Saturday, March 10, 2012

The unification of internal and external flow

The persistence of exhaust-blowing in contemporary Formula 1 is a significant philosophical development, for it signals a step towards a more effective unification of internal and external airflows. It is, as Mark Hughes points out in Autosport this week, a way of "using the upper-body airflow to seal the exhaust flow into going to the right place, where it in turn seals the underbody airflow into going to the right place." But as well as being an aerodynamic zip, it also constitutes a karstification of Formula 1's aerodynamic landscape.

External flows on racing cars are familiar and comforting territory, conveniently idealised as incompressible. Beneath this, however, lies the dark, disturbing realm of internal flow. In this subterranean domain the flow is often compressible, is characterised by changing temperature and density, and is bedevilled by Mephistophelean harmonics.

External and internal flow were almost treated as independent worlds for many years, but the resurgence of exhaust-blown diffusers heralds a new era in which the two regimes are becoming ever more tightly integrated. What we already have is a hydrodynamical topology matching that of a Karst landscape, with multiple sinkholes and outlets, and mysterious, hidden networks interpolating between.

External flows are swallowed by engine airboxes and radiator intakes, discharged from exhausts and cooling outlets, and then re-ingested by transgressive brake ducts and diffuser orifices; like trains on the Piccadilly line, shooting out of some fetid tunnel, briefly scuttling through a graffiti-ridden cutting under a sunless sky, then anxiously diving back into the darkness.

There's even the suggestion that Mercedes are using the greater temperature of the exhaust and radiator flows above the diffuser, to create a pressure differential with the cooler, denser air below. One imagines a semi-permanent band of frontal rainfall, hanging gloomily over the driveshafts.

In combination with this is the equally secretive and speluncal world of the F-duct. Nothing is solid any more; everything is potentially hollow, permeated with channels and rills and flues and pipes.

Visible aerodynamic surfaces are no longer just external-vorticity generating solid boundaries; they are also the separation between the internal and the external.


barryD said...

Can you put an educated approximation on the possible permutations of airflow at the rear of an F1 can in the light of curent regulations. (Even as I type this it seems an impossible question to answer, but what the heck!)

Gordon McCabe said...

Blimey! Yes, that is a good question.

Recall the complexity of McLaren's abandoned 'octopus' exhaust solution, which appeared to have multiple outlets. (In fact, Paddy Lowe now claims that "We actually called it a 'Fan tail' internally because it was a fan tail pipe that went into a slot, which blew through the floor.") Exhaust outlets in such positions are no longer permitted, but you could have passive F-duct outlets just about anywhere, depending upon what you're trying to achieve.

barryD said...


Can we have an iterpretaion of today's news re. the Mercedes "F-duct"?

Gordon McCabe said...

Well, I'm astonished that it's been deemed legal. It appears that the driver-activated DRS is being used to trigger a fluidic switch; that has to be driver-activated aerodynamics. I suspect a number of other teams will feel the same way.

Ingenious, though. Mercedes were forced by circumstances to route their 2010 F-duct through the floor and rear-wing endplates, and it seems that the knowledge gained there has been re-employed now.