43 replies to this topic

[Pascal]

For some time now, I've been asking myself that question... Let me explain.

It is commonly known that nowadays overtaking is made difficult by the aerodynamic turbulence created by the car in front, and some people advocate smaller wings to cure the problem. That seems rather odd to me considering that wings have been in use for the last 30 years... Therefore I tend to believe that the blame should be put on the diffusers that have become more and more complex in design over the last decade. Besides, their low position seems more likely to affect the downforce created by the front wings of the second car.

I might be completely wrong of course, but I think that getting rid of diffusers might be a solution to the difficulties most drivers seem to experience when trying to overtake a slower car. Tell me what you think.

[petronas]

I totally agree.

I had the opportunity to talk to a Sauber aerodynamic developer and he explained me, well I don`t understand to much of it, that the diffusors are the main problem of the overtaking problems - not the wings itselfs.

Maybe there are some other opinions out there?

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http://www.pitlane.ch

[IndyIan]

Pascal,
I don't know anything about how diffusers work but I think you are right. Indy cars don't have them and even with the handford device on their back wings on the super speed ways indy cars can follow relatively close on the corners.

[DangerMouse]

Yes they should be banned! along with Gurney Flaps and Barge Boards, also wings should be restricted to 1 element running at a defined maximum plane of only about 5 degrees.

Then stick big fat sticky slicks back on!

[SlowDrivr]

I'm not sure what the diffuser is exactly. Is it the entire underbody of the car or just some part of the rear underbody? What's it for? I guess it has something to do with creating downforce, but what it for exactly?

[PDA]

SD - under car venturi for creating downforce were banned many years ago by mandating a flat underbody. The flat underbody only goes as far as the rear wheels, so the designers can sort of flip up the undertray at the end of the flat undertray. By carefully proportioning that air expansion path, the air flow is accelerated and a low pressure area created in turn creating downforce. I have seen comments in various places that the diffusers contribute up to 30% of the available downforce. Banning them would obviously reduce downforce, and may well reduce the low level turbulence of their wake which seemingly effects the front wings of following cars so adversely.

I tend to agree with DM that multi element wings should be restricted to additionally reduce downforce, permitting the return to slicks. The rear wing should also be moved forward and reduced in size to greatly reduce its effectiveness.

To limit the pitch sensitivity of the cars, and further reduce under body induced downforce, ground clearance should be increased, significantly IMO.

Suspension and damper engineers will then take on a much increased importance.

[MPH]

I agree that the diffusers should be banned if they are creating the turbulence problems, but I don't know why down force from the rear wing should be limited. Increased down force increases drag so there is a trade off that the teams have to make. If you mandate less down force you will see higher straightaway speeds, which the FIA does not want to see.

[Zeus]

They should be banned or at least drastically reduced in size. F1 still needs to be fast to maintain it reputation as the top form of motorsport and if they were totally removed from the car, the speed would probably suffer too much.

[Christiaan]

Now a good question to ask is why then does the Ferrari seem to create the greatest turbulence in its wake. I say this because ever since the Ferrari adopted their exchaust style, I have hardly ever seen it being overtaken on a high speed section. This means that either its top speed is very high (which I doubt as compared to Mclaren and Prost) or that anyone behind a Ferrari has an even harder time slipstreaming.

[Pascal]

Well Christiaan, it might then be interesting to watch if that apparent phenomenon is duplicated with the Stewart cars during the next few races, as they now use a similar exhaust system...

[SlowDrivr]

Is the FIA concerned with straightline speed or cornering speed? Probably both I guess, but limiting the diffuser and wings would slow down the corners. And as was pointed out, greatly increase straightline speed. Wouldn't this make for easier passing? Having to slow down more from an even higher speed?

[PDA]

SD - cornering speed is the main concern, because that is where nearly all of the crashes happen.

Smaller wings can be induced to give higher downforce, but only by increasing drag, which slows the car on the straight.

[Pascal]

SlowDrivr, I don't fully agree with your point. One of the main limitation factors to straight line speed is the aerodynamic drag of the car.

In the case of wings, the bigger they are the more drag they generate. But in the case of diffusers, the downforce they give to the car is obtained through a venturi effect. I might be mistaken, but I believe that the main advantage of that setup is to create almost no additional drag. Therefore removing diffusers would not give any significant improvement in terms of top speed to F1 cars.

Correct me if the lines above are incorrect or incomplete, as they might well be...

[SlowDrivr]

Pascal, that's a good point. I'm not sure how much of an effect the diffuser has on drag. If the diffuser creates a low pressure area behind the car, then it should create drag since low pressure behind the car would tend to pull it backwards, which is drag. If the low pressure mostly pulls it downward then there wouldn't be much drag.

I don't know which it is.

[Pascal]

The low pressure area is mainly contained within the diffusers themselves, so I don't believe they're likely to create that much drag in "sucking" the car from behind...

[baddog]

maybe some aerodynamics buff is going to contradict me here, but if you generate downforce, you must be using something to generate it, and the only thing going is speed.. it might be more fluid-mechanically efficient than wings, but surely it must generate drag? conservation of energy and all that crap, unless F1 cars are quantam devices ;)

Shaun

[ket]

Diffuser create less drag than wings is because it does not increase the frontal area of a car whereas wings do. Diffuser is also less affected by dirty air. The stepped bottom ruling by FIA force the constructors rely even more on wings to create down force. If you look at the post ground effect cars, you will find that they have flat and flushed bottom with big diffuser just after the read axle, the cars also have very low ride height. They do not generate as much down force as ground effect cars but they still produce much more than today's stepped bottom cars with FIA's plank which enforce the minimum ride height.

[Christiaan]

Baddog, you are right about that. I am not familiar with the true mechanics of the diffuser but I will say that with wings, the greater they deflect the incoming streamlines of air, the greater the drag. It is impossible for a wing to function without drag. The Meweys of this world have the know how of reducing this drag.

It is important for you to note that the rear diffuser is out of the "drag area" of the car and (I think) no air flows into it directly. This means that it will not contribute to the drag.

[Achim Kuhl]

Hi you exhaust pipe sniffers!

I'm lucky to have a Formula 1 fan around who also is a graduated in physics. I will try to put his explanation down in English.

A F1 car as a whole is aeroynamicaly like a wing. The components are the nose, the underbody and the difusor. They work together. Under the high nose you let as much air as possible in (more air is more effect). It streams allong the underbody. To make it stream there as fast as possible (creating underpressure = downforce) you got to have the difusor. So there is no resistance for the underbody airflow and it can stream off without turbulences. Then the car ends an air can do whatever it wants without affecting the car. Sadly it tends to build great turbulences.

So it realy doesn't matter which kind of aerodynamic downforce you create, it always affects somebody behind. Best solution is more mechanical grip.

Enjoy next race

Achim

[Christiaan]

Achim, yes and no. If you create a vortex 7m after your car the somebody who is 2m behin you can take advantange of the slipstream. If you create the vortex 1m behind your car the somebody is better off drivingout of the driving line because the vortex will be like a nasty crosswind.

So in principal, YES, any car will create some sort of turbulent flow behind it. The distance behind the car, that the turbulence is great, is the key factor. In todays cars this distance certainly is very small, making slipstreaming much harder. I don't know what role the rear diffusor plays in all of this but I suspect its a large one.

[narhuit]

Baddog - agreed. Diffuser are probably more efficient than wings (have a better downforce/drag ratio), but they do generate drag.

Ket - I disagree. The frontal area is not the main reason. Of course the frontal area is of great importance in aerodynamics. But the reason you give is a little bit simplistic. Imagine that the air flow coming into the diffuser is blocked. you have the *same* frontal area, you do not have downforce (from the diffuser, of course), and you have *less* drag. However, a diffuser is less affected by dirty air than wings are, because dirty air makes a wing stall, whereas it only reduces a diffuser efficiency.

Christiaan - Do you really believe you can generate downforce without generating drag? Then you solved the problem of finding new energy ressources: why don't you use this downforce not to pull the car down, but forward? If you create downforce, then you have drag. Then, the way you create downforce can be more or less efficient, and disturb what follows you for 1 meter or 7 meters, and allow it to slipstream within the disturbed area behind you. The more drag you have, the more easy is the slipstream to catch, all right.

[Achim Kuhl]

Hi!

Let me rephrase: The current aerodynamic concept derived from the underbody. So it resulted into this high nose plus difusor thing. To change this you have to change the rules for the underbody which would result in new means for incomming and offgoing air. Just banning difusors would create a car with aerodynamics closer to NASCAR than formula racing.

Achim

[ket]

Narhuit,
My reasoning about diffuser generating less drag in relation to frontal area is based on the fact that an F1 car, with or without diffuser, has already paid the penalty of drag from having a certain frontal area due to its dimension. By having diffuser, the frontal area of the car will not increase and thus no extra drag. The result is not exactly something from nothing, but rather exploit what you have already paid for.

[PDA]

Ket - another way of saying that is that cleaning up the airflow under the car actually reduces drag somewhat. Then putting that air through a diffuser and generating 30% of the downforce with very little additional drag is quite a bonus. To get the same amouint of downforce using wings would require much bigger wings (which are not allowed) and higher angles of incidence, both of which would result in a large increase in drag. In comparison, the diffuser is "something for nothing"

[Christiaan]

Er, Narhuit, I don't understand what you are trying to say.

What I want to point out is that the air entering the bottom of the car is less than the air leaving the rear bottom of the car, because of the presence of the rear diffuser pumps out exhaust gases. Based on this , simple conservation of momentum is not valid because the mass is entering is not equal to the mass leaving.

Ket also explianed that the diffusor is not incident on the frontal area, so it cannot create drag.

The Ferrari exhaust style has one very big advantage. It significantly reduces the drag created by the rear wing. It blows a jet of high speed exhuast gases just a little under the theoretical stream boundary so that boundary layer seperation does not occur and a turbulent stream is not created in above the wing. This significantly reduces the drag. I am guessing that this effect makes it even harder to slipstream behind a Ferrari.

[narhuit]

Ket & Christiaan - I am afraid my English will not enable me to put what I mean in understandable words in a so little place (right now, I am make wide moves with my arms to try and give better explainations ;-) ). Let's try with an example. [Ooooh if only I could draw...] Take a basic wing; made of a flat, square underplane, and a bent "overplane". It has got a precise frontal area. If you cut it vertically, through all its length, where it is the most thick, you do not change its frontal area at all, but you certainly increase the drag a lot, don't you?
The diffuser creates drag, because it creates a low pressure air pocket behind (and preferably under) the car, which "sucks" the car down, *and back* (well, this is pretty simplictic too, but I hope this is not that inaccurate...).

[Christiaan]

Narhuit, I understand what you mean by cutting the wing section but I don't see its relevence to the diffusor since the diffusor is not hit by the incident airstream. By cutting the wing you described you increase the drag because you have increased the Cd value which is a function of the actual shape of the body that cuts the airstream. The diffusor is "inside" the car so its dimensions do not have any contribution to the Cd value.

The extra downforce does createfrictional drag, which is a result of the increase in the normal reaction force on the tyres. This drag is frictional, and not aerodynamic. The diffusor will only create aerodynamic drag if the low pressure zone results in some sort of turbulence under the car. I doubt that is the case because of its proximity to the rear of the car.

[SlowDrivr]

I don't understand what you mean by the diffuser being "inside" the car and so it cannot change the Cd. If air flows on any given surface, can't its shape change the Cd?

[PDA]

SD - it is a relative rather than absolute statement. The diffuser does not project out into the airstream so does not increase the frontal area. This means that any downforce it generates is "free" compared with an aerofoil, which must add to frontal area.

Also, the diffuser cleans up the airflow under the car, so probably contributes to a reduction in Cd cmpared to a car in which the underbody just stops at the rear axle line.

[narhuit]

Christiaan & PDA - What I mean is that the drag coefficient does not depend on the frontal area only. Hence although the diffuser is not involved in the frontal area, it *is* involved in the drag coefficient. And I am talking about aerodynamics, not friction. The diffuser is somewhat like the contrary of a airplane reactor: a low pressure that pulls backward instead of a high pressure that pulls forward.

The purpose of the diffuser is to have as little air as possible under the car, so that the car is "sucked" down. Rules about the shape of the underbody of the cars prevent them from having an efficient underbody in this regard. Hence the importance of the diffuser, which can create downforce, but, of course, at the expense of more drag.

[PDA]

Narhuit - Drag coefficient is completely independent of frontal area. Total drag is Cd x (frontal area).

My contention is that the diffuser provides a lot of downforce in exchange for a small Cd penalty. It will be very small because the diffuser assists exhausting air from beneath the body and will effectively reduce Cd in that area. Compared with the drag penalty for any other legal method of inducing downforce (wings) it is practically something for nothing.

[narhuit]

PDA - Oops. Of course the drag coefficient does not depend on the frontal area at all. I quite mixed the drag and the drag coefficient together! Apart from that, I think I can easily acknowledge that the diffuser brings a lot of downforced in regard to the drag it generates.

[Christiaan]

Okay Narhuit, I see what you are trying to say. The pressure difference between the front and the back of the car is directly related to the Cd. I can appreciate that the diffusor contributes also to this "negative propulsion effect" but I cannot see it doing so greatly, to the extent that it might create a large pocket low pressure immediately behind the car instead of under the car.

Also, there is something you might have confused. The rear diffusor (as I understand it) creates a low pressure zone under the car by acceleration the airflow using exhaust gases. Therefore it is not as you said "ensure that as little air as possible flows uner the car". If this was true then from Bernoullis principle, (slow moving air = high static pressure.) the pressure differantial will be more like a wing on an aeroplane and cause the car to lift.

[This message has been edited by Christiaan (edited 08-20-1999).]

[Pascal]

What follows is barely understandable, as I would even have trouble saying in French... I'll nevertheless try to be as clear as possible.

Correct me if I'm wrong, but when the low pressure area is "released" from within the diffuser itself by exiting rearward, the atmospheric pressure will quickly fill the gap from all direction, except from the car which will be running too fast and leave in its trail a stream of unstable air because of its rear wing. That is why it seems to me that the drag penalty of using a diffuser is close to negligible.

Now back to my original question. Will not the mere fact of getting rid of the contraption force the engineer to put more emphasis on mechanical grip? Will it force them to use the wings with higher angles therefore reducing straight line speed? In my opinion, implementing a step bottom without banning diffusers looks like a half-baked measure from our beloved FIA...

[narhuit]

Christiaan - There might be a little misunderstanding (hey, Pascal, mind going on in French ;-) ?). As little air as possible indeed. To have a low pressure under the car, so that it is "sucked" down. And Bernouilli can live happy with this, because "a little air" does not mean it has to travel slowly. On the contrary: the faster it goes, the lower is the pressure (Bernouilli is still happy!).

Pascal - "The atmospheric pressure will quickly fill [the low pressure area] from all the directions, except from the car". This is exactly the very explaination of the drag: the car tries to fill in the gap too; it is "sucked" backward: drag! Its speed will not allow him to "escape" this force.
Now, on the original question, I am not sure. I would even think that diffusers could be privileged over wings. If they produce less drag, they produce less "dirty" air too. They are less sensitive to dirty air. So, cars with diffusers and small wings might very well be able to "easily" closely follow each other, increasing the overtaking chances. On the other hand, behind a wing which is big and/or has a high angle of attack, the air is very dirty, and cannot be efficiently used by an other wing (of a following car, that must then stay at bay).

[PDA]

narhuit - One of the perceived problems of Diffusers is that the vortex created is at ground level and consequently causes more problems for the front wings of a closely following car than the higher vortex coming off the rear wing, which is significantly higher in comparison to the front wing.

One of the really bad characteristics of the current breed of flat bottomed, diffuser rear cars is their pitch sensitivity. Look what happened to the MB cars at Le Mans. If the front of the car is lifted, the whole car goes off like a Frisby. Very dangerous. The same thing happened to the Porsche GT1 at Road Atlanta in the Petit Le Mans last year.

[narhuit]

Hum... These are pretty much the same arguments as for the pros and cons of the ground effect. Less sensitive to dirty air, more sensitive to pitch...
But I am not a diffuser fan. I just tried to figure out why they were so popular amongst the car designers.

[PDA]

Narhuit - they are popular with the designers because they can get a hell of a lot of downforce from them. When underbody venturis were banned by introducing flat bottoms, they made the mistake of only demanding that the flat bottom extend to the rear axle line. Behind the axle line, the dsigners were allowed to design diffusers. They now contribute as much as 30% of the overall downforce.

[Christiaan]

If you have very little air under the car then you have low static pressure, that is true. Only, at the speeds that F1 cars travel the static pressure is very very low compared to the dynamic pressure. Also, to maintain a given dynamic pressure, you have a square relationship with the velocity that the air travels to volume so that a certain pressure gradient is maintained.

I believe that the drag created by the car as result of the rear diffuser is small. A drag is always created by the low pressure behind the car exists even without the diffusor. This low pressure zone is a result of the body of the car disturbing the airstream.

I agree that the diffusor might increase pressure gradient between the front and the back of the car, but by how much? I am speculating that the velocity of the pressure wave which drives ambient air to fill the low pressure zone is much much faster than the car itself.(afterall, sound is only a pressure wave ,and it travels at 330m/s) This implies that the low pressure behind the car is immediately resolved. So I propose that the flow immediately behind the car is turbulent but the pressure is the near that of the front of the car, which is ambeint pressure.

[This message has been edited by Christiaan (edited 08-30-1999).]

[Pascal]

Christiaan, you've succeeded in putting in understandable English the point I was trying to make in my confusing last post.

Thanks!

[narhuit]

An other point of view: we saw last week-end that Monza is not a track where it is any easier to overtake than anywhere else. And this despite the fact that the cars run with nearly no wings! Hence I assume that something else than the wings prevents the cars from being able to overtake one another. The diffuser makes a good suspect, doesn't it?

[PDA]

In spite of Monza being (like Hockenheim) a low dawnforce circuit, the performance of the wings is still critical. The "problem" that makes overtaking so difficult is that turbulence behind the leading car disturbs the airflow over the front wings of the following car. If the following car gets to close in a corner, the reduced downforce, particularly at the front causes massive understeer. This slows the car considerably (and wears the tyres ). Because of theis, the following car cannot drive up close behind a car through a corner and then pass on the following straight.

So even on a low downforce circuit, the same effects take place. Because many of the corners at Monza are relatively high speed corners, the effect is even worse. In recent years, overtaking by anyone at Monza has been very rare, making RB's performance in the first section of the race very praisworthy

[BADGER]

I have some things I feel are worth mentioning with regards to diffussers. First of all, the largest component of drag comes from the following equation which I think is referred to as planer drag:

drag=.5x(air density)x(frontal area)x(velocity squared)

Because wings increase the frontal area, it is obvious from this equation that drag is significantly increased with wings. Also, since a diffuser does not increase frontal area, they do not increase this function of drag. However, along with planer drag, there is also a function referred to as induced drag. Induced drag is made up of friction between the air and the surface of the car. It also is affected by how the air flows around the surface of the car (whether it is turbulent or laminar). The diffuser does not create additional planer drag but it most likely increases induced drag by causing at least a small amount of low pressure air behind the car.

I also want to say that the venturi (ground effect) principle does not rely on preventing air from being under the the car in order for it to work. It is not the absence of air that causes the pressure drop, it is the higher velocity of air under the car that causes the pressure differential between the top and bottom of the car. This is why the exhaust used to be blown into the diffusers of F1 cars. It was done to increase the velocity of the air under the car which reduced its pressure. I think it is now illegal to exhast out the diffusers or else it caused too many stability problems as drivers got on and off the gas.

For anyone interested, the best book that I have seen on aerodynamics is "Race Car Aerodynamic" by Joseph Katz. If you like the technical side of racing it is a great book.

I saw where someone said that the ride height or the step plane should be increased in order to reduce downforce and reduce turbulence. I agree with this absolutely. Even with an true ground effect car like used in CART, the greatest air velocity (lowest pressure) is reached at the venturi section closest to the track. Interestingly, this is why the current CART cars have almost a flat bottom nearly all the way to the rear axial. When you maximize the area of the fast moving air, you are maximizing downforce. A recent picture of a CART underbody (Racer) showed that it is actually very similar to the flat bottom/diffusser used in F1. Really, the only difference was the Cart car had vanes at the leading edge of the sidepod.

By increasing the height of the step plane, you will reduce the downforce and it should also make the cars more stable. The problem with the current cars riding so close to the ground is that when they hit a bump or pitch under braking, the airflow under the car changes radically. As an example, if you are one inch off the ground and the nose dips .5 inch, suddenly the underside is recieving much less air. However, if you go from 3 inches of clearance to 2.5 inches of clearance, the airflow is still reduced but not by as large of a percentage. I think one of the largest benifits of the raised nose is that the higher nose gives a more repeatable type of airflow under the car.

My final thought is to limit the diffuser and to raise the step plane.

[Yelnats]

Not being trained in fluid dynamics I have had to use simpler concepts than venturis to understand diffusers. I think of the diffusers as a coupler between the low pressure area behind the car and the underside which produces the downforce effect.

This was first detected when the fan on Jim Hall's Sucker car failed and he proceeded to race anyway only to find the cornering of the car was little effected at high speed. Presumably this was because of the coupling of the low pressure area behind the car through the fan orifices to the underside of the car. The sucker car used dragging, sliding lexan skirts to maintain the low pressure effect but these were soon banned as moving areodynamic devices as they were very effective but they caused the cars to fly off the road when they lost their seal on bumps or during spins.

[This message has been edited by Yelnats (edited 09-26-1999).]