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.