Monday, May 04, 2009

Adrian Newey and Rayleigh-Taylor instability

After dominating the recent Chinese Grand Prix, a race run in treacherously wet conditions, Red Bull designer Adrian Newey was asked by Autosport's Edd Straw to explain why his car is so quick in the wet:

"That's a good question!" he replied, "The fundamental characteristics of the car - it seems to be reasonably well balanced with a decent level of downforce - are a good start for the wet. But it gets into the fine details for the last couple of tenths and it's a bit harder to nail down why."

Whilst the Red Bull is undeniably competitive this year, for the moment it is bereft of a double-decker diffuser, and the evidence from the other races this year suggests that the Brawn is intrinsically a quicker car; the Brawn is also 'reasonably well balanced with a decent level of downforce'. Hence, the advantage of the Red Bull in wet conditions requires something of an explanation.

One could take Adrian's response at face value, and accept that Red Bull don't really understand it themselves. Or, more intriguingly one might recall that Adrian has spent some time looking at the fluid mechanics of yachts, and has often flirted with the idea of working on an entry for the America's Cup. What fascinates Adrian about yachts, in particular, is the fact that they cleave through two different fluids: air and water. In this sense, the hydrodynamical challenage of yachting design is quite different to that faced by an automobile aerodynamicst, who must study only the effect of one fluid, the air. Except, that is, when it rains, and a film of water lies upon the surface of the road...

One might hypothesise, therefore, that Adrian has learnt something from his yachting research, which can be applied to good effect in setting-up a Formula 1 car for wet-weather conditions. When it rains in motorsport, the generic response is simply to increase ride heights and wing angles. It might, however, be possible to exploit the complex processes that occur at the contact boundary between the air and water when accelerated by the aerodynamic surfaces of a Formula 1 car. Liquid water is denser than air, and when fluids of different densities are subjected to acceleration, Rayleigh-Taylor instabilities result in turbulent mixing of the fluids. Could one, for example, use the mixing of the air and water on the underside of the wings and beneath the floor of the car, to create a turbulent boundary layer that keeps the flow attached for slightly longer, thereby increasing downforce and reducing drag? The effect would be the same as that created by the dimpling on the surface of a golf ball.

Perhaps the Red Bull's wet-weather advantage over the Brawn is simply due to the fact that it is harder on its tyres in all conditions, and therefore capable of generating the necessary heat to 'switch the tyres on' in wet conditions. And it should also be noted that once the layer of water on the road surface has been transformed into a spray, the water droplets in the spray do not follow aerodynamic streamlines. Nevertheless, one feels that the hydrodynamics of Formula 1 cars displacing both air and a surface layer of water, is a science which has yet to be fully explored or exploited.

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