CFD versus tunnel versus track – part 1

Friday, April 26, 2013

Tags :  aerodynamics

Formula One aerodynamics is driven by several toolsets – track testing, full-scale tunnel, model-scale tunnel testing and CFD. The debate as to which method is most accurate or productive seems to be never ending; debate is healthy but sometimes it helps to step back and look at first principles.

Of the four toolsets, only one – track testing – is truly correct, the rest being approximations of real-car aerodynamics at the track. But track testing is restricted, expensive and has poor repeatability/resolution due to changing conditions (weather, temperature, tyre and so on). So the track has all the physical processes that contribute to or influence aerodynamic forces and moments.

At first glance this is simply car design and set-up, car state (ride, roll, steer, yaw) and wind speed. But there are other factors that drive aero forces too. As the need for higher-fidelity aerodynamics increase, so does the importance of looking further than car design, car state and speed. The mind map below aims to illustrate some – hopefully most – of the other physics or influences on aerodynamic forces.

Working through the list on the left-hand side, the first three are the obvious ones, as stated above. Given a car design, attitude and speed we can get steady aero forces from either tunnel testing (scale or full) or CFD. From there we’ll see differences and debate CFD turbulence models, wind tunnel design and geometry differences forever. But the bigger picture is: what else should we be looking at to get the real forces and moments the car would see on the track? Let’s go back to the mind map and work through the list.

Acceleration of the car: There are two things happening here. The first is that the air around the car has mass, so to accelerate the car you need to accelerate that air too – it’s a small but significant force change that steady flow won’t show. The second is where aerodynamics can play tricks, where a sudden change in flow speed can cause a different flow structure compared to a slow increase in flow speed. This doesn’t happen often but imagine the effect of having a front wing that works at a range of speeds but stalls if the acceleration of the air is greater than, say, 1 g.

Using wind tunnels it is probably very hard to match the high accelerations seen on track, whereas with CFD it’s fairly straightforward. Are the teams looking at this as part of their process? Probably not, as it is considered low priority and tricky.

Static aero-elastic bodywork deflections: The pressure exerted by the air on the car as it passes over it causes things to bend or deflect under load. ‘Static’ means these deflections move and stay moved – that is, no flutter. It’s pretty routine using today’s tools to take forces from CFD and plug them into stress analysis codes and calculate the deflected shapes. These shapes can then go back into CFD or wings made for tunnel testing that are “as deflected at 180 mph”.

So the effects can be studied, gains exploited and front wings that peek outside rule boxes created. This has led to more stringent load tests on car components, and so the game continues.

Given the deflected shape, the tunnel or CFD toolset can equally perform a role. Full-scale tunnel testing has the upper hand in that the wings will naturally deflect in the right way. Most static aeroelastic work in Formula One is probably done in CFD, since the link to the stress codes can be made slick and seamless. It is very likely that all teams are using these tools as part of their standard process.

Dynamic aero-elastic bodywork deflections: This is similar to the above but here the deflection doesn’t stay put and things flutter. This motion can be all aero-driven, like a flag in the wind, but the motion of the car (bumps and vibration) can contribute too. The full dynamic aeroelastic problem is very difficult in either scale of tunnel. Even in CFD with complex stress codes, this problem is only just becoming solvable, largely because the motion (such as pitching) of the car is very hard to model. Very few teams will be looking at true dynamic aeroelastic work, and probably fewer will be including the car motion in the calculation.

Tyre shape, roughness, flash and deformation: At the track the tyres don’t stay smooth, the edges get sharp with rubber flash from wear, and they deform significantly under load. Structural models of the tyre can help to predict and provide tyre shape under load, and these can then be fed into CFD directly.

For scale tunnel testing, the tyres used are designed to try to mimic the shape changes seen on track. In either part-scale or full-scale tunnel testing it is possible to get tyre deflections through vertical load, but generating correct lateral forces and deflections in the tunnel is very difficult or impossible.

Tyre shape is a key driver for a car’s aero performance; the tyre sheds vortices and wakes, and interacts strongly with wings and underfloors. Flash and roughness can be applied in CFD or tunnels, and the effects shown. True cornering tyre shape control can probably only be achieved in CFD; tunnel testing can get close but struggles to fully match the track. Most teams put large efforts into getting tyre shapes right in tunnels and CFD.

Track roughness: As the air is squeezed under the car it can be travelling faster (or even slower) than the track, so the surface finish of the track can play an important role in the car’s aero. Teams run rough belts in their tunnels and apply roughness to the ground in their CFD models. It’s mostly straightforward stuff, and tuning of track roughness for different circuits isn’t really necessary.

Dirt build-up, surface finish: Tunnel testing and CFD tends to be done with a clean model. At the track, the cars can build up dirt and rubber deposits in key aerodynamic places. Mostly this makes little difference and can be ignored, but a finely tuned front wing might fall over if there are a few specs in the wrong places. This sort of flow change is largely small-scale boundary layer physics, which CFD struggles to resolve, and tunnel testing provides an easier route to gluing dead flies in the wrong places. Teams have probably rarely looked at this in the past, although this has almost certainly changed now given the amount of rubber build-up of recent years!

In Part 2 we’ll continue through the list and discuss how the curvature of the wind in a corner can make a big difference to Formula One aerodynamics.

Written by Rob Lewis

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thanks for information sharing
The devil is really in the details, isn't it. With the new 2014 regulations forcing down the number of wind tunnel hours and CFD processing resources, does this mean that there will me even more unknowns about the design than before? And what does this mean for safety in the long run?