Press Room - Red Hot Braking News

	Red Hot Braking News

	Stephen Ferguson, CD-adapco

	CFD makes Formula One cars faster. Every major F1 team uses Computational Fluid Dynamics (alongside wind-tunnel testing) as a standard tool for optimizing the aerodynamic performance of their racing cars. Many of the most successful teams, including Renault F1 Team, use STAR-CD for this purpose, endorsing its accuracy, robustness and ability to deal with complex geometries. As they prepare for a title challenging 2005 season however, the CFD team at Renault F1 are using STAR-CD for an altogether different purpose. CD-adapco is proud to announce that STAR-CD plays a critical role in slowing down the cars of the Renault F1 team. In F1, slowing down is an important part of going faster. In a sport now dominated by driver aids, braking remains one of the sternest tests of an F1 driver's ability. In order to shave vital fractions of a second from lap times, drivers are forced to brake deep into the corners, maintaining a fine balance between speed and cornering stability. Without modern aids such as ABS or powered brakes, drivers need to apply braking pressures of up to 100bar through the pedal, while modulating the effort to prevent the inside front wheel from locking. If an F1 driver consistently brakes too early, he'll lose time, and ultimately race position; if he brakes too late then he'll end up out of the race, with his car embedded in a tire wall. " When an experienced racing driver first drives an F1 car, almost without exception, their first comment concerns the power of the brakes", explains Pat Symonds, Renault F1's Executive Director of Engineering, "A modern F1 car can achieve 5.5g under braking whereas even a high-performance road car will probably not reach 1g." The brakes of a Formula One car can provide brutal deceleration, able to slow the car from 200 mph to 50 mph in just 3 seconds, during which time the car has covered just one hundred meters of track. Each application of the brakes results in a huge transfer of energy, as the kinetic energy of the car is converted into heat by the friction of the brakes, and stored in the brake disc as thermal potential energy. The braking temperatures generated are staggering. If they are not properly managed, then rate of brake wear increases significantly, at best reducing the competitiveness of the car, at worst endangering the safety of the drivers. Whereas typical road cars use cast-iron brake discs with an organic brake pad, an F1 car uses a material known as "carbon-carbon" for both the disc and the pad. Composed of carbon fiber within a carbon matrix, carbon-carbon is often used in airplane brakes. Carbon-carbon has two advantages over alternative braking materials; it is much lighter and has a much higher coefficient of friction if used at the correct operating temperature. Maintaining the brakes at this correct operating temperature is therefore critical, and it is here that CFD plays an important role. At Renault F1 Team's Enstone base, the CFD team use STAR-CD to analyze the effectiveness of different brake cooling strategies, as well as providing critical information on how much impact each cooling solution has on the overall aerodynamics of the car. Optimal braking occurs at temperatures above 650°C; once the temperature has reached this critical threshold, the driver is able to press the pedal with confidence that the braking effect will be both immediate and consistent throughout the braking event. Below 400°C braking performance is poor as the brakes experience an effect known as "bite" during which braking is non-linear. Carbon-carbon brakes need to generate sufficient heat before they will operate effectively. " The use of carbon brakes requires a little time to get used to" says Renault F1 Team driver Fernando Alonso, "during the first milliseconds after pressing the brake-pedal; it feels like nothing is happening. This delay is in fact the length of time required by the disc/caliper tandem to reach operating temperature, which increases by 100°C per tenth of a second for the first half-second of braking, after which it can reach up to 1200°C. After that short period, deceleration is immediate, and brutal." Although "non-linear" braking at 200mph might seem like a frightening prospect, a Formula One driver is able to cope with different braking performance on different parts of a circuit almost instinctively. Far more critical from an engineering point of view is the fact that at elevated- temperatures the surface of the brake disc begins to oxidize. Put simply, oxidization means that the surface of the brake disc begins to burn. The rate of oxidization increases rapidly with brake temperature; at temperatures beyond 600°C it becomes the most significant mechanism of wear. Since the temperature of the brakes can reach around 1200°C in a typical braking event, it is obvious that effective brake cooling is a critical factor in maintaining brake performance throughout a race. In order to combat oxidization, the brakes are cooled by air directed through the braking ducts onto the brake pads and through the radial center-vents of the brake-disc itself. Although the ducts do little to cool the brakes during the braking event itself, they work by feeding cool air to the brakes on the high speed straights of a circuit, dropping their temperature below the oxidization threshold and reducing the rate of wear. The amount of brake cooling required is very circuit dependent. Smaller ducts are used at circuits that demand less braking (such as Spa and Suzuka) in order to manage the temperatures of the brakes and achieve the correct balance between high performance and acceptable wear rates. Choosing the correct brake ducts for a circuit is essential, as the added cooling effect doesn't come for free. The penalty for using the largest (most cooling efficient) brake ducts is an increase of aerodynamic drag of around 1.5%. This corresponds to a 1kph reduction in top speed, or the difference between winning and losing a Grand Prix if you choose incorrectly. Circuits such as that used for the Canadian Gilles Villeneuve Grand Prix, which combines very low speed corners with long straights, provide the biggest challenge: finding the right balance between braking performance and aerodynamic efficiency. " Achieving braking performance alone is easy, as is minimizing the aerodynamic penalty, but the secret, as ever in Formula 1, is to achieve maximum braking performance and minimum aerodynamic losses simultaneously", says Matthew Laight, Head of CFD Development for the Renault F1 Team, "STAR-CD is a good tool for this job due to its advanced robust solver and ability to handle very complex geometries.” Whereas, for pure aerodynamics work, CFD complements wind tunnel analysis, in the field of brake cooling, CFD is the only really viable tool, allowing engineers not only to calculate the distribution of heat transfer coefficients, but also to visualize the subtle changes in flow structure that influence the cooling behavior of a particular design. The simulation of F1 brake cooling presents several challenges, not least of which is the geometric complexity of the braking system. Small features, such as the vents in the brake, need particularly fine mesh resolution to model accurately the flow through them. The brakes are also located in aerodynamically "dirty air". In order to simulate the flow through the ducts, it is first necessary to correctly predict the upstream aerodynamics of the car and, in the case of the front brakes, the flow over the front wing. Rear brake studies are further complicated by the influence of the driveshaft, the rotation of which must also be accounted for, as well as the need to represent the complete geometry of the car. Model reusability is also critical. The front wing, brake-disc venting and the brake ducts themselves are all liable to change on a race-by-race basis. Here CD-adapco's trimmed cell meshing provides a significant advantage over alternative approaches. Jarrod Murphy, Head of CFD Applications for the Renault F1 team, explains: "Because STAR-CD's trimmed cell technology is based around a core-hexahedral structure, blocks containing entire components can simply be "lifted out" of the mesh and replaced with revised meshes of just that section. This dramatically reduces run-times and ultimately makes CFD the only viable tool for this type of analysis. "