Importance of the diffuser?

[xpensive]

This subject became more interesting and should thus be rewarded a thread of its own, or so I believe anyway.

If you behold an air-stream separated by an object such as a wing-profile, or an F1 car for that matter:

Total pressure-pt, is static pressure-ps plus dynamic pressure-pd. Then if total pressure is held constant:
ps1 + pd1 = ps2 + pd2
The above gives a static pressure differential, ps1 - ps2 (which creates Force), equal to pd2 - pd1.

When pd is density * speed squared / 2 and Force is static pressure differential times Area, the resulting force is:
area * density * (speed2^2 - speed1^2)/2.

This force can be substantial, if you can arrange the air-speed under the car to be 100 m/s, while the speed on top is only 60, downforce would be about 7700 N with a floor-area of 2 sq. meters.
But if you find a way, through a novel diffuser for example, to speed up the air under the car with 10%, downforce increases to 10200N, or with 32%.

[Belatti]

Even Bernoulli wouldnt say that better!

[kilcoo316]

Best of all, its virtually drag free.

[Callum]

i used to think that the diffuser created downforce by causing the air to expand and drop in pressure, until i read Simon McBeathe's book.




what i still can't quite get my head around is why the air increses velocity from the outside to section 2. I know the cross sectional area of the space between the air and the ground decreases => an increase in velocity which gives the low pressure.But i keep thinking that there would be a high pressure zone at section 1.

Can anyone clear things up?

[kilcoo316]

what i still can't quite get my head around is why the air increses velocity from the outside to section 2.

You said it yourself in the next line.

I know the cross sectional area of the space between the air and the ground decreases => an increase in velocity which gives the low pressure.

Conservation of mass.

But i keep thinking that there would be a high pressure zone at section 1.

The diffuser sucks air through. It is not forced under the car as such - unlike say, the airbox, which forces air into the engine.

Yes, the air will have a relatively higher total pressure at 1 due to its lower speed, however the ram effect (which is what I think your getting at) can be dismissed.

[Callum]

ah right, the ram effect is what i was getting at.


Oh, another thing, which may be a reallly simple/stupid question..

How is the car travelling through static air the same as moving air through a static car...?

Thanks Conceptual.

[avatar]

How is the car travelling through static air the same as moving air through a static car...?

Don't think of it as moving through, more that there is a relative difference between the velocities (speeds in given directions) of the car and the air.

To put that in a broader sense:
"Relatively speaking" there's no difference between:
- object A moving towards to object B - (air towards the car)
- object B moving towards to object A - (car towards the air)
(search for "general relativity" for some physics answers)

My understanding however, is that wind tunnel testing is similar to track running, but not identical, due to other factors affecting the airflow such as:
- tyres spinning (hence rolling roads)
- track surface moving relative to the car (hence the giant treadmill type rolling road)
- heat from live running (could change airflow and viscosity)
- texture variations in track surface
- bumps & cambers on track surface
- walls/barriers/other trackside objects
- other cars
- cross winds

[xpensive]

Right, no objections i principle to the application of Bernoulli on an F1 floor above then.

The purpose was of course to picture just how sensitive to downforce the diffuser can be. In the above example, a ten percent increase in air-speed under the car resulted in a 34% increase in downforce.
But with a more realistic scenario, let's say the car is moving at 144 km/h(40 m/s) through a corner with an air-speed undeneath of 70 m/s, resulting downforce is 4000N, when a 7% higher speed of 75m/s would increase downforce with 20% to 4800 N.
With virtually no drag-penalty to carry for the coming straight, unbeatable.

No wonder the teams are bickering over the interpretation of diffuser-rules.

[kilcoo316]

Right, no objections i principle to the application of Bernoulli on an F1 floor above then.

Nah. No objections from me anyway.


It'll not be 100% right, but who cares, its only to demonstrate possible magnitudes, it doesn't need to be exact.

[xpensive]

You are right of course, the numbers do not pretend to be precise, only pointing out the effect of small changes.

However, they might not be that far off in the case of Brawn, check this out, kilcoo:

http://www.autosport.com/news/report.php/id/74034

[NormanBates]

what % of total downforce is a regular difusser prividing?

just to know how much those 800N (or 3000N) can matter

(in that article briatore says the brawn difusser is increasing downforce by 14%, but I wouldn't believe his number)

[BorisTheBlade]

Very interesting topic.
Coul you please tell me your opinion to that:

Besides, one very experienced F1 aerodynamicist told me the following: "In aerodynamic terms, seven tenths of a second implies about 70kg of extra downforce for a given drag.
"The size of the diffuser as defined in the regulations means it creates only about 10% of the car's total downforce - about 120kg.
"If their diffuser is worth seven tenths of a second it implies that they are getting 60% more downforce from their diffuser than anyone else. That is ludicrous.
"I'd say if you could get a diffuser to be giving you an extra 20kg over everyone else - about 0.2 seconds-worth - you would be doing incredibly well."

Source: http://news.bbc.co.uk/sport2/hi/motorsp ... 972046.stm

I was always under the impression that in previous years the diffuser accounts for roughly half the total downforce of a F1. So a cut to one-fifth seems to be totally out of line.

[kilcoo316]

Source: http://news.bbc.co.uk/sport2/hi/motorsp ... 972046.stm

I was always under the impression that in previous years the diffuser accounts for roughly half the total downforce of a F1. So a cut to one-fifth seems to be totally out of line.

Diffuser providing 1/10th total downforce?


Dismiss that report out of hand. Either the aerodynamicist was telling fibs or Mark Hughes got the numbers mixed up.

[xpensive]

That's very interesting numbers Boris, while I have to agree with my Irish buddy as usual. However, when aerodynamical downforce in general increases with the square of the speed of the vehicle, perhaps they were talking about the Monte Carlo hairpin?

120 kg (1200N) of diffuser and underbody downforce sounds awfully low, that would mean some 0.006 atmospheres over a 2 sq. meter area, if we stick to that figure for now.

[Ogami musashi]

The 2009 baseline diffuser was aimed at 15% of total downforce.

But first the total downforce is certainly not 1200kg (we are speaking of max downforce right?) the actual figures are already around 80% of last year and with future developments it could be raised to 95%.
We are more into the 2200-2500kg region.

Conservation of mass works very well for underfloor aerodynamics but an add from newton's second law is better if you are to take boundary layers effects in consideration.

Of course the full NS system is the best solution but hum well...It requires me at least 30 seconds to solve it...(joking).