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Thread: Diffuser Stall

  1. #1
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    Diffuser Stall

    Following on from the discussions in the Fiorano test thread:

    Pages 10 and 11 of this thread:
    http://www.thescuderia.net/forums/sh...210#post266210

    The topic of problems with the diffuser came up.

    The following is an explaination based on various websites and my own knowledge, and may be amended in future if it is either incorrect or not explained adequately.

    edits shown in red

    The diffuser
    Firstly, the diffuser is basically a device that expands the air from under the car and slows it down. This creates a lower pressure under the floor of the car, effectively 'sucking' the car down (term sucking used loosely).
    This is Bernoulli's principle of a higher velocity generating a reduction in pressure. Since the pressure above the car is higher than that bellow it, the car is pushed down onto the track.
    The diffuser is made up of a series of sloped 'ducts' increasing in height as they leave the floor section of the car.




    Stalling
    In aerodynamics, stalling is when flow separation occurs.

    What this means is that the air flowing closely around the aerofoil starts to detach from the surface and become turbulent.

    This occurs when the angle of attach (angle of the aerofoil to the flow direction) becomes too great, and the air cannot remain attached to the top surface of the aerofoil.

    (below is an image of the flow around a stalling aerofoil - note the turbulent flow above it)


    This causes the drag and lift to become eratic.


    Diffuser Stall
    On an F1 car diffuser stall is where the air separates from the surface of sloped exits or by 'vortex burst'

    Since the angle of attack will not change significantly on the diffuser, flow separation will happen when the speed of the air drops such that it will detach.
    The task is therefore to match the slope of the diffuser sections to the expected flow speeds through those areas.

    Stalling will occur during the following conditions:

    Under braking, the car pitches forward, and the back of the car will get further from the ground. This (again through Bernoulli's principle) will increase the pressure by allowing more air in from the sides, and thus slowing the air.
    At this point, the diffuser will stall if poorly designed.


    Stalling can also occur when the car is running level.

    The side parts of the diffuser are fed with air from under the sidepods, which will have partly mixed with slower air from either side of the car. This will slow the overall flow, and if the slope of the diffuser is too high - lead to stalling.

    In the diffuser there are various fences which control the flow.
    When overloaded aerodynamically, the vorticies generated off the fences can burst releasing slow 'lossy' air right in the middle of the diffuser.


    Finally, the diffuser can stall if the car ‘bottoms out’. This is when the car runs so low under high downforce and over bumps that the skid block/plank grounds out on the track, and air flow to the diffuser is significantly reduced if not interrupted for brief periods.


    Having thought about it further, the stuff below is probably not totally correct - please see further discusion for latest thoughts on the Ferrari 248 F1 diffuser.

    Ferrari 248 F1 Diffuser
    In the case of the new Ferrari, they appear to be complaining of increased drag at high speeds.
    This would suggest stalling during level running, probably due to the arrangement of the fences within the diffuser.
    Alternatively, they may have gone too aggressive with the design of the diffuser and made the exits too steep – leading to stalling.

    Of these the first is easier to solve with a decent bit of windtunnel time, the second could require a major re-design of the diffuser.

    It is unlikely to be due to ‘bottoming out’ as this is simply resolved by increasing ride height slightly, or increasing the bump stiffness of the suspension.
    Last edited by coysht; 10th February 2006 at 12:29.
    Disclaimer: The views expressed by this forum member are purely opinions and observations and should not be interpreted as fact, or indeed as anything other than a cheap gag for my own amusement.

  2. #2
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    Cheers for that Tom....

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    Nice one, mate. Thank you...
    Dr. Wicked

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    Great, thanks for that !!

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    Thanks. The discussion was leading towards saying the stalling was induced to help top speed, which didnt make any sense to me. This all makes sense. Which car is that in the diffuser pic?

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    Dare I say that picture was possibly taken by me with a telephone ? I don't have those pictures myself anymore but I remember me photographing the diffuser of a BAR at an Autoshow in Amsterdam just like that....

    I might be completely wrong

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    Originally posted by S.F. 1
    Dare I say that picture was possibly taken by me with a telephone ? I don't have those pictures myself anymore but I remember me photographing the diffuser of a BAR at an Autoshow in Amsterdam just like that....

    I might be completely wrong
    The website I got it from said it was from a Ferrari, but I'm not sure if it is.
    Disclaimer: The views expressed by this forum member are purely opinions and observations and should not be interpreted as fact, or indeed as anything other than a cheap gag for my own amusement.

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    It appears to have white flipup ears. Also that diffuser has a box much like Ferrari is using on the 248 F1, and where the heck is the rain light?

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    Originally posted by SS454
    It appears to have white flipup ears. Also that diffuser has a box much like Ferrari is using on the 248 F1, and where the heck is the rain light?
    I would guess its a 2004 or 2005 car.

    The rain light is probally right above the camera, and out of shot.
    Disclaimer: The views expressed by this forum member are purely opinions and observations and should not be interpreted as fact, or indeed as anything other than a cheap gag for my own amusement.

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    Sorry coysht, but contrary to what you say stall does not necessarily increase drag. Studying what happens to drag after the stall is rather difficult, because the behaviour of the wing or airfoil becomes unstable, so small changes in the angle might lead to big changes in drag. That does not mean a big increase, might mean a big decrease too. Source: http://www.grc.nasa.gov/WWW/K-12/airplane/inclind.html

    That being said, stall is almost always undesirable because it leads to the loss of downforce. In a wing it's also undesirable because stall occurs at rather big angles of attack, which means huge drag. So people normally try to avoid reaching the stall angle.

    If we go by coysht's explanation, Ferrari simply screwed up. It means that they designed diffuser that stalls without wanting that. It well might be true, but then there is no potential huge gains for Ferrari, because every team can and knows how to play with that. There is no new idea here.

    If we are to believe Autosprint that "Ferrari haven't got right yet but once it works properly it could be a decisive advantage", we must assume that they indeed are trying to use the stall effect to their advantage, not to get rid of stall.

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    Yeah, after reading...
    Originally posted by ferrarista
    It's known that the major part of the downforce is produced by the diffuser, it's very useful in the corners but in some parts of the circuit you don't want it. The principle that Ferrari wants to exploit works to stall the car at a certain speed on the straights to reach higher top speeds, since the downforce on the straights is useless. Ferrari and Mclaren have been working on this for some years.
    ..in this thread.. http://www.thescuderia.net/forums/sh...&pagenumber=10

    I was under the impression that we were actually trying to induce diffuser stall on the straights to reduce downforce and hence reach higher top speeds...
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  12. #12
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    Originally posted by Tobes
    Yeah, after reading...

    ..in this thread.. http://www.thescuderia.net/forums/sh...&pagenumber=10

    I was under the impression that we were actually trying to induce diffuser stall on the straights to reduce downforce and hence reach higher top speeds...
    Its an odd one that.

    As some of the quotes were saying that their top speed was actually being impaired by the diffuser not functioning as it should.

    Needs some more research.
    Disclaimer: The views expressed by this forum member are purely opinions and observations and should not be interpreted as fact, or indeed as anything other than a cheap gag for my own amusement.

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    So the question is... how can one predictably induce diffuser stall and use it to their advantage....?

    That was what i meant when i said about the car virtually bottoming out, remove the air to the diffuser and it ceases to function properly, in doing so it would reduce downforce and hence go quicker down the straights.. now i appreciate the moment the diffuser ceases to function 100% the car will probably lift due to lack of suction and the gap is opened again, and i can't see a way to control or predict that...

    Just for a change that's a completely wild idea as to how it may be beneficial to induce the stall...
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  14. #14
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    Originally posted by Tobes
    So the question is... how can one predictably induce diffuser stall and use it to their advantage....?

    That was what i meant when i said about the car virtually bottoming out, remove the air to the diffuser and it ceases to function properly, in doing so it would reduce downforce and hence go quicker down the straights.. now i appreciate the moment the diffuser ceases to function 100% the car will probably lift due to lack of suction and the gap is opened again, and i can't see a way to control or predict that...

    Just for a change that's a completely wild idea as to how it may be beneficial to induce the stall...
    The problem with bottoming the car out is that you loose lots of speed through friction with the skid block (not to mention giving you driver a rather warm backside).
    You could also end up loosing traction and steering for a bit, as the tyres would not be in full contact with the road if the car is running along on its bottom.
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    Originally posted by coysht
    The problem with bottoming the car out is that you loose lots of speed through friction with the skid block (not to mention giving you driver a rather warm backside).
    You could also end up loosing traction and steering for a bit, as the tyres would not be in full contact with the road if the car is running along on its bottom.
    Yeah, but i mean it virtually bottoms out, but doesn't quite, just enough to reduce the air flow to the diffuser sufficiently so as to induce the stall, do you see what i mean...?

    Would that be possible, and would it be maintainable for enough time in a straight line for it to be advantageous....?
    "Racing is life. Anything before or after is just waiting."
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  16. #16
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    Originally posted by Tobes
    Yeah, but i mean it virtually bottoms out, but doesn't quite, just enough to reduce the air flow to the diffuser sufficiently so as to induce the stall, do you see what i mean...?

    Would that be possible, and would it be maintainable for enough time in a straight line for it to be advantageous....?
    If it did what it was supposed to, then that would be advantageous - you'd gain a decent chunk of straight line speed for over taking, or you could run more downforce overal, knowing that it'd be reduced down to the levels you previously had on the straights (meaning you could get more in slow speed corners)

    But that assumes that you didnt actually bottom out and loose the advantage or spin off.

    They do run the cars so low that they already do bottom out over some bumps - any lower and they'd bottom out even more (and possibly wear the skid block out and get excluded).


    I'll have a think and a look and see what I can come up with. Its an interesting concept, and if stalling the diffuser works, it'd be very clever.
    Disclaimer: The views expressed by this forum member are purely opinions and observations and should not be interpreted as fact, or indeed as anything other than a cheap gag for my own amusement.

  17. #17
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    some body have the pic of the new diffuser on the 248?


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    Originally posted by coysht
    But that assumes that you didnt actually bottom out and loose the advantage or spin off.

    They do run the cars so low that they already do bottom out over some bumps - any lower and they'd bottom out even more (and possibly wear the skid block out and get excluded).


    I'll have a think and a look and see what I can come up with. Its an interesting concept, and if stalling the diffuser works, it'd be very clever.

    Yeah, the idea has obvious flaws, and like i said, just chucking an idea into the pot, but if it's not that, is there another way that this stall can be induced...?

    Will see what you come up with.....
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  19. #19
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    Originally posted by Tobes
    Yeah, the idea has obvious flaws, and like i said, just chucking an idea into the pot, but if it's not that, is there another way that this stall can be induced...?

    Will see what you come up with.....
    I do wonder if you could somehow tune the slope of the diffuser exits and the little fences to make it so that it would stall when the air speed going through it when over a certain value (as opposed to below, but I'm damned if I know how you'd do it at the moment.
    Disclaimer: The views expressed by this forum member are purely opinions and observations and should not be interpreted as fact, or indeed as anything other than a cheap gag for my own amusement.

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    I think you could have the right idea Tobes, but very hard to say, as the teams run the car as low as possible anyways. Its not just to lower the center of gravity and reduce drag going over the car, but it speeds up the air under the car as far as I know, and increases downforce. The lower you can get an F1 car, the faster it is on the straights, and faster in corners too. It would also have to stall at a certain speed of like 280 kph, because you certainly dont want a reduction in diffuser downforce in a corner like the parabolica or turn 3 at Spain. I suppose if you stalled the air at the point of going under the car, any slow air coming in from the sides wouldnt act like brakes on the air as much. But how do you do that? Or maybe its how the air flows out the back of the car around the mainplane and the top of the diffuser, to limit the expansion of air out of the diffuser, and hense creating a stall effect explained in coysht's original post, not in the so much it stalls the overal airflow, but in a sense straightens it out and makes it cleaner, which should provide a reduction in drag. You can see in the rain, that the incredible "suction effect" the diffuser creates makes a big rooster tail wall of turbulant are out the back of the car, perhaps what i just mentioned would smooth that out at high speeds, and be faster in top speed. Plus they could run the rear of the car lower so you dont have such a difference in ride height between front and rear, as normally the rear end will compress about 4.5 cm's from downforce.

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    Hmmm i think coysht came up with a simular idea while i was writing my post.

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    Tobes, I think you pretty much explained why "bottoming out, but not quite" is unlikely to work. My idea would be to make diffuser slightly deform with speed. Don't know how legal it is though.

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    Ok, i'm wondering if there is a point where the restriction of airflow under the car will cause the diffuser to begin to stall, but not actually 'stall' completely, at this point the 'stall' may be predictable enough so that the downforce settings on the car can be adjusted so that there can be a seting found that can maintain this point and hence provide the possibility of reaching the higher top speeds, i just can't quite see how that balance can be found, or even if....?
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    Ok, another idea....

    As the car goes down the straight, the faster it goes the lower it gets, but what if the airflow under the car to the diffuser can (from a specified point) be reduced at exactly the same rate as the extra downforce is applied to the wings, then the car would reach a point where the downforce will cease to increase at any speed, untill it does finally bottom out, but you don't take it to that extreme, in effect the more the car is pushed down by the wings, the less it's being sucked down by the diffuser, and at that point there is an advantage gain....

    Is that a possibility...?
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  25. #25
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    Without bottoming out the front of the car (never gonna happen), the amount of airflow under the car theoretically will always increase as the speed increases. As the faster you go, the faster air goes under/through/around the front wing, nose, and deflectors, and all that air speeds up as hieght between the floorboard and the ground decreases. I dont think that would work.

    I've considered fermy's suggestion too, which is perhaps logical, as I dont recal any flex regulation on diffuser parts.

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    Re: Diffuser Stall

    Originally posted by coysht
    The diffuser
    Firstly, the diffuser is basically a device that expands the air from under the car. This creates a lower pressure under the floor of the car, effectively 'sucking' the car down (term sucking used loosely).
    This is Bernoulli's principle of a higher velocity generating a reduction in pressure.
    The diffuser is made up of a series of sloped 'ducts' increasing in height as they leave the floor section of the car.
    Sorry to rain on your parade, coysht, but a diffuser creates a slower velocity of fluid/air. If you think of it in terms of volumetric flow rate, you see what I mean... Unless I'm wrong.
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    Moving on from that, JBI...

    I know they speed up airflow underneath a car to induce low pressures (hence, downforce); does the diffuser then reduce the velocity of the air from underneath the car once it exits through the rear? Not sure why they'd want to do that but I am quite certain that fast air exiting from the rear + the relatively "slower" air it hits = turbulence.

  28. #28
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    Originally posted by killer
    Moving on from that, JBI...

    I know they speed up airflow underneath a car to induce low pressures (hence, downforce); does the diffuser then reduce the velocity of the air from underneath the car once it exits through the rear? Not sure why they'd want to do that but I am quite certain that fast air exiting from the rear + the relatively "slower" air it hits = turbulence.
    I don't think that would create turbulence. The two flows should just mix together. I have to say, I'm not totally sure about it, though.
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    Originally posted by fermy
    Tobes, I think you pretty much explained why "bottoming out, but not quite" is unlikely to work. My idea would be to make diffuser slightly deform with speed. Don't know how legal it is though.
    If we are trying to make the diffuser airflow less effective as speed increases, thereby decreasing downforce and increasing top speed, this sounds like a valid explanation.

    But what part would flex. The diffuser ducts get bigger as they go to the back of the car. My guess, is they could make the front of the diffuser ducts more flexible than the rear of the ducts. Which means the front of the ducts would increase in size, and decrease the steepness of the angle of the diffuser duct, decreasing the effectiveness of the diffuser. But I don't think that would cause the stall effect we are looking for. (this could be completely stupid and impossible, its just an attempt to explain the "Flex" theory, which I like.)

    Seriously, this problem is best left to the experts like Byrn and Tombasiz.:


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  30. #30
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    Originally posted by Cavallino

    Seriously, this problem is best left to the experts like Byrn and Tombasiz.:
    Yeah I think we are a bit over our heads on this one.

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