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Old 27th September 2002, 03:02 PM   #81 (permalink)
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It depends how severe the area change is.

A venturi in the classic sense is a means of createin a low pressure region in a pipe without severly restricting the flow. If you've ever seen the choke in a carburettor you'll know what I mean.

The whole concept (and much of fluids theory one way or another) is built around the Bernoulli theorum. Which states a fluid (including gases and liquids) in a closed system (ie no work done on or by it) has a constant amount of energy. It consists of the potential energy due to gravity, the kinetic energy due to velocity, and the pressure energy.

So it follows that for a quantity of fluid, that remains at the same level (no change in potiential energy) has only the velocity and pressure to work with. If you increase the velocity of the moving fluid, it's kinetic energy is increase, so the pressure energy must be reduced.

Back to the "venturi", we are trying to do is speed up some air under the car or a wing, and benefit from the drop in pressure, creating downforce. I have attached some pics to try and explain.

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Old 27th September 2002, 03:06 PM   #82 (permalink)
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Actually, thinking about it, the peak low pressure on the rake example will be at the front, but the overall centre of downforce will be somewhere behind the front wheels due to the large area where the pressure is low but not as low as it is at the front.

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Old 27th September 2002, 04:38 PM   #83 (permalink)
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In your third sketch, if you raise the leading edge on the 'tray' between the 2 pointers marked as 'VERY GENTLE RAD OR FLAT SECTION' you can ppromote stability just before the front wheels and as speed increases it will spread gining quite a lift to the steering resonse, the use of my 'brushes' under the car is to give the flow no way out, being so close to the ground and to some extent 'sacrificial' should you cause them to scrape the ground. What we have then is exactly what is on my car, and that generates 55kgs of downforce at 160kmh, 100kg at 225kph.

This design can also be used to assist in cooling the car if you have an FMIC as the 'slatted scoops' can be behind the FMIC and can 'hit' the rad with cool air to supplement the air that has past the FMIC and been warmed over.

So potentially you can achieve a two-fold gain, additional stability and cooling to the rad/engine bay, again I have this on my car so know it works.

In addition I have splayed my brushes a little so as to encompass the width of the engine bay 'hole' again achieving greater cooling for a very hot space.

Sketch 4 shows a 'depression' under the belly of the car, if you use the brushes then you have no need of this throat as it is achieved by a slightly skewed method rotating it through 90deg to the perpendicular.
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Old 27th September 2002, 07:03 PM   #84 (permalink)
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I plan view of your brush positions would be informative.

There is a another trick using dive planes on the the front, I think some one mentioned it. WHat it basically does is generate 2 big vortices (1 each side), the right hand one, as sitting in the car would rotate clockwise and vice-versa. Anyway, they can suck air from the bottom of the car and dump it at the top, kinda reversing the effects flow over the top causeing lift on the roof section.

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Old 27th September 2002, 07:15 PM   #85 (permalink)
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I have to admit that inducing vortices worries me slightly as they can become very energy consuming and (I'm told) tend to trail around and upset things on long fast bends, they can in these circumstances slip under the car.

But the idea of trying to force the air up rather than along the side of the car is something I hadn't considered. My thoughts have always gone along the lines of pushing the flow from the sides 'out and away' from the car, but perhaps this maybe a more discreet way of doing things.

I'll see what I can do about pics/drawings etc. my camera skills (read; lack of) are legendary on the LSOC
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Old 27th September 2002, 09:11 PM   #86 (permalink)
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center of downforce

Looking at the flat undertray and rear diffuser sketch, would it be
correct to assume that the center of down force would be increased by using the larger type of diffuser wings as this would keep the air flow more stable as it exits the rear of the car ?
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Old 27th September 2002, 10:41 PM   #87 (permalink)
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From a completely non-aerodynamicist standpoint, presumably this...



..must have some purpose which fits into the arguments being presented? Wouldn't this undertray have the effect of lightening the front of the car? If so, surely (to my mind anyway) this is counter productive?
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Old 27th September 2002, 10:42 PM   #88 (permalink)
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Thing is

wouldn't our GTR's have to be 'slammed' to the floor for a full undertray to have any effect. Also the downforce needs to be right or we get the flying CLK at Le Mans .

Also I thought that any rear fins/diffusers need to be nearly dragging on the floor otherwise if they are too high they can actually cause turbulence at the rear of the car. I only say this as all the cars I have seen with these 'effects' are very, very low to the ground and nearly always scrape.

Personally I think their is no happy medium for us with fixed 'effects' as we nearly all tend to use our cars in different ways from week to week.

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Old 27th September 2002, 10:44 PM   #89 (permalink)
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Hi Durzel

Just seen your pic. The Top Secret front splitter I have has a 'venturi' halfway down it and I suppose this must be their for some reason . Mycroft ?

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Old 27th September 2002, 11:10 PM   #90 (permalink)
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The above pic that shows the undertray should produce downforce. The idea is to suck the air out the back of the diffuser, in generall it will exit at the same speed as the bulk flow (because it becomes part of the bulk flow as it rejoins it) and in order for this to happen flow must be maintained along the length of the diffuser (what goes in must come out).

But with the ground being close to the diffuser, there will be a constriction at it's throat (point lowest to the ground), so air velocity is increased, and pressure drops. If the ride height is large though the percentage decrease in area is small.

It is just like a wing, except the pressure differential on a wing just kinda bleeds away. Which the ground close to the lower surfaces, the air available to equalise the pressure is vastly reduced, and it must come from the sides. If you then put skirts on the sides, you have fully utilised the ground effect.

The problems the CLKs had when they flipped at LeMans (and it was a BIG flip) were possibly down to being greedy. There is a fine line between not discouraging airflow under the car, and ramming it in as fast as possible. I think they crossed it, hit a bump and whoooosh, they was up and away.

Paul
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Old 28th September 2002, 12:37 AM   #91 (permalink)
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I may be wrong here, but wernt the Le Mans CLK's (and dont forget Porsche had two flips on the GT1 as well, in America) flat bottomed due to the GT regs?
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Old 28th September 2002, 10:32 AM   #92 (permalink)
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Gary, everything I'm told and almost everything I read promotes the 'high ahse' stance. the pressure difference between cutting thru the air (that is the air close to the ground) and that in its wake is quite small, if there were skirts on the rear that were similar to those on the front, the car would be quite unstable at speed, the more turbulent the air exiting the car the less value it has, but the larger the gap from the ground to the rear valance the greater the opportunity for this turbulence to happen, the 'fins' are an attempt to allow the pressure differential (front/rear ratio) to escape smoothly without too much hindrance.
The wheels will try to force a vortex to form and this causes problem at their inducement point, so although useful in some places they are generally bad news at the exit.

If we can 'shroud' off the wheels from the 'throat' formed by 'my' skirts for example then the air has a good chance of exiting cleanly, most Le Mans cars do this and because of the stable platform such cars suspensions can be cropped close to the tarmac, we dont have that 'luxury' , but we are getting ahead of our selves a little so it is probably best to return to this in a few pages time.

Durzel, no, that 'tray' system will help the car, but only marginally unless other things are done, if you were to extend that centre 'tray' further under the car and vented it so as to cool the Engine/Diff area it would do more, additionally that picture does tie up a loose end in my thoughts, the side pods will almost certainly benfit from enclosure, this is common to almost all performance cars, that fact that Nismo has done this probably means they intended to keep the 'Factory' cars one step ahead of the run-of-the-mill motors. (common practice)

Glen, yep, and they need to become 'slab-sided' so as to stop air trying to enter the low pressure area under the car, if you look at the Skyline its sides' are curved, the entire shape of the side of the car would have to change to make a full flat under tray to work effectively.

Suspension with insufficient travel hitting a bump will change the angle of attack and the wing which once (effectively) pointed to the ground, then points to the air and you have take-off, at 200+ the car was past both the V1 and V2 thresholds and she tried to fly, if you drop a wing section from a height then it will flutter or spin around its on centre point, the CLK did this precisely, it was predictable and inexcusable.
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Old 28th September 2002, 09:53 PM   #93 (permalink)
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Mycroft,

The problem with using the brushes as you describe, is the pressure drop will not be equal across the width of the gap. What it will do is encourage higher velocities next to the brushes themselves. Of course some of this will lead to low pressure distributed under the car, but if will be concerntraed at the brushes.

The use of a ground effect venturi would promote low pressure on the under skin of the car, where you really want it. A low pressure region just randomly under the car is not nearly as a low pressure region attached to the bottom surfaces of the car.

As a solution for a roadcar however, it could be pretty good, as it does seem quite practicle from your discriptions.

A flat undertray should work no problem on a curved side road car. I don't think the GT racers use flat sides purely for functional reasons, rather they are very easy to implement and design. Unless you run skirts or whatever, you are going to get leakage from a flat undertray, but it's still a damn site better for both drag and downforce that not having one.

The fins on the underside of the front venturi/diffuser show above are probably their to prevent the air in the diffuser from being sucked out sideways, and generally stopping it oscillating from left to right. Depending on how critical it is, you only need a slight cross wind or bit of understeer/oversteer and the diffuser gets stalled on the edge and it screws it all up.

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Old 29th September 2002, 01:31 AM   #94 (permalink)
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#The problem with using the brushes as you describe, is the pressure drop will not be equal across the width of the gap. What it will do is encourage higher velocities next to the brushes themselves. Of course some of this will lead to low pressure distributed under the car, but if will be concerntraed at the brushes.#

I'm not sure I understand this, but the brushes run along the underside of the car
-------------------------------------------

#The use of a ground effect venturi would promote low pressure on the under skin of the car, where you really want it. A low pressure region just randomly under the car is not nearly as a low pressure region attached to the bottom surfaces of the car.#

The principle is to continue the venturi effect along the length of the car.
-------------------------------------------

#A flat undertray should work no problem on a curved side road car. I don't think the GT racers use flat sides purely for functional reasons, rather they are very easy to implement and design. Unless you run skirts or whatever, you are going to get leakage from a flat undertray, but it's still a damn site better for both drag and downforce that not having one.#

Surley it is the other way round, the low pressure zone atracts the other air into it, it don't leak out! Slab sides are easy to design but are great at preventing this flow under the car, especially as they are usually just 3-4inches off the ground
-------------------------------------------

The fins on the underside of the front venturi/diffuser show above are probably their to prevent the air in the diffuser from being sucked out sideways, and generally stopping it oscillating from left to right. Depending on how critical it is, you only need a slight cross wind or bit of understeer/oversteer and the diffuser gets stalled on the edge and it screws it all up.

Agreed, that is why most Le Mans cars run a deep trimmer near the wheels and shallower ones in between, the ratio here is important and so is how far back they extend.
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Old 30th September 2002, 09:31 AM   #95 (permalink)
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Perhaps a different perspective on the issue:

wouldn't it be more effective to go for an active system with fans like Chapparral used in the Can Am of the Seventies?
Their system was fairly simple but clever: a separate engine ran the fans and moveable skirts were used to shield off the vacuum area.

Obviously you can't create those extreme levels of downforce on a road car but you would be able to generate a constant level of downforce (more or less independent of speed).
If I remember correctly, the McLaren F1 uses fans like this for the same purpose.

Any thoughts?

MDG.
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Old 30th September 2002, 09:48 AM   #96 (permalink)
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while we are looking at other alternatives,

has any manufacturer ever played about with using the wheels themselves to suck the air out from under a car ?

If the wheel spokes were twisted in such a way as to produce a fan effect they could either force air in or out from under an enclosed wheel chassis.

The only problem I can see with this is that on slow cornering (when you need max down force) there would be the least amount of air being pulled from under the car.

maybe just another hair brained scheme but it would be free downforce. (dunno how much more fuel the car would use as a result of the increased drag though...

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Old 30th September 2002, 10:50 AM   #97 (permalink)
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More artz required, will do some more!

Mycroft, low pressure leaking out is, I suppose, the same as air leaking in, granted.

The Brabham fan car represents the ultimate in active aerodynamics! Blip the throttle and it sucked itself to the ground.

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Old 30th September 2002, 12:05 PM   #98 (permalink)
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Also highly illegal, it is a road sweeper/vacuum without a bag to collect the rubbish, course getting behind such a car would preclude the need to sand your car down prior to a respray.

Using the wheels is no good, except maybe on the front (even then I would think its a little risky and noisy.) but you would disturb the line of flow under the car badly.
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Old 2nd October 2002, 12:00 PM   #99 (permalink)
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Group C sports cars and Group B rally cars used to use 'spinners' on the wheels - big flat discs with vanes attached to the outside of the wheels. Mainly for brake cooling I think but might have been for downforce on the Group C racers at least. The powers that be banned them for some reason.
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Old 2nd October 2002, 09:31 PM   #100 (permalink)
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Hmmm, interesting, do any of you remember such a thing as a gyroscope? It was a toy, it was just a heavy disc with a shaft running thru the centre of it attached to spherical wire frame, at each end of the shaft, the shaft had a hole at one end where a length of string is threaded thru and wound round the shaft and when fully wound you pulled hard on the string and this thing wound stand on a pencil tip perfectly upright, do any of you recall hold the G'scope in your hand and trying to rotate it?

There was resistance wasn't there?

Well when we turn our steering wheel we have to overcome this, and if you add to this a directional flow of air which would also resist the movements of the steering wheel.

That is one of the reasons unsprung weight is important, and also why wheels on road cars are carefully designed to be near neutral in the wind vane effect as well as those described previously.
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