Regulations and CFD development part 2Tags : aerodynamics
In my previous article on this subject I examined whether the Formula One regulations still make it possible to be creative with simulation methodology. In this article I want look at what those regulations implicitly prohibit, focusing predominantly on thermal applications.
The option to couple solvers is not limited to structural solvers. The new set of technical regulations in Formula One has put greater emphasis on cooling, and in turn on understanding heat transfer. This is an area where CFD methodology has been developed extensively by roadcar manufacturers, with engine bay cooling airflow and exhaust modelling simulations now commonplace. Such coupling would also allow Formula One teams to evaluate their cooling package as part of their aero simulations and accurately model the flow of the air escaping from the rear of the car.
This methodology could also have proved more effective in recent years, with the prominence of exhaust-blown diffusers. Wind tunnel models are rather limited in their ability to mimic how the exhausts truly behave, as it is impossible to put a scale engine in a wind tunnel model that produces the same mass flow and temperatures through the exhaust. Even accounting for the correct temperature of an exhaust, it would have been hard to correctly model the mass flow through it. In reality, an exhaust’s behaviour is linked to what the engine is doing, and what the engine is doing is changing at an alarming rate.
This again leads to the argument for using transient methods, although it should be noted that a fully transient coupled simulation – where ride heights, yaw, steer and roll trajectories and exhaust parameters are changing – is still beyond the time constraints of a Formula One team. However, a compromise has been developed by at least one automotive OEM.
This was developed by taking a car around a race track and monitoring the velocity and dynamics of the car together with the exhaust parameters. A filtering process is then applied to identify the most significant velocities and exhaust parameters, and from this a sample of points is taken to simulate 20 or so steady-state RANS simulations. From these results an understanding can be developed of how the car is truly behaving around a circuit. The time needed to complete this analysis is within a working week – a potentially acceptable timeframe in Formula One – although at the moment this approach would take up most of the restricted CPU time available just to analyse a single car configuration.
Staying with the theme of coupling with a heat transfer solver, brakes obviously get hot and cool down, and it is important to keep a racecar’s brakes within a specific operating temperature range. In recent years, front and rear brake ducts have been an area of aggressive aerodynamic development for teams, but when optimising these ducts for aerodynamic gain it is perhaps easy to overlook their cooling requirements. Coupling the braking system geometry with a heat transfer solver would allow teams to analyse the extent to which they are keeping within operating temperatures, while also analysing the aerodynamic effect of modifying their brake duct design.
The CPU time penalty incurred by coupling solvers implies though that this is not a methodology that is currently attractive to Formula One teams. Moreover, brake cooling effect is inherently associated with wheel shape, so modelling the rotation of the wheel as accurately as possible via sliding mesh techniques should greatly enhance the accuracy of such simulations.
Formula One has undoubtedly added to mainstream automotive technologies over the years, with ABS and semi-automatic gearboxes being prime examples, and the modification of the powertrain regulations for 2014 will hopefully once again improve the series’ relevance to roadcar technology. In terms of manufacturing, Formula One also continues to drive the development of new techniques.
With regard to aerodynamics and CFD, however, Formula One’s relevance to the wider world is arguably diminishing. The 2014 regulations aim to reduce costs, but by linking the wind tunnel usage limit to the CFD usage limit, the FIA is surely overlooking the fact that CFD is cheaper than using a wind tunnel. So, in the context of cost-efficient development, should CFD not be encouraged?
The phrase in the regulations most damning to the development of CFD methodology is surely the one that states that the CFD limit line will change every three years “to take account of changes to CFD hardware ownership and running costs”. This suggests that the FIA is being naive in its view of the rate of development of both CFD methodology and computational hardware, so Formula One needs to ensure that the rule makers are not prejudiced about the benefits of CFD.
The fact remains though that some teams are not using their current hardware to its full potential. The hardware is expensive but brain power is free – provided you are employing the right brains!
Written by Sam Wakelam