Diffuser Stall

[Greig]

Idiot (payback for Variante Alta) haha

[coysht]

Originally posted by stefano
Does that mean that I am on the right track then?

If I'm just an idiot, go ahead and tell me, I'll take it all in stride

:

You're not wrong, but just how right you are I'm not sure.


(that helps doesnt it! : )

[antares]

Yes, the air does need to be sped up underneath, but the air leaving the underbody channels needs to be not exactly, but about the same speed... I guess similar to the speed differential between the lower and upper surfaces of the wing. Otherwise, and I'm sure you all understand this, there will be LOTS of turbulence, because the air will mix and swirl ==> eddies ==> drag ==> bad. Not saying you're wrong here Tom, but I think that's the principle it works on... speed it up under the car, so you get the downforce, and then make sure you slow it down right, so you don't get too much drag.

[stefano]

Originally posted by Greig
Idiot (payback for Variante Alta) haha

Fair play...

: : :

It would be nice if we could get some "definitive" information somewhere on what the diffuser's primary goal is - creation of downforce vs. reduction of turbulence (and therefore drag). Unfortunately, I wouldn't know where to look...

[SS454]

Originally posted by stefano

It would be nice if we could get some "definitive" information somewhere on what the diffuser's primary goal is - creation of downforce vs. reduction of turbulence (and therefore drag). Unfortunately, I wouldn't know where to look...

Mostly definitly to create downforce. The undertray/diffuser is worth about 40% of the cars downforce.

[antares]

Definitely to create downforce.

It serves the same purpose as underbody tunnels would if they were allowed. They're not, so they have to make a diffuser, behind the rear axle. It definitely reduces turbulence, but hell, any simple flap could be made to do that.

[coysht]

Originally posted by antares
Definitely to create downforce.

It serves the same purpose as underbody tunnels would if they were allowed. They're not, so they have to make a diffuser, behind the rear axle. It definitely reduces turbulence, but hell, any simple flap could be made to do that.

I'm not so sure "reduces turbulence" is correct as any turbulence is created by the diffuser in the first place.

[stefano]

SS and Antares, you seem to be certain that the main goal of the diffuser is to create downforce. But I reiterate my doubt:

The principle with which downforce is created on the underside of the car is the Bernouilli principle whereby air that is SPED UP decreases in pressure and therefore the car is "sucked" to the ground.

But if the aim of the diffuser is to SLOW THE AIR DOWN (by giving it more "space", hence the cone shape), then what kind of downforce is it generating?

That's why in my mind, the diffuser serves not so much to create downforce as to "clean" the airflow at the rear of the car... i.e. once the air reaches the diffuser the downforce has in reality already been generated by the underside of the car.

Correct?

[coysht]

Originally posted by stefano
That's why in my mind, the diffuser serves not so much to create downforce as to "clean" the airflow at the rear of the car... i.e. once the air reaches the diffuser the downforce has in reality already been generated by the underside of the car.

Correct?

Yes the downforce is basically created under the car, but the diffuser is their purely to "pull" that air through. Without the diffuser you would have no downforce under the car, therefore it is the diffuser that generates the downforce no matter where the force is actually felt.

Also, not so sure that the air coming out of the diffuser is that "clean" anyway, if anything the process of expanding it will make it quite turbulent, but due to the amount of time gained through downforce the any extra drag is acceptable.

[antares]

I agree Tom, except for the turbulence bit.
Diffusers, if badly designed, could create huge amounts of drag, because the flow becomes detached (a whole lot worse than turbulent), if the angle at which it slopes up is wrong. I'll try to illustrate this when I get back to my room later today. The next worst thing is turbulence, but if I'm not mistaken, that range is very small, because of the way it transitions from laminar to turbulent to detached with that geometry. Again, I need a picture, and I will edit this post in a few hours. So yes, it CAN be turbulent, but it's more likely to be detached, which will lead to stall (eventually, same principle as a wing.. when the detachment point comes too far forward, the wing stalls). And most teams avoid that with their diffuser geometries, so on the whole the airflow is pretty clean, except in some small speed ranges, where it becomes somewhat turbulent.

[SS454]

You want the diffuser to pull as much air from under the car as possible, to keep as much air moving as fast as possible, then get the air to vent out of the diffuser and join the speed of the air above the lower mainplane/behind the car to keep the drag at a minimum. It make sense if the speed of the air is massively different, it could cause the air to "detatch" like antares says.

[antares]

I just got back... I'm not in a mood to draw any kind of diagram of this. So I guess you guys will have to wait a day or two, till I'm in a mood for this.

[coysht]

Originally posted by SS454
You want the diffuser to pull as much air from under the car as possible, to keep as much air moving as fast as possible, then get the air to vent out of the diffuser and join the speed of the air above the lower mainplane/behind the car to keep the drag at a minimum. It make sense if the speed of the air is massively different, it could cause the air to "detatch" like antares says.

The air detaches from the diffuser if the diffuser ramps are too steep for the speed of the air passing through, or if the intermediate fences are overloaded and the vorticies in the diffuser burst (as it says back in my first post of this thread).

[stefano]

Alright, I'm getting there (slowly : ).

I understand the concept of the diffuser "pulling" the air from the undertray, since this further speeds it up and creates more downforce.

Now (you're going to start hating me ) by what mechanism / principle does the diffuser "pull" the air from the undertray?

[Tobes]

Originally posted by stefano
Alright, I'm getting there (slowly : ).

I understand the concept of the diffuser "pulling" the air from the undertray, since this further speeds it up and creates more downforce.

Now (you're going to start hating me ) by what mechanism / principle does the diffuser "pull" the air from the undertray?

Stef...

The diffuser has two functions, primarily it creates low (negative) pressure under the car that creates downforce, and by doing that, the area of negative pressure under the car then in effect 'pulls' the air from in front of that area into the area (of negative pressure) and so on...

As far as I understand it (and please correct me if i'm mistaken) there is in effect static air molecules that attach to long flat areas of the bodywork of the car, when at speed, this static air creates friction along these surfaces including the diffuser, the closer to the ground the car gets, the more influence this static air has in the aero of the underside of the car untill eventually the static air in effect disrupts the airflow under the car to such an extent that the downforce created becomes minimal and the diffuser 'stalls'...

I think that's about right...

[JustBringIt]

Originally posted by Tobes
Stef...

The diffuser has two functions, primarily it creates low (negative) pressure under the car that creates downforce, and by doing that, the area of negative pressure under the car then in effect 'pulls' the air from in front of that area into the area (of negative pressure) and so on...

As far as I understand it (and please correct me if i'm mistaken) there is in effect static air molecules that attach to long flat areas of the bodywork of the car, when at speed, this static air creates friction along these surfaces including the diffuser, the closer to the ground the car gets, the more influence this static air has in the aero of the underside of the car untill eventually the static air in effect disrupts the airflow under the car to such an extent that the downforce created becomes minimal and the diffuser 'stalls'...

I think that's about right...

I don't know about the diffuser creating a low pressure. I was under the understanding that it blends the air from under the car into the air going over the car.

On the second point you make, when any fluid flows around a body, a boundary layer is formed where the flow speed has a parabolic (I think) profile from zero at the surface (i.e. the air is effectively attached to the surface) to the flow speed at the outer edge. So, as you say, if the road and diffuser surfaces get too close together (i.e. the two boundary layers meet), its possible that air would stop flowing through the diffuser. Whether this is actually "stall" or if its just a convenient term to use, I'm not sure, because its not how its typically used.

What I still haven't got is how the diffuser doesn't vibrate itself to bits when it goes through this "stall" because of the rapidly changing back-and-forth of the pressure in the diffuser as it stalls and un-stalls. Vibrations tends not to be that good from a drag viewpoint so I'm not sure about it.

[stefano]

Originally posted by Tobes
Stef...

The diffuser has two functions, primarily it creates low (negative) pressure under the car that creates downforce, and by doing that, the area of negative pressure under the car then in effect 'pulls' the air from in front of that area into the area (of negative pressure) and so on...

But you see, mate, that's exactly what baffles me. (I wish I knew how to draw something and then post it up here, but unfortunately I don't : ).

See, the diffuser, IN AND OF ITSELF, DOES NOT create low pressure under the car. Low pressure is created when you try to "squeeze" air through an opening. The carb is the simplest way of picturing this. The engine is sucking air through the carb, but the passage is profiled such that the air gets "squeezed" into the main body, thus creating the low pressure and sucking the petrol through the nozzle.

By its nature, the diffuser does the opposite. It provides MORE space for the air that has ALREADY been squeezed underneath the car. The air moves from the underside of the car to the diffuser where it can "expand" as such. I would therefore expect the pressure within the diffuser to be significantly higher than the pressure underneath the car. This is what I mean when I say that the diffuser does not create downforce in and of itself.

Does that make sense?

And for Heaven's sake, techies out there (Thomas, I'm looking at you ) correct me if I'm wrong!

[coysht]

Originally posted by stefano
But you see, mate, that's exactly what baffles me. (I wish I knew how to draw something and then post it up here, but unfortunately I don't : ).

See, the diffuser, IN AND OF ITSELF, DOES NOT create low pressure under the car. Low pressure is created when you try to "squeeze" air through an opening. The carb is the simplest way of picturing this. The engine is sucking air through the carb, but the passage is profiled such that the air gets "squeezed" into the main body, thus creating the low pressure and sucking the petrol through the nozzle.

By its nature, the diffuser does the opposite. It provides MORE space for the air that has ALREADY been squeezed underneath the car. The air moves from the underside of the car to the diffuser where it can "expand" as such. I would therefore expect the pressure within the diffuser to be significantly higher than the pressure underneath the car. This is what I mean when I say that the diffuser does not create downforce in and of itself.

Does that make sense?

And for Heaven's sake, techies out there (Thomas, I'm looking at you ) correct me if I'm wrong!

You're totally right about the pressures, but without the diffuser you would have no low pressure under the car, so it is pretty safe to say that the diffuser creates the downforce, even if its not acting on the diffuser itself.

[JustBringIt]

Originally posted by stefano
But you see, mate, that's exactly what baffles me. (I wish I knew how to draw something and then post it up here, but unfortunately I don't : ).

See, the diffuser, IN AND OF ITSELF, DOES NOT create low pressure under the car. Low pressure is created when you try to "squeeze" air through an opening. The carb is the simplest way of picturing this. The engine is sucking air through the carb, but the passage is profiled such that the air gets "squeezed" into the main body, thus creating the low pressure and sucking the petrol through the nozzle.

By its nature, the diffuser does the opposite. It provides MORE space for the air that has ALREADY been squeezed underneath the car. The air moves from the underside of the car to the diffuser where it can "expand" as such. I would therefore expect the pressure within the diffuser to be significantly higher than the pressure underneath the car. This is what I mean when I say that the diffuser does not create downforce in and of itself.

Does that make sense?

And for Heaven's sake, techies out there (Thomas, I'm looking at you ) correct me if I'm wrong!

It puzzles me too. I thought I understood it until you used that carbie example a long time ago and then I understood why my reasoning was wrong. Remember when I told you about the assumption that air is incompressible (obviously it doesn't hold when its stored under pressure, or when it travels at near or above the speed of sound, for that matter)? Its approximately true for most "normal" flows. Anyway, so that begs the question, why do they have it? The only answer I can come up with for that is to reduce the drag.

Tom, what is it that makes you say that without the diffuser there would be no low pressure under the car?

[coysht]

Originally posted by JustBringIt
Tom, what is it that makes you say that without the diffuser there would be no low pressure under the car?

Technical articles, discussions with someone i know who's quite hot on their aerodynamics and the fact than when someone breaks their diffuser they suddenly find that they have alot less downforce than they had before.

The shape of the under side of the car is flat (together with the road) that generates a parrallel walled tunnel which on the Bernoulli principle does not change the pressure of the air under the car.
To generate a low pressure you would need a profiled (like the upper surface of an aerofoil) floor shape to modify the flow, by reducing the size of the tunnel and forcing the air to speed up and reduce in pressure.

Small note: When I say tunnel, obviously on an F1 car this is open sided, but the principle is the same in that without a change in the size of duct that the air passes through, there would be no change in pressure.


Therefore the Diffuser is used to "suck" air through from under the car, creating a low pressure under the car - generating downforce on the car as a whole.

[stefano]

Hi JBI

Good to see you around, mate. Boy you have a good memory. I don't remember that particular discussion, but since it was also about carbs it goes to show that I've been resorting to the same old boring examples for years now

:

Originally posted by coysht
The shape of the under side of the car is flat (together with the road) that generates a parrallel walled tunnel which on the Bernoulli principle does not change the pressure of the air under the car.
To generate a low pressure you would need a profiled (like the upper surface of an aerofoil) floor shape to modify the flow, by reducing the size of the tunnel and forcing the air to speed up and reduce in pressure.

We may be getting somewhere...

Let me make another example. If you take a plane wing, the wing creates lift because, due to its profile, the air has to travel a greater distance over the wing than under it, which is what creates the speed increase and lower pressure, right?

So if the wing was instead a rectangle with 2 curved ends, it would create no lift because the distance the air has to travel is the same both under and over the wing, right?

Therefore, if this reasoning is correct, is it fair to say that the reason the diffuser creates downforce is because it changes the profile of the underside of the car (well the rear, anyway) thereby increasing the distance the air has to travel to escape from underneath the car???

We are going to get to the bottom of this!

[coysht]

Originally posted by stefano
We may be getting somewhere...

Let me make another example. If you take a plane wing, the wing creates lift because, due to its profile, the air has to travel a greater distance over the wing than under it, which is what creates the speed increase and lower pressure, right?

So if the wing was instead a rectangle with 2 curved ends, it would create no lift because the distance the air has to travel is the same both under and over the wing, right?

Therefore, if this reasoning is correct, is it fair to say that the reason the diffuser creates downforce is because it changes the profile of the underside of the car (well the rear, anyway) thereby increasing the distance the air has to travel to escape from underneath the car???

We are going to get to the bottom of this!

The argument about why wings work, related to larger distances is sort of true, but flawed when it actually comes down to it. But in essence what you have is true.


The diffuser does not change the distance the air must travel, but rather the cross-sectional area through which it travels.

If you imagine the gap under the car as a tube (ie ignor the fact that there are big gaps at the side) it is of constant area from front to back (ish) at the diffuser it has an increasing area.
This means that for the same volume of air to pass through the whole under-car "pipe" it must be faster under the car than at the diffuser.


In this very quick diagram, you can see the changing height of the gap under the car, and if you assume that in 3D the width is the same, the area at various points will change.

[stefano]

Originally posted by coysht
If you imagine the gap under the car as a tube (ie ignor the fact that there are big gaps at the side) it is of constant area from front to back (ish) at the diffuser it has an increasing area.
This means that for the same volume of air to pass through the whole under-car "pipe" it must be faster under the car than at the diffuser.

Wicked mate, and thanks for the effort

Oh s.hite... now I'm getting other doubts... but I will articulate them better once I manage to make sense of them in my own head...

[JustBringIt]

Originally posted by coysht
The argument about why wings work, related to larger distances is sort of true, but flawed when it actually comes down to it. But in essence what you have is true.


The diffuser does not change the distance the air must travel, but rather the cross-sectional area through which it travels.

If you imagine the gap under the car as a tube (ie ignor the fact that there are big gaps at the side) it is of constant area from front to back (ish) at the diffuser it has an increasing area.
This means that for the same volume of air to pass through the whole under-car "pipe" it must be faster under the car than at the diffuser.


In this very quick diagram, you can see the changing height of the gap under the car, and if you assume that in 3D the width is the same, the area at various points will change.

Sorry to be such a pain about this but that is exactly the point where I lose the whole notion. If the air slows as it goes through the diffuser, that means it has a higher pressure than the air under the flat part of the car, right? So I don't understand how that leads to downforce.

I believe you when you say about the technical articles, etc. but I want to understand it, not just take it as said. I don't like it when I don't understand why something like that is happening.

Also, when you talk about the flat bottom not creating any downforce, I think that's only partially true. It doesn't create and downforce on its own but I think the front of the cars are designed to feed as much air as possible under the car, effectively making the air go from a large volume in front of the car to a small volume under it to speed up the air flow -> lower pressure -> downforce.

[coysht]

Originally posted by JustBringIt
Also, when you talk about the flat bottom not creating any downforce, I think that's only partially true. It doesn't create and downforce on its own but I think the front of the cars are designed to feed as much air as possible under the car, effectively making the air go from a large volume in front of the car to a small volume under it to speed up the air flow -> lower pressure -> downforce.

Very true, but without the diffuser itself the effect is very small, and that is as you say caused by the profile of the front of the floor.

Originally posted by JustBringIt
Sorry to be such a pain about this but that is exactly the point where I lose the whole notion. If the air slows as it goes through the diffuser, that means it has a higher pressure than the air under the flat part of the car, right? So I don't understand how that leads to downforce.

I'll get back to you on that one later - not got time to write a full explaination now.

Bare with me....

[antares]

Alright here's an image I found, it's about the CDG, but it gives a good pressure map (false colour) around and under a car... hopefully looking at that will make something clearer. I'm sorry I haven't responded here, it takes time to think of stuff to write and explain, rather unlike the brainless comments I make elsewhere on the forum.

http://www.fia.com/resources/images/...ow_graphic.jpg

[stefano]

That's quite an interesting image, but... what does it have to do with the diffuser?

[mikey_s]

It seems to me that the confusion relates to mixing up two separate (albeit related) principles.

it becomes a little easier to visualise if you think that volume and pressure are related, and that equal air pressure requires equal air density (i.e mass of air per unit volume). Just of the sake of argument consider that there are 10 molecules of air under the floor of the car - although there are in fact hundreds of millions!...

In the drawing by Coysht the floor of the car is flat, but normally there is a slight upwards slope towards the back.... as the car moves forward the floor of the car starts to slope upwards suddenly the diffuser appears and the volume under the car increases dramatically, but there are still only the same number of air molecules under the car (for the purposes of the argument just ignore the fact that they can slip in from the side...). So at the front of the car you have x molecules of air in a particular volume, at the diffuser you still have x molecules of air, but the volume they have to occupy is much greater, thus the air pressure is lower, therefore the relative difference in air pressure on the top of the car is much greater compared to the underside of the car so the car is essentially pushed down onto the road. In summary, a low pressure zone is created under the car by changing the volume whilst limiting the passage for air to enter the space. Hence they try to run the car as close to the ground as possible to minimise the air coming under the car from the sides and thereby increasing the air pressure under the car and reducing the downforce on the vehicle.

Just a minor correction to a comment by JustBringit - they dont try to force as much air under the car as possible, they try to manage the air flow and ensure that the air is not turbulent - and to make sure it goes where they want it to go...

Skirts are now banned, but all of the teams make the floor of the car as wide as possible to minimise the effect of air entering from the sides of the floor.

btw, in a carburettor the venturi works the other way around; the air enters the trumpet as a consequence of the inlet manifold pressure being below atmospheric (because the piston displacement creates a vacuum). In other words the air is forced into the carburettor because there is a partial vaccuum in the manifold. The air speeds up in the venturi and there is actually a pressure increase at the narrowest point, but there is also a substantial velocity increase, which is what drags the fuel out from the jet. The high velocity air is required to vapourise the fuel as it travels through the manifold.

Hope this helps...

[stefano]

That's a very good explanation, mikey, a very easy way to visualize what is going on.

Unfortunately, I'm not sure your theory holds



By your same reasoning, if we take a carb, the air inside the trumpet should actually be at a higher pressure than the surroundings because you are in effect "cramming" air into a narrow tube. In reality it is the opposite because the increased speed at which the air is travelling creates a lower pressure...

Applying this to the underside of the car (though again, your example makes perfect logical sense), what you are saying is that in theory there is a HIGHER pressure underneath the car because the air there is squashed... this would lead to the underside of the car actually creating lift rather than downforce!

I am no expert and therefore cannot say with certainty... perhaps your way of visualizing what is happening can help the other "techies" to come up with a definitive explanation???

[mikey_s]

Stefano,

let's take the carb example first; the engine can be thought of as an air pump, the induction stroke of the piston draws air into the cylinder from the inlet manifold. Some very basic calculations; 2.4L engine displacement at 19000 rpm means 9500 induction strokes per minute (once every two revolutions) - this means the engine is sucking in 9500 x 2.4L of air every minute = 22800 litres of air essentially being sucked into the engine by the pistons. The air is pushed into the inlet manifold because there is a lower pressure in the inlet manifold than on the outside and it does increase in pressure as it is travels through the trumpet from the wide part to the narrower section - but there is a big pressure gradient across the inlet manifold which determines the direction of flow, the inlet manifold is below atmospheric pressure, so air flows from outside into the engine..

The diffuser is working in the opposite direction; underneath the car, somewhere just behind the front wheels there is a splitter; a sharp, flat area which serves to divide the airflow that is to go under the car from that which is to be directed around the sides and into the sidepods. The splitter only permits a certain mass of air under the car (and they try to run the car low to the ground to maximise the effect). Once the air is under the car the floor of the car starts to slope gently upwards, increasing the volume and thereby reducing the pressure - remember that there is effectively a fixed mass of air under the car, but the space is getting bigger so the pressure drops. At the diffuser, there is a very rapid increase in volume and a correspondingly much bigger drop in pressure, because when the car is travelling at speed there is just no time for the air surrounding the car to move in and fill the vaccum.

The pressure drop in the diffuser area means that the car is basically pressed down onto the track because the air density beneath the car is much lower than the air density above the car.

The diffuser is a passive device and doesn't generate any downforce when the car is static because the air pressure is equal beneath and above the car - at high speed the splitter under the car traps a given mass of air under the vehilce and then, as it travels forwards increases the volume that the air must occupy. Travelling slowly there is more than enough time for air to 'leak' in from the sides and back of the car, but at high speed the wide floor of the car limits the flow of air under the side and the hole that the car makes as it moves throgh the air limits the ability for it to sneak in from the back (that's also why they put the complicated flukes in the diffuser, to limit ingress from the side). It's a very elegant concept, the faster the car goes the more downforce it will generate... right up the the point wher the diffuser stalls (that's because of turbulent flow) and then the downforce becomes highly unpredictable...

If you can remember the Chapman suction cars, they put skirts around the edge to further limit the air ingress.

So the air is not squashed under the car, it is forced to occupy an ever increasing volume and therefore the pressure is reduced - creating a partial vacuum and the mass of the air above the car presses it down onto the track.

Let me now if I'm still not making sense

Mike