Blown Diffuser: How does it work actually and why it's there

[hardingfv32]

Wouldn't the high velocity of the exhaust be resistant, for a small distance after leaving the tip, to forming a vortex?

Brian

[DaveW]

Wouldn't the high velocity of the exhaust be resistant, for a small distance after leaving the tip, to forming a vortex?

I'm inclined to agree. The trailing vortices behind a 747 (for example) form fully some way aft of its tailplane. The delay is often observable from the ground when an aircraft is "contrailing" at high altitudes. Aircraft that operate at high incidences, such as the F16 look-alike posted by n smikle, are different. Here the principal vortex forms as shown in his diagram. However, in that case the vortex travels over the wing some distance above it (which it must do to maintain the circulation). I happen to know that because I once spent some interesting times chasing an HP 115 trying to fly through its trailing vortices in order to measure their velocity profile. "Vortex" lift generation trades efficiency (lower L/D) for robustness (the wing won't stall).

I'm not sure how that translates to an F1 diffuser. I don't know if enough "space" is available for "wingtip" vortices to develop within the length of the diffuser &, if they did, logic would suggest that the effectiveness of the diffuser would probably be reduced.

On the other hand, sector times, particularly in qualifying, demonstrate that some blown diffusers work rather better than others, so it is quite possible, I suppose, that different teams have implemented different solutions.

[PlatinumZealot]

I think the solution is black and white because of the gap in qualifying. That is why I supposed that vortices are being used. Just something of an educated guess..

However, in that case the vortex travels over the wing some distance above it (which it must do to maintain the circulation). I happen to know that because I once spent some interesting times chasing an HP 115 trying to fly through its trailing vortices in order to measure their velocity profile. "Vortex" lift generation trades efficiency (lower L/D) for robustness (the wing won't stall).

Hmm interesting that you mention that. It could be the reason why the RedBull ride height in the rear is much higher than the others.



These are the strakes on the RedBull. Different from the Ferraris. You can sorta see the angle of attack as the exhaust shrouds are slightly pointed downwards.

[DaveW]

An interesting photo that I have puzzled over for a while. I have been unable to convince myself that the undertray shown is (or ever was) symmetrical. Date & location might be useful information.

[jake_m]

Those are Monaco garages if I'm not mistaken. Sure is an interesting part.

[godlameroso]

Very clever, they included a hollow section in the floor, I assume they leave a small gap next to the wood plank, to use the sides of the floor as additional diffusers.

[hardingfv32]

Why the large hole just after the front bib?

Brian

[godlameroso]

Probably to feed those ducts with the yellow tape, and then further back, right before the diffuser kink there are two slots to feed the gap in the rear of the diffuser.



Also look at the winglets on the wheel it almost looks like they're the blades on a turbine.

[PlatinumZealot]

Gurneys on those winglets too. That is going to the extreme.

[hardingfv32]

1) What are the winglets mounted to?

2) What would be the quality of the airflow so close to the wheel? Are they getting assistance from the exhaust location?

Brian

[jake_m]

The winglets are mounted to the wheel hub and generally are used to control turbulence around the rear wheels and provide some downforce directly to the rear axle. If the teams were allowed they would apply more downforce directly to the hub to limit aero loading on the suspension. For example the Chaparral 2E from 1966 Can-Am applied the downforce from it's massive wing directly to the wheel hubs.

[DaveW]

The revealing photographs posted by n smikle & godlameroso, together with the inspirational comments posted by them & others have lead me to some tentative conclusions that I would like to test here, if I may. Thanks to all.

First, courtesy of some sharp people I work with, I have been convinced that the RBR undertray is symmetrical.

The undertray image showed a fairly obvious heat stressed outer edge behind the exhaust that suggests the exhaust flow is directed slightly away from the vehicle centreline & is split so that some passes over, & some under, the plane of the undertray. The rear view shows that the diffuser outboard of the rear wing end plate has a protective finish to its underside suggesting that the lower component of the exhaust flow passes close to it, probably without much dilution. I suspect this implies that the lower component is used to seal the outer edges of the diffuser. That would imply, I think, that "east-west" diffuser expansion is at least as important as "north-south" expansion, & it should also make the diffuser more tolerant of ride height changes.

That leaves the upper component of exhaust flow. I would guess that is used to "pull" air through the space above the undertray & between the wheels & gearbox cover. That would be expected to increase the effectiveness of the cascade attached to the rear uprights & the rather neat slat placed immediately above the trailing edge of the diffuser. It could be argued that the slat is the vestigial remains of last year's double diffuser & it would be expected to help keep flow inside the diffuser attached to its upper surface.

If the above withstands scrutiny, then the RBR rear aero is highly complex. It would probably explain why RBR has to fly its front wing closer to the ground than the competition & also why it can, with the help of increased rake &, perhaps, a flexible tea tray.

I have a question. Has the upright cascade & the diffuser slat been in place all season, or is it a recent, perhaps "high downforce" development?

[mep]

I wonder how those cascade wings on the upright are influencing the rearwing. They seem to point quite aggressive towards the rear wing endplates. Wings with such small spans can hardly be efficient for creating downforce. They don’t even have wing tip endplates so they probably create a big vortex passing by the rear wing endplate.

For sure the rake of the floor is very important for creating downforce. The current car probably already has a higher area expansion due to the rake then due to the diffuser. The FIA could completely disallow the use of diffusers and the floor would still generate downforce just by tilting the car. It’s just a matter how efficient you can seal the edges of the floor. Creating vortexes at the front for example by the front wing and the tuning vanes is on way to achieve this. Using the exhaust is another. Interesting to see Renault went the way to use the exhaust already at the front of the floor. They might have a very good seal at the front of the floor but lose control at the rear where the distance between floor and road is highest. A mid mounted exhaust might be a further option.

[hardingfv32]

1) "the lower component (of the exhaust flow) is used to seal the outer edges of the diffuser."

How is this done, air screen or vortex formation?

2) Why are the winglets on the hubs not movable aero devices and thus restricted?

Brian

[DaveW]

I don't know the answer to your Q1 (although I would favour your airscreen idea).

I would have thought that Regulation 3.15 "Must be rigidly secured to the entirely sprung part of the car (rigidly secured means not having any degree of freedom)" would have been more relevant for the upright cascade, but (fortunately) I don't have to interpret the FIA Regulations.