PROJECT REPORT: VA170

Optimisation of a Motorbike Front Windshield and Fairing Using Sculptor

 

Haakon Dahle Smith
22 October, 2004

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SUMMARY

This report presents the results of an optimisation carried out on the front windshield and fairing geometry of a Yamaha R1 motorbike to reduce drag. Sculptor was used to directly deform the CFD case file therefore removing the need to re-mesh.

The fairing geometry was controlled using four parameters, which were optimised using a response-surface method. Twenty-five runs were initially used to define the preliminary response surface and thereafter a total of four cases were run attempting to optimise the fairing for low drag.

The motorbike and rider were modelled in an upright position with rotating wheels and a rolling road. The model was run full scale at a speed of 27.8 m/s (100 kph)

The optimised front fairing / windshield geometry reduced drag by 3.75 N, 2.5 % of the total drag.

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CONTENTS

SUMMARY

1. INTRODUCTION

2. RESULTS

3. ANALYSIS AND CONCLUSION

4. FUTURE WORK

APPENDIX A: Model Setup

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1. INTRODUCTION

This report follows on from previous studies carried out by ACFD on a Yamaha R1 motorbike, VA109 and VA27. The previous reports noted that although there is no clear requirement for downforce in motorbike racing the reduction of drag is vital in improving the performance of the bike.

A 2.3 million cell case was taken from VA109 and Sculptor was used to deform the fairing / windshield geometry in the case file directly. A response surface methodology was then used to optimise the deformation parameters.

Figure 1.1 identifies the front fairing and windshield (coloured in blue) that were modified as part of the study. Figure 1.2 shows the surface mesh on the bike.

Figure 1.1: CAD geometry of motorbike with fairing / windshield in blue

Figure 1.2: Surface mesh of bike

During the optimisation there were four parameters used that would alter different aspects of the fairing / windshield. The parameters were:

• Height of the rear section of the windshield glass.
• Height of the front and middle section of the windshield glass.
• Width of the top section of the fairing.
• Height of the top section of the fairing.

An overview of the deformations that were carried out is shown visually in Figures 1.3. The “net” surrounding the fairing and windshield, used to make the geometry deformations, can also be seen. An MPEG movie of the deformations is available on the web copy of this report. Once these parameters had been defined the deformations could be made interactively – directly to the case file without the need to re-mesh. Note that Figure 1.3 shows the limits used in the study.

Figure 1.3: The maximum and minimum deformations of the 4 parameters

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2. RESULTS

The response-surface method required 25 geometries to be analysed to define the initial response-surface for the 4 parameters. A further 4 cases were subsequently recommended by the response-surface method in order to search for an optimum solution based on the performance criterion of lowest drag.

The man time used in setting up of all 30 runs in Sculptor was very small in proportion to the computational time taken to solve the case. The initial ASD volume allowing deformations to take place took 2-3 hours to setup, thereafter each change in the fairing geometry only took 2-3 minutes plus 10 minutes to save each case. A summary of all the drag and lift forces that were generated can be seen in Table 2.1 and are plotted in Figures 2.1 and 2.2.

Each case made use of the baseline data file as an initial flow solution and was therefore only required to be run-on 1000 more iterations over the previous 2500 of the baseline.

Table 2.1: Summary of forces over all runs

Figure 2.1: Variation of Lift Over All Runs

Figure 2.2: Variation of Drag Over All Runs

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3. ANALYSIS AND CONCLUSION

The run chosen for the best drag characteristics was RUN-it-04. It lost 3.76 N of drag (2.5% of total) whilst gaining 6 N of lift (28%). Figure 3.1 shows the difference in fairing / windshield geometry between the baseline (blue) and RUN-it-04 (red).

Figure 3.1: Difference in fairing / windshield geometry between baseline and RUN-it-04

Table 3.1 shows a summarised version of the full force table (Table 2.1) highlighting the differences in forces between the baseline and RUN-it-04.

Table 3.1: Summary of forces for baseline and RUN-it-04

Table 3.2 shows the difference in cooling between the baseline and RUN-it-04. This is represented by the drop in pressure, DP, across the radiator. It can be seen that the change is negligible.

Table 3.2: Summary of cooling flow for baseline and RUN-it-04

Figure 3.1 shows that the optimised geometry has a slightly lower rear section to the glass whilst the front and mid section of the glass has been raised more significantly. The width of the top section of the fairing has been narrowed and the height of the top section has also been lowered.

Figure 3.2 shows a comparison of contours of static pressure coefficient acting on the motorcycle and rider for the baseline and optimised cases. It can be seen on the fairing / windshield of RUN-it-04 that there is a larger area of lower pressure on the windshield (coloured in dark purple) and therefore a lower drag force. The corresponding area on the baseline case (coloured in blue) is at a higher pressure, meaning that there is a greater drag force on the component.

Figure 3.2: Banded contours of static pressure coefficient for baseline and RUN-it-04

The following conclusions can be drawn from the study:

• The time taken for all of the Sculptor stages of the optimisation process was approximately 1 man-day, a very small percentage of the total time of the project. This method eliminated the need to re-mesh the case. A manual fairing modification would have taken ¾ days for each run.
• Increasing the height of the rear section of the windshield glass increases the downforce but also increases the drag.
• Increasing the height of the mid and front section of the windshield glass reduces the downforce but also reduces the drag.
• Narrowing the central section of the fairing reduces the downforce a little but the also reduces drag.

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4. FUTURE WORK

Further work to be investigated could take the form of:

• A different Sculptor ASD volume, which would deform different areas of the fairing, to further reduce drag.
• Validation the reports findings through manufacture of the fairing and subsequent test in a wind tunnel or track test.
• Optimisation on other components e.g. radiator ducts, rear wing, riders helmet.
• Optimisation for a combination of lift, drag and cooling could be investigated.

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APPENDIX A: Model Setup