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Old 9 Jun 2006, 10:08 (Ref:1630505)   #16
browney
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Denis Bassom
I guess it would depend on the front spoiler as well. A front 'lip' can effectivly shelter some of the protrusions at the bottom of the engine bay also reducing underbody drag. I guess it is possible for flow to seperate in the underbody if you have protrusions, and that would reduce the effect of the diffuser as well as increase drag.
Although, as I have demonstrated, I'm not an expert.

Locost47
I guess that another advantage of a diffuser is that by raising the pressure at the back and 'boat tailing' the profile of the rear, you will propably decrease drag too. Does anyone ever use a 'nozzle' shape on the front of the car to try and get faster underbody flow, or will this increase front end lift too much?

Do you know where I would be able to get some info on diffusers? I've read Hucho's book on the 'Aerodynamics of road vehicles' but our library doesn't seem to have much more and I can't find any journals that I have full text access for on the subject. It seems like getting answers on bluff body aero is really difficult (unless you hang aroudn on ten-tenths ;-) ).I'd love to find out more about the difference between mixing the air at the rear of the car at high speed vs. bringing back down to closer to the free stream.

I hope you love your job, I dream of doing stuff like that!

Last edited by browney; 9 Jun 2006 at 10:16.
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Old 9 Jun 2006, 10:46 (Ref:1630533)   #17
Locost47
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Denis (with one 'n' now, sorry about that)
As Browney says, things like nuts & rivets won't matter so much, within reason, but it's normally big stuff like chassis members or suspension gubbins which scupper the effectiveness of a diffuser but i would also add poorly planned cooling system outflows and the disturbances from the wheels.

Ideally the cooling airflow should be completely ducted to the radiator/intercooler etc and then compeltely ducted out of the car, although some provision may have to be made for direct cooling of engine/gearbox or specifically sensitive components. Regardless, this cooling flow must not be allowed to leave via the underside of the car if the diffuser is to work well.

For the wheels it would be nice to have endplates which extend forwards to the front of the rear wheelarches in order to try and stop the peak suction at the apex of the diffuser drawing in air through and around the rear wheels. This will make an unholy mess, though will be handy for brake cooling.

Also important is to have a smooth-ish transition from the floor to the diffuser. A lot of people, including specialist car companies who should know better, manufacture it out of two flat pieces, with a join exactly at the apex of the diffuser. Unless this panel match is perfect you will get a gap, or even worse a step, in the floor which can knacker the diffuser. Best to have it made from a single flattened v-shape piece with the seam 100mm or more upstream, so that the disturbance from any mismatch has some time to sort itself out before it reaches the critical bit at the start of the diffuser.

browney
Yes a rounded front end will increase front lift but it doesn't have to be much of a radius at all since air is more forgiving when it first arrives at the car and hasn't had any of its energy stripped from it through viscosity and turbulence. Also you benefit becaus this further reduces the energy losses through the underbody and so increase massflow etc and you do get some of that benefit at the front of the floor. The 360 Challenge Stradale is quite a good example of this but you can spot it on the newer breed of Le mans cars too, which all tend to have a raised and sometimes radiussed centre section to the front splitter.

For more information you could try:

Competition Car Aerodynamics
Simon McBeath
Haynes Publishing 2006
ISBN 1 84425 230 2

or

Race Car Aerodynamics: Designing for Speed
Joseph Katz
Robert Bentley Publishers, 1996
ISBN: 0 8376 0142 8

Simon's book is more modern and has lots of cool pictures (done by our competitors at Advantage CFD, grrrr!) but has a misleading and technically incorrect explanation of how wings work, although unfortunately this is the version which is often written in textbooks. We're actually working with Simon on his next set of articles for Racecar Engineering magazine so we're putting him straight. Apart from that though his book is excellent. Both books are available from places like Amazon or Waterstones etc.

Cheers

Rob
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Old 13 Jun 2006, 12:45 (Ref:1633457)   #18
zac510
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Interesting read. I've recently purchased CCA but have only covered the first couple of chapters.

I'd like to hear thoughts on the use of strakes in diffusers. The current GT (endurance) trend seems to be more, more, more! Sometimes over half a dozen strakes that run all the way to the trailing edge.
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Old 13 Jun 2006, 13:47 (Ref:1633514)   #19
Locost47
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I've not done any real parametric tests with the number of strakes in a diffuser so i can't say what the optimum is likely to be. I know the primary function they serve is to reduce the amount of cross-flow in the diffuser and try to keep it more like the 2-dimensional idealised situation.

The more strakes you have the more '2D' the flow should be. I presume there would be a point though where the amount of effective diffuser area you're losing due to the unpleasant boundary layer goings-on in all the 90-degree corners betwwen strake & diffuser surface outweighs the other gain.

Also there are occasions where cross-flows can be beneficial, however, such as when you want to run a wing/diffuser steeper than you could normally get away with. By allowing some flow around the side edges (making it more '3D') you can re-energise the tired boundary layers and keep the flow attached for longer. There is a fairly hefty drag penalty for this but, if done cleverly, the overall balance between downforce/drag can still work in your favour.

If you're being really cunning you can deliberately manipulate the cross-flow to generate strong vortices whose extremely low pressure core acts on a conveniently located nearby surface and pulls the car down. A bit like how delta wings work on, though there are other things going on as well. It's also now a commonly used trick on the top flaps of F1 car rear wings.
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Old 14 Jun 2006, 12:10 (Ref:1634249)   #20
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Thanks, that's some more food for thought
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Old 14 Jun 2006, 13:12 (Ref:1634303)   #21
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an interesting read, that for all the input. Now I know why my F4 was rubbish when the diffuser fell off!!
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Old 31 Jul 2006, 15:26 (Ref:1669015)   #22
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I was reminded of this thread by this picture, which a forum-friend took during a visit to the Ferrari Gallery at Maranello.




However I think the 'pressure paint' is merely airbrushed on to represent distribution.
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Old 1 Aug 2006, 08:10 (Ref:1669661)   #23
Locost47
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LOL! Yes that's a highly stylised interpretation of some CFD results as painted onto a model. It's what you would *want* it to look like...

I was disappointed to see that pressure sensitive paint only works in monochrome. You have to shine a fancy light at the model whilst in the wind tunnel and then measure the light intensity reflected back from it. The paint absorbs light in different amounts depending on the pressure exterted upon it so you end up with a kind of black & white version of those pictures above, only with real results!
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