Analyzing the Aerodynamics of a Competition Kart

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dynamics Magazine V2.02

Viewed from trackside, kart racing shares absolutely none of the glamour associated with the multi-million dollar world of Formula 1. However, with the fastest karts capable of attaining speeds in excess of 160mph (250kph), when viewed from a kart seat located just a few inches from the ground the racing is every bit as competitive. Kart racing is often seen as the first step on the ladder of a motor racing career, with almost all top F1 drivers, from Michael Schumacher to Lewis Hamilton, having graduated from competition karting in the early stages of their careers.

From an engineering point of view, F1 is dominated by cutting edge technology with aerodynamics being a key consideration in producing a championship winning car; the F1 industry was an early adopter of CFD technology, and the needs of the industry continue to play a key role in the development of CAE tools.

Obviously, by its very nature, a small open wheeled kart constructed largely from steel tubing is never going to be the most aerodynamic of vehicles. However the authors of the current study were confident that, using CFD simulation, they would be able to better optimize the configuration of the kart and help to squeeze the extra couple of hundredths of a second that could ultimately be the difference between winning and losing a race.

The Kart
The kart in question is a regular small competition kart with a typically high power to weight ratio. It is equipped with a water cooled engine mounted on a tubular chassis and is connected to the wheels via a chain with no differential or suspension. A typical kart track contains many tight bends, but despite this, the average speed of a race is 90kph with a top speed of around 130kph.

In keeping with the “rough and ready” nature of the sport, the kart itself had no original CAD drawings, so a series of digital images of the kart were taken including measurement references and were then imported into CATIA and scaled accordingly. Finally, a mannequin representing the driver was inserted into the seat of the kart to complete the assembly.

Once complete, the model was imported into STAR-CCM+ with a bounding aerodynamic box. Boundary conditions were set up and the model re-surfaced ready for generation of a polyhedral volume mesh generated containing approximately 2 million cells. The automated nature of the STAR-CCM+ meshing was particularly important in this study as multiple configurations were analyzed, with proximity refinement heavily used to ensure suitable mesh density in areas of small clearance e.g. low ground clearances of the tubular frame.

The steady state flowfield around driver/kart geometry was analyzed with a 90kph inlet velocity and ground and wheel movement to match. The standard k-? model was used throughout with the radiators on the kart modeled using the porous media formulation.

Three different configurations were simulated with modifications made to side-pod protection and radiator position in line with CIK-FIA regulations (Commission International de Karting - Federation International de l’Automobile). The first configuration analyzed was the existing, commercially available model with two modified versions, type 08a, which has a traditional radiator positioning, but at a shallower angle to the flow and type 08b, which has two smaller radiators positioned either side of the driver, each with an inlet duct. Slight modifications to the faring, front bumper and side pods are also implemented on type 08a and 08b.  

    Fig:03a Streamlines showing the improved heat exchange of both radiators, thanks to the good quality of air flowing through them.     Fig:03a Streamlines showing the improved heat exchange of both radiators, thanks to the good quality of air flowing through them.      Fig:03b Streamlines showing the improved heat exchange of both radiators, thanks to the good quality of air flowing through them.     Fig:03c Streamlines showing the improved heat exchange of both radiators, thanks to the good quality of air flowing through them.
Fig:01 Fig:03a Fig:03b Fig:03c

Fig:03a-c  Streamlines showing the improved heat exchange of both radiators, thanks to the good quality of air flowing through them.

Results
A key indicator for success in the study was an overall decrease in drag, increase in down-force (vital for the tight twisting tracks seen in karting), improved radiator efficiency and finally better cooling of the brake system.

The results (shown in Figure 3) immediately demonstrated that the modified front bumper and faring immediately have a significant effect on the pressure field around the kart:

  • Less force is generated over the helmet of the driver due to the deflection of the air due to the new faring.
  • This masking action also shows an overall reduction in pressure on the drivers body, something likely to make driving easier at higher speeds.
  • Less influence of the airflow of the front and rear wheels again due to the deflection of airflow by the front bumper and re-designed side pods.

The modified radiator design for configuration 3b shows an improvement in the “quality” of air through the two radiators due to the ducting and their higher position. Figure 3c demonstrates that both new configurations cause a slight increase in drag; this is specifically due to increased airflow (and hence drag) through the radiators, over the engine and the brakes. It is also seen that a significant increase in downforce is seen on the newer configurations, indeed it is almost doubled in the ‘a’ configuration.

Conclusion
Although an increase in drag was seen in the new configurations, this is offset by the improved downforce and better cooling of the engine and brakes. It is clear that, although an improvement is seen in the new configuration, it is not yet optimized. Significant design changes have yet to be carried out on the bumpers which have been developed to conform to international regulations which are open to interpretation. This means that the design of such karts is constantly changing and evolving year on year



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