mph As expected the sources of the largest levels of broad band noise creation occur at the sharp changes in surface curvature such as at: the intersection of the front of the car and the bonnet, the wheel fairings, the A-Pillar and the wing mirror. It must again be stated that these plots only serve as an indication of the noise created as the broad band noise model accuracy is limited. The model also does not determine the strength of the noise propagation, which could be determined through transient analyses. 18. DISCUSSION AND FURTHER STUDY18.1 WIND TUNNEL TESTINGThe wind tunnel testing conducted for the purpose of this study provided useful information for the comparison and validation of the CFD results. The sources of error were attributed to the fact that the values of velocity determined through the use of the Pitot tube were consistently greater than the actual velocity over the wing mirror, due to the position of the Pitot tube at the flow inlet. To improve the wind tunnel testing method it may have been beneficial to place the Pitot tube nearer the position of the wing mirror in the test section. This would have provided a more realistic value of the flow velocity in this region and therefore produce more accurate drag readings. It may have also been useful to test the wing mirror on a flat plate rather than on the A-Pillar mock-up. This may have resulted in better correlation between the wind tunnel and CFD drag values. However, without the need for further wind tunnel testing, the effects of the cardboard A-Pillar structure on the wing mirror drag cannot be quantified.
The second optimization effort was also successful in reducing the drag and the sources of high acoustic power creation on the wing mirror surface. With more time available it may have been possible to apply further changes to the wing mirror geometry to further improve and enhance the wing mirror’s performance. One change that could have been made would have been to widen the channel running through the wing mirror mount, to further reduce the flow stagnation on the leading faces. It may have also been beneficial to apply general streamlining alterations to the wing mirror mount to reduce the bluffness of the body thus reducing drag. This would have also smoothed out the sharp changes of surface curvature on the mount and would therefore (in theory) reduce the amount of high level aero acoustic noise created at the edges. With the restrictions on available computational power, it was only really feasible to model the optimizations accurately on a small scale such as the flat plate model. If more processing power was available it would have been useful to model the optimizations on the full car model to gain a more accurate real life representation of the optimization effects on the wing mirror/A-Pillar flow interaction. However, the comparison between the wing mirror drag results for the flat plate model and the whole car model showed that although the values were not precisely concurrent there was very little difference. This would suggest that although the effects of the optimization efforts on the whole car cannot be reviewed when modelling the flat plate configuration, it still represents an accurate method for the study of optimizing the wing mirror in isolation. It was also encouraging to observe similar flow patterns downstream of the wing mirror in both the flat plat and the whole car models, with both exhibiting a region of low velocity flow on the surfaces aft of the wing mirror.
The reduction of lateral flow over the windshield may have also resulted in a reduction of the acceleration of the flow over the A-Pillar onto the wing mirror, therefore reducing the flow velocity over the wing mirror and consequently the wing mirror drag. 18.4 AEROACOUSTICSAlthough using the Broad Band Noise model in Fluent proved useful for assessing the ‘noisy’ features on the wing mirror and the A-Pillar, its accuracy was fairly limited. Once again, the use of greater computational power would have provided a means of applying a finer mesh and employing one of the more sophisticated acoustics models that can be used with transient analyses in Fluent. The use of such a model could be utilised to determine the nature of sound propagation from the A-Pillar and could potentially be used to find the levels of noise audible to the driver/passenger. 19. CONCLUSIONAlthough the limitations on the work and progress of this project with regards to available computational power have been detailed and discussed, it is apparent that the fundamental aims of the project were met. The wind tunnel testing provided a means of validating the results provided by the CFD analyses and successful design optimizations were applied to the wing mirror geometry. An analysis of the flow over the whole car which detailed the flow over the A-Pillar was also successfully performed. Upon completion of these initial aims it was then realised that with additional time, there would be numerous possible avenues of study which could be focussed on the wing mirror/A-Pillar aerodynamics. The most constructive path of study would most likely be to alter the A-Pillar and windshield design in an effort to minimize lateral flow over these regions. A reduction in lateral flow over the A-Pillar could result in not only a reduction in the wing mirror drag due to decreased flow acceleration onto the wing mirror, but also a reduction in the induced drag on the whole car as a consequence of minimizing the vortex shedding from the A-Pillar. Recognizing the possibilities of further improving the wing mirror and A-Pillar design emphasizes just how useful CFD software can be when the necessary computational power is available and the correct expertise is applied. REFERENCESHucho, W.H, Aerodynamics of Road Vehicles. 4th Edition, 1998. P196 & 294 http://mayfco.com/tbls Stickland, M. Aerodynamic Performance Course Notes. The University of Strathclyde Dolek, O., Ozkan, G., Ozdemir, I.B. Structures of flow around a full scael side mirror of a car with relevance to aerodynamic noise Stickland, M. Flight & Spaceflight 2 Course Notes. The University of Strathclyde Modelling Turbulent Flows (Presentation), Introductory FLUENT Training, www.fluentusers.com Sovani, S. Acoustics Modelling, 2004 CFD Summit, Fluent Automotive UGM Download 3.93 Mb. Share with your friends:
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