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PostPosted: Tue Sep 12, 2006 4:04 am 
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Guys,

Have some questions regarding the aerodynamics of front wheel wells. I have been reading both Joseph Katz's book `Race Car Aerodynamics' and Simon McBeath `Competition Car Aerodynamics'.

I understand that you put vents in the side of the front guard behind the wheel, to vent the high pressure air in the well that builds up because of the `scoop' shape of the guard behind the wheel. That makes sense.

You also put a vent on top of the guard right above the wheel to vent the same built up air. This has the added benefit of increasing the static pressure on top of the guard and reducing lift over the front wheels. That makes sense too.

You then put a small diffuser on the front splitter/underpan in front of the wheels so it doesn't have to do a sharp 90 degree turn to head up the front of the wheel and out the top vent. That makes sense too.

My question is, do you think this diffuser should have a sharp 90 degree inside edge, ala rear diffusers? or be smoothed into the flat underpan that runs under the engine? The idea of sharp edges is to generate vorticies INTO the diffuser to energise that flow and delay flow separation.

The only one I have seen (in S. McBeath) had smoothed edges. I was wondering if the sharp edge might not generate enough of a vortex to enable much steeper front diffuser, so maybe its not worth it?

I was also interested in the flow-down effect of any underbody vorticies. Would this allow me to run a longer/deeper/sharper rear diffuser?

I think thats all I can think of for now.

Dave

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PostPosted: Tue Sep 12, 2006 5:24 pm 
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Hell Dave, it sounds like you are the one to [I]answer[/I] questions on the subject, not ask them. The only comment I can make is that on examining a Supercar front air dam I noticed the 'channels' underneath had fairly sharp edges. But how they related to the mugduard shape I do not know.

Its interesting about the front guard vents behind the wheel. A lot of old performance cars had those, the 260Z was modded to incorporate them and something similar has been done with the ZX.

On the rear diffuser, given we are talking about a road car that is not going to have a covered in bottom, I'd design the diffuser to be effective as possible and leave it at that.

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PostPosted: Tue Sep 12, 2006 8:21 pm 
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thehelix112 wrote:
You then put a small diffuser on the front splitter/underpan in front of the wheels so it doesn't have to do a sharp 90 degree turn to head up the front of the wheel and out the top vent. That makes sense too.

My question is, do you think this diffuser should have a sharp 90 degree inside edge, ala rear diffusers? or be smoothed into the flat underpan that runs under the engine? The idea of sharp edges is to generate vorticies INTO the diffuser to energise that flow and delay flow separation.


Are you looking at saloon/road cars when you are making these observations?
On a road car with the additional road clearance I'd be considering putting a fence there instead. Ultimately you want to stop the low pressure underbody air from being screwed up by the wheel well air.
A vortex might achieve this if you are running with millimetres of ground clearance on an LM prototype but I'd be sceptical of my ability to control the thing and its effectiveness at low speed, where a saloon car typically operates.
Of course they run the underbody as close to the wheel as possible to maximise the area.


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PostPosted: Wed Sep 13, 2006 12:47 am 
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This is for my race car, which will have a smooth underbody.

So Richard, the channels that head between the front wheels had sharp edges you say... interesting.. Something that just occurred to me was that any vorticies generated from a sharp `step' side to the front diffuser would be in no-mans land (inside the wheel well with LCA's, brake ducting and all sorts of rubbish to interfere with.

I was also only thinking of a straight (longitudinally) step, but on second thoughts an edge curving inwards to the rear slightly might help to accellerate the flow more so than a vortex, and more importantly, direct it under the car where it can do some good.

Bah, damn complicated stuff. I suppose its the sort of thing thats too application specific to correctly make large general statements about anyway. Real-world testing might be my only hope. Ie, never-ish. :(

Dave

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AUS Race Car: 1973 240Z, L28ET, Autronic, GT35R.
AUS Project: 1972 1600, 3200km old S15 SR20DET, ground up rebuild.


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PostPosted: Wed Sep 13, 2006 12:50 am 
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Zac,

So you reckon a fence that goes all the way from the inside side of the diffuser/wheel well down the chassis rail and around the rear of the wheel well.. Interesting. Perhaps thats the thing you see in V8SC where the bottom of the front of the sills head WAY inwards to the wheel well. Maybe they are endeavouring to spit that turbulent air out?

Dave

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PostPosted: Wed Sep 13, 2006 5:37 am 
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The curved sill to extract the wheel well air is definitely what they are trying to do. Sucking it out there is better than it interfering with the underbody air. I wasn't really thinking of a fence quite as large as that but that sounds good!

I'm not sure that diffuser is the right word for a sedan/coupe touring car front aerodynamic device - it doesn't really slow the air flow (by strict definition of the word). so you really just have a splitter with a flat underbody.

This is a real front diffuser on a touring car but being a bespoke silhouette chassis, I don't know how you'd get that around all the 240Z front end! :)

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PostPosted: Wed Sep 13, 2006 8:35 am 
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That is interesting. Any ideas on what they are trying to do there? I've seen similar pics on mullanes corner, but the layout of that site annoys me. Impossible to find any introductory information.

I understand the hole above the diffuser, which I imagine is to route a portion of air that would be going over the car and accellerating to form low-pressure/lift, out the side of the car where it can do no harm. But whats the point in routing air that would otherwise be going under the car (the more the merrier!) outwards?

Dave

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AUS Project: 1972 1600, 3200km old S15 SR20DET, ground up rebuild.


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PostPosted: Wed Sep 13, 2006 9:15 am 
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Not sure of the theory behind the low-slung wheel well vents. I'm guessing there's something in the rules that screws up the underbody, so it's a work-around?

FWIW, the Aston Martin Le Mans cars have a similar-ish thing.


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PostPosted: Wed Sep 13, 2006 11:21 am 
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thehelix112 wrote:
But whats the point in routing air that would otherwise be going under the car (the more the merrier!) outwards?

Dave

Interesting comment Dave.
Shoot me down in flames if I am wrong...

If the if the pressure build up in the wheel arches was to be forced UNDER the car, then does this not create a pressure build up under the car?
(From the air not being able to escape fast enough?)
I would have thought that one would be trying to minimize the pressure build up under the car so as not to induce lift - I.e. a high pressure area?

The openings in the vehicle that Zac posted would be doing a better job,
but routing the high pressure air OUT of the wheel well,
therefore creating a negative pressure zone and then doing it's part by 'sucking' the car closer to the ground?

BTW, if you were to look at the top of the guards on the latest generation of 'Radicals' out of the UK,
you would see that they have slats/vents in the top of them to vent air out of the wheel well's...

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PostPosted: Wed Sep 13, 2006 12:10 pm 
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Lurchybaby,

Yep. You hit the nail on the head. Gotta get as much air out from there as possible as its all high pressure. Atleast thats what I think at this stage, am coming up on the wheel well bit of Katz's book in more detail now.

What I wasn't sure about was the function of the front diffuser which appears to take air from the front underside of the car and spit it out the side. This is a bad thing. The more air you have flowing nicely under there, the more it will be forced to accelerate and the lower the static pressure will be.

I posted something on another forum about maybe the front diffuser's job is to minimise the air going into the wheel well. It could do this by having its forward facing section narrower than the rear one (as per the pic in S. McBeath). In this case both the high pressure in front of the wheel, and the higher pressure in the channel itself would, maybe, create a wall for the faster moving air on either side to be diverted under the car where we want it..

Too complicated, but interesting to think about none-the-less. Need testing.

Dave

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USA Dail..err..Project: 1978 280Z, minor suspension upgrades, VK56DE conversion in progress.
AUS Race Car: 1973 240Z, L28ET, Autronic, GT35R.
AUS Project: 1972 1600, 3200km old S15 SR20DET, ground up rebuild.


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PostPosted: Wed Sep 13, 2006 6:04 pm 
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Was just reading Katz and something clicked.

In the pic zac posted you WANT high pressure air on top of that horizontal plate because, its a horizontal plate! High pressure on top, low pressure on the bottom = downforce.

They get this high pressure by venting a large part of the air from the wheel well, as well as the higher pressure air as delivered from the end of the front diffuser!

Just thought I'd let you know. :D

Dave

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USA Dail..err..Project: 1978 280Z, minor suspension upgrades, VK56DE conversion in progress.
AUS Race Car: 1973 240Z, L28ET, Autronic, GT35R.
AUS Project: 1972 1600, 3200km old S15 SR20DET, ground up rebuild.


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PostPosted: Wed Sep 13, 2006 6:21 pm 
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There is an alternative to the usual horizontal and flat front splitter and air dam and thats a central venturi arrangement. Recall seeing a picture of a racing MBenz sedan with one. Looking at the front back there was a central channel shaped rather like a rear diffuser.

I've always thought such an arrangement may be more practical for a road car because there is more ground clearance in the middle. But perhaps the exact ride height would be too crucial to maintain in all situations for the venturi to work consistently?

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PostPosted: Wed Sep 13, 2006 8:16 pm 
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thehelix112 wrote:
I posted something on another forum about maybe the front diffuser's job is to minimise the air going into the wheel well. It could do this by having its forward facing section narrower than the rear one (as per the pic in S. McBeath). In this case both the high pressure in front of the wheel, and the higher pressure in the channel itself would, maybe, create a wall for the faster moving air on either side to be diverted under the car where we want it..


I disagree entirely. The front diffuser is all about downforce and aerodynamic balance. You can tell they are aimed at downforce because the front of the car is designed to feed air to the diffuser. If you ran a super low splitter in front of a diffuser it would stall and not produce downforce, thus it is ok to feed a little bit of air under the car for downforce generation. This technique probably decreases the size of the stagnation point at the front of the car with the associated drag decrease aswell.

One influential factor in sportscars are the strict rule limitations that are continually being introduced on underbody stuff like diffuser length and width, flat floors (with no skirts). There are two effects:
a) Smaller rear diffusers require less air
b) Need efficient way to gather front downforce with low drag

Proper front diffusers have only been around since late 90s - before that they had front mounted wings (see Jaguar XJRs etc). But they produced tons of drag for all their downforce. Not very efficient L:D ratio.

The rules are often more influential than you'd think and interfere with the perfect aerodynamic design. On the V8SC, the cars are so heavily restricted I'm sure there would be some rule that says you can't modify the factory bodywork above the sill line. Again, they have practically no rear diffuser and shithouse control front tyres so anything gained at the front end is absolutely crucial. This is probably slightly less true for the Aston GT1 car as they have a huge rear diffuser and a little less rule limitation.

I can't speak for the DTM rules governing the Audi photo above, but it is still a cool example :) I wish I'd taken a photo of the actual underbody :(

One thing you could do is pull the wheel arches tight in around the wheel and extract all the air from the top. I've got a photo of an Audi R10 wheel arch I can show you when I get home. The leading edge is sharp whereas the trailing edge is radiused.

Lurchness, I think you should read up on Bernoulli's principle particularly the conservation of mass part ;) Air under the car is always low pressure (when the car is moving)!


Last edited by zac510 on Thu Sep 14, 2006 12:19 am, edited 1 time in total.

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PostPosted: Wed Sep 13, 2006 8:25 pm 
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sorry I have to...

how 240Z goes that aston look!!!
....ok I know I'm "dat-addticted", but do you see it?... lik eg-nose lights and all....?
ok never mind me.

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PostPosted: Wed Sep 13, 2006 8:32 pm 
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Nice reply Zac. I can see your point.
zac510 wrote:
Lurchness, I think you should read up on Bernoulli's principle particularly the conservation of mass part ;) Air under the car is always low pressure (when the car is moving)!


Correction taken on board - looks like I've got some reading to do :wink:
Although this is all WELL over my head...

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