Aerodynamic Analysis of a Motorcycle and Rider on a high speed corner

Giorgio Pagliara and Giuseppe Ganio

 

Pro S3 was founded in November 2001 by three shareholder coming from Polytechnic of Turin University. After 3D CAD design, other services like FEM analysis and CFD simulation have improved the company range of activities. Autovotive, Aerospace, Defense and General Industry are the typical areas where Pro S3 gives best demonstration of its capabilities. Nowadays Pro S3 is a mature reality offering 360 degrees engineering support to its customers.

The analysis of motorcycle race aerodynamics can present a significant challenge, requiring the simulation of many different configurations and positions of both bike and rider. Because of the manner in which the rider maintains stability by leaning into corners, wind tunnel analysis with a rolling road is often impractical. The deployment of CFD within the design process, however, enables such studies to be carried out with relative ease.

The current study demonstrates that, using surface wrapping and polyhedral mesh technology, it is possible to run an aerodynamic optimisation study using only a relatively small 2 CPU machine. Although widely used throughout the automotive and transport industries, the analysis of motorcycle aerodynamics using CFD remains relatively rare; by coupling STAR-CCM+ to the geometry creation and manipulation package Blender, it was possible to perform multiple studies automatically, providing detailed insight into the behaviour of both rider and bike through a high speed corner.

Geometry preparation
The geometry consists of both bike and rider separately. Before constructing a computational mesh the two separate geometries needed to be combined and placed in an aerodynamic domain. This combination was achieved using the blender software, a package primarily designed for 3D rendering and visualisation as opposed to engineering design and analysis. The use of this software presented several challenges as it is not designed to fit into a CAE process and so the generic CAD formats exported from it were not of a high quality and contained many errors preventing volume meshing without surface cleanup.

Fig:01 Slice view of final polyhedral mesh with volume source visible around the bike.

To ensure a suitably high quality surface for aerodynamic analysis, the STAR-CCM+ surface wrapper was used to perform geometry cleanup and repair, as well as connecting the rider, motorbike and ground together. During the wrapping process free edges, self intersections and surface mismatches were removed in order to provide a clean closed surface ready for surface re-meshing and volume meshing.

The computational mesh was constructed automatically using polyhedral cells mesh, surrounded at solid boundaries by three prismatic extrusion layers. Because polyhedral cells fill space more efficiently than tetrahedral elements, fewer cells were required than might otherwise have been needed, significantly aiding the goal of using a small desktop machine to perform such aerodynamic analyses.

Two different configurations of bike and rider were studied firstly a high speed straight line analysis with the bike perpendicular to the ground. Once complete the angle of the bike was changed to represent the position during cornering, the replace mesh feature in STAR-CCM+ allowed an updated geometry to be imported with original mesh settings retained for faster turnaround time.

Fig:02 Different configurations or both bike and ride angle were studied automatically using the mesh replace feature and the
surface wrapper

Results
The results of the simulation predicted that at a straight line speed of 120 Km/h, the motorcycle is well balanced with neither excessive lift or down force experienced. During a turn, however, the rider and bike are at an angle to the ground, generating large amounts of lift and a rolling moment that acts to straighten the bike.

Plots of pressure coefficient show that, during cornering, the rider produces aerodynamic downforce while the bike produces lift. The L/D ratio (lift over drag) ratio of the bike and rider is around 0.4 which may be compared to a typical value of between -3.5 to -2.5 of an F1 car, a difference which is largely accounted for by the lack of any lifting surfaces (front and rear wings) and the effect of rider on the overall aerodynamic performance.

After the preliminary study of the original geometry in both configurations, modifications were made to the bikes shape to improve the L/D ratio with the aim of increasing downforce and reducing drag. The ultimate goal of the optimization study was to produce a negative L/D ratio, as with F1 cars, to help “hold” the bike to the road and so enable faster cornering. Although this was not achieved it was possible to reduce the L/D to 0.1 (compare with 0.06 on the straight) so representing a significant reduction relative to the original configuration which yielded a value of 0.4

Conclusions
One of the key aims of the study was to prove that it was possible to easily produce and turn around aerodynamic studies of vehicles using relatively modest computational resources. By using the surface wrapper and polyhedral mesh technologies in STAR-CCM+ it was possible to take a tight budget and perform studies on a racing motorcycle and rider in multiple configurations. It is the intention of the authors to continue this work and expand it to including engine cooling of the bike whilst further refining the process.