## Diffuser functions

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Why don't people just pick up a book on basic road vehicle aerodynamics before saying things like "diffusers don't produce suction"?

Ask yourself this. Is there suction on a wing (Hint, the answer is yes)? On a Pressure coefficient vs x/c plot you can easily identify the suction peak occurring on (surprise!) the suction surface. A diffuser is not dissimilar to a wing in ground effect. Yet apparently according to some thermodynamic law the wing does not do work on the flow thus there cannot be any suction. Work has little-to-nothing to do with it. I think the aero definition of suction has been completely misunderstood by some - it simply implies a relatively reduced local static pressure. There ARE suction peaks on diffusers, one at the floor 'entry'/'inlet' and one at the diffuser kink line. And they DO produce a hell of a lot of downforce.

Heres a few handy links, they will save us from half of the preposterous stuff posted on these forums! I appreciate that most people here don't have degrees in aeronautics but the downright assertiveness that some people have when presenting fundamentally incorrect concepts is rather...fascinating.

http://www.amazon.com/Competition-Car-A ... =8-1-fkmr0
http://www.amazon.com/Race-Car-Aerodyna ... 811&sr=1-1
void1
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Joined: 31 Mar 2011

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void1 wrote:Yet apparently according to some thermodynamic law the wing does not do work on the flow thus there cannot be any suction. Work has little-to-nothing to do with it. I think the aero definition of suction has been completely misunderstood by some - it simply implies a relatively reduced local static pressure. There ARE suction peaks on diffusers, one at the floor 'entry'/'inlet' and one at the diffuser kink line. And they DO produce a hell of a lot of downforce.

Maybe I'm wrong..please correct me...As I understand this work/not work thing : Maybe the diffuser itself doesn't make work, but you can't get downforce for free. So if the diffuser causes drag ( even considerably little ), the it must do work too, because it slows down the car, hence the engine must do work too to overcome this drag. As seen from opposite direction : to slow down a car, you must do work. And DF does slows down the car, because it's always causing drag too....It's sure not just he diffuser that causes this, it's a pressure difference on whole car that plays, but it's mostly caused by the diffuser.
Now please rip this idea in pieces if it's wrong, just for me to understand better
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Joined: 18 Feb 2010

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kalinka wrote:Maybe I'm wrong..please correct me...As I understand this work/not work thing : Maybe the diffuser itself doesn't make work, but you can't get downforce for free. So if the diffuser causes drag ( even considerably little ), the it must do work too, because it slows down the car, hence the engine must do work too to overcome this drag. As seen from opposite direction : to slow down a car, you must do work. And DF does slows down the car, because it's always causing drag too....It's sure not just he diffuser that causes this, it's a pressure difference on whole car that plays, but it's mostly caused by the diffuser.
Now please rip this idea in pieces if it's wrong, just for me to understand better

For an ideal diffuser total energy of gases does not change along it's path (no temp change, no momentum change, no total pressure change), so no work done.

For any given slice of time, the mass of air at inlet and outlet is the same. If inlet area is smaller then outlet area, air have no choice but to go faster at inlet, then slowing down along the way to outlet, ideally arriving just at external flow's speed at the end.

Faster in comparison to external flow speed means lower then ambient static pressure.

For this ideal diffuser downforce is free from car's point of view - the work of pressing the car against the tarmac is done by all the air above the car (up to 10,000 kg/sqm if there is no static pressure (vacuum) under the car to support the floor).

Of course there are no ideal diffusers in real world, so you have to add some energy to the system (drag) to overcome skin friction, generate vortexes at separation points, carry some air molecules behind the diffuser with you ...

But unlike a wing, diffuser does not significantly change flow's direction, so they have very little induced drag and are far more efficient then wings in terms of downforce/drag ratio.
marekk
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Joined: 11 Feb 2011

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+1

Quite simply, diffusors allow devices upstream to do the work. All a diffusor does it return gas to ambient pressure without shock....ideally.
Raptor22
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Joined: 7 Apr 2009

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n smikle wrote:
marekk wrote:
bot6 wrote:It had been a while since we'd seen a turbine...

And - again - in a completely irrelevant context.

And Ac system. But no saxophone horn this time.

A saxophone horn is a diffuser too.

Yes. But all of your devices (AC diff, turbine diff, rocket engine nozzle, even saxophone horn) work from high pressure at inlet to lower pressure at outlet.

We are discussing opposite scenario in this thread.
marekk
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Joined: 11 Feb 2011

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Lindz wrote:I just think that while most teams went a more conventional way of creating as much downforce as possible at the diffuser, Adrian Newey and his team have searched for efficiency and downforce everywhere BUT the diffuser. It's not nearly as important as it was with DDDs. By removing the diffuser out of the equation for a bit, I think they were able to find gains in many more places.

That's proved quite prophectic I think, especially last year with the double diffusers people were using. I think most were a little surprised that RB stayed with their pull-rod suspension because most at the start of last year seemed to think that the cars would have the largest diffuser possible strapped to them and that's where the gains would be made. RB seemed to stay away from the diffuser, and it didn't look as well developed as other teams at times, but concentrated on conditioning airflow forward of it and making it as efficient as possible.
segedunum
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Joined: 3 Apr 2007

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and with only (relatively) small gains from the diffuser, when they lost the 2nd deck, they lost the least.

raymondu999
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Joined: 4 Feb 2010

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Thanks marekk, that makes sense. So running a diffuser indeed requires power, but significantly less than a wing-system of same DF. I forgot that what goes in - must go out too
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Joined: 18 Feb 2010

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raymondu999 wrote:and with only (relatively) small gains from the diffuser, when they lost the 2nd deck, they lost the least.

I can't help but think that they had a lot of the rear end concepts sussed out with the RB5. To compare the two see the RB5 in 2009 testing and a latest of the RB7:

Obviously the diffuser is smaller and further back this year, but I think the exhaust positioning provides some replication of the extended endplates. The real shocker is just how much the airflow to the beam wing and diffuser Gurney was top of the list in the design requirements of the RB5. It's like they had a 2 year head start on the rest of the field.

What is a surprise is the HRT style front wing it had!

"Words are for meaning: when you've got the meaning, you can forget the words." - Chuang Tzu
horse
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Joined: 23 Oct 2009
Location: Edinburgh

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I'm not convinced that HRT didn't just copy off the RB5. I mean, the RB5 spent most of, if not the whole year, on that wing anyways

raymondu999
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Joined: 4 Feb 2010

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Try to copy you mean! I think RB might be a bit insulted if you claim last years HRT was an RB5.

I had a look about for a front wing change and although the wide nose made a debut at Silverstone I would say it wasn't until Singapore that the "present" version of the front wing appeared.

http://www.formula1.com/news/technical/ ... 9/700.html
"Words are for meaning: when you've got the meaning, you can forget the words." - Chuang Tzu
horse
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Joined: 23 Oct 2009
Location: Edinburgh

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Raptor22 wrote:+1

Quite simply, diffusors allow devices upstream to do the work. All a diffusor does it return gas to ambient pressure without shock....ideally.

I think this is what I said earlier in the thread in a round-about way.

Simple question, wouldn't the car be more better at producing more downforce if the underbody of the car wasn't flat ? I know this can't be done in F1, just a general question.

Also, are there two mandatory car weights in place this year for cars that run KERS & those that don't ?
hollowBallistix
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Joined: 13 Mar 2011

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hollowBallistix wrote:Simple question, wouldn't the car be more better at producing more downforce if the underbody of the car wasn't flat ? I know this can't be done in F1, just a general question.

Sure that's the whole point of the 2013 aero regs, shift the downforce production to being primarily produced by the under-body of the car by the reintroduction of a profiled floor.
Last edited by mx_tifoso on Wed Apr 06, 2011 8:25 am, edited 1 time in total.
Reason: split from RB7 topic
"Words are for meaning: when you've got the meaning, you can forget the words." - Chuang Tzu
horse
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Joined: 23 Oct 2009
Location: Edinburgh

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horse wrote:
hollowBallistix wrote:Simple question, wouldn't the car be more better at producing more downforce if the underbody of the car wasn't flat ? I know this can't be done in F1, just a general question.

Sure that's the whole point of the 2013 aero regs, shift the downforce production to being primarily produced by the under-body of the car by the reintroduction of a profiled floor.

Ah cool, cheers !

Didn't realise that was that the plan was for 2013 !
hollowBallistix
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Joined: 13 Mar 2011

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hollowBallistix wrote:
Raptor22 wrote:+1

Quite simply, diffusors allow devices upstream to do the work. All a diffusor does it return gas to ambient pressure without shock....ideally.

I think this is what I said earlier in the thread in a round-about way.

Simple question, wouldn't the car be more better at producing more downforce if the underbody of the car wasn't flat ? I know this can't be done in F1, just a general question.

Also, are there two mandatory car weights in place this year for cars that run KERS & those that don't ?

Simple answer: you get about twice the downforce for given airspeed, angle of attack and airfoils area if you use real airfoil instead this "barn door" one. Interesingly, drag increases more then that, about 2,5 times.

KERS use is not mandatory, in fact some teams don't have one. There is one minimal weight for all cars.
marekk
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Joined: 11 Feb 2011

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