Press Room - CFD for fun

	CFD for ‘fun’

	Alex Leong, Ford Motor Company, UK

	The original concept of soapbox racing is simple - a homemade car built from orange crates, tin sheets, buggy wheels and almost everything of ‘junk value’, but it can only be powered by gravity alone. The name ‘soapbox derby’ was christened back in 1933, when a newspaper photographer called Myron E. Scott encountered three boys racing homemade carts down a back street in Dayton, Ohio. He was fascinated by the amusing spectacle, and arranged a coasting race for the boys and their friends. In the same year, with the financial backing from his boss, Mr. Scott organized another race. This time, 362 kids showed up with their homemade cars and the event attracted more than 40,000 people watching along the hill. Very soon, the Derby blossomed into a fiercely contested competition across North America. At its peak, the All-American Soapbox Derby attracted 25,000 entrants in 120 races and drew an estimated crowd of 1.5 million people nationwide. In Britain, there was also a strong following in this sport and the National Soapbox Association (NSBA) organized annual championships until 1994. In 2000, the organization committee of the Goodwood Festival of Speed, which is already acknowledged to be the world’s biggest annual historic motor racing event, decided to adopt soapbox racing as one of its major events. In that year, the Goodwood Gravity Racing Club invited 20 teams from both race and road car manufacturers, plus a handful of enthusiastic privateers, to take part in the race. And since then, the Downhill Soapbox Challenge has become one of the most talked-about events in the Festival. Although the Soapbox racing started from a very humble beginning, the modern version is much more sophisticated. In the Goodwood downhill soapbox challenge, the design and construction of the ‘gravity-powered vehicles’ are strictly regulated. Apart from stating that no pedal power or push-starts are allowed, the six-page rulebook also specifies things such as the overall size, shape, driving positions, etc. There are two classes of soapbox, either open-topped roadsters or roofed streamliners. The competition comprised of two timed runs for each competitor with the result decided by the lowest sum total time of both runs. Although the downhill course is just over 1.1 km long, it will take most of competitors about 80 seconds to descend and the vehicle will notch up speeds in excess of 95 km/hr out of the final corner. The aerodynamic efficiency, low friction losses and handling finesse are the critical factors of a winning design. Although this may not be the organizers original intention, with only top engineering firms being invited to take part, the race is not just about winning but the prestige of the company is also at stake. As a result, the competition is fierce and intense. However, this is also a good opportunity to train the new recruits (of those companies) who may not have been involved in any competitive projects before. At Ford, most of the team members are drawn from first and second year engineering graduates who undertake projects such as this to further their knowledge of the company and engineering skills. This also reflects in the name adopted by Ford’s entry: Centennial EGG box. The acronym of EGG stands for Engineering Graduate Group and 2003 was also the centennial year of the company. The Ford team consisted of 14 core members (the author is one of the senior members who provided critical support to the team). Each member was assigned to a group that was responsible for certain aspects of the vehicle, such as the design and construction of the body, wheels, brakes, etc. The streamlined body is built from carbon fiber with a Nomex constructed chassis. The wheels used in this design are 20-inch conventional spoke wheel with wheel covers to reduce aerodynamic drag. However, due to package and cost restraints (a familiar concept within a car company!), it was not possible to use CFD to conduct a complete design iteration exercise. Instead, CFD was used as a tool to study the airflow around the vehicle and to provide quantitative data for further improvement of the basic design. In the CFD study, the full sized model of the vehicle was simulated at a zero yaw angle (i.e. on level ground). Only half of the computational domain was meshed and a symmetry boundary condition was applied along the center plane of the vehicle. In order to provide suitable inflow and outflow conditions, the flow domain was extended to an upstream and downstream location approximately 0.5 and 1.5 times the vehicle’s length, respectively. As an open-topped design, the driver would be sitting in an inclined position with his body partially exposed to the airflow. To take this into account, the CFD model includes the driver and his helmet. However, the parts of the driver’s body that are shielded by the vehicle were excluded. The ground and the body of the vehicle (plus the driver) are treated as fixed wall boundaries, but a moving wall boundary (with a rotating speed corresponding to the forward motion of the vehicle) was applied to the wheels. To simulate the boundary layer flows around the wall regions, two layers of prism cells were created which covered all the wall surfaces. Trimmed cell technology was used to create the mesh. Based on a near wall cell size of 8mm and additional refinement in the wake and ground regions, the final mesh contains approximately one million computation cells. A fixed velocity inlet condition and a zero gradient outlet condition were applied. The inlet flow velocity corresponded to the case in which the vehicle was traveling at 95 km/hr. The flow is assumed to be steady state, isothermal, incompressible and turbulent. The widely used high Reynolds k-e turbulence model with default wall treatment was used. The results demonstrate how CFD can be used to enhance the design. In these pictures, the pressure distribution (expressed as pressure coefficient) on the car and driver body is shown as color-filled contours. The CFD predicted that a stagnation region was formed at the front face of the helmet but, as the flow sped around the helmet, it suppressed the flow stream coming through the underside of the vehicle. A small recirculation zone was formed immediately behind the driver’s helmet. In contrast, Figure 01 shows the pressure coefficient and velocity vectors for the case with a small deflector fitted. As expected, the driver and his helmet were shielded by the presence of the deflector. Although the stagnation zone had gone, the strength of the flow within the recirculation zone was much weaker. When this flow was mixed with the stream coming though the underside of the vehicle, the latter dominated the resulting flow and it created a significant amount of turbulence. In fact, the design with the deflector had increased the drag value by 18%. In hindsight, a common sense approach would choose the design with the deflector, however CFD has predicted that if the combined effects of the whole vehicle are taken into consideration, the design without the deflector has a better overall performance. Despite all the efforts, the Centennial EGG box failed to reach the top three at the Goodwood race. After some fine-tuning; the same vehicle won the downhill race at Prescott, Gloucestershire in the same year. Overall, CFD has been successfully used to enhance the aerodynamic performance of the basic design. Furthermore, the valuable lessons learned from this exercise were passed onto the team who prepared the design for the following year.

CFD for ‘fun’