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Old 26 Sep 2008, 18:56 (Ref:2298599)   #16
tristancliffe
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The 7į is between the ground (or reference plane) and the 'ceiling' of the diffuser (the upper wall). The internal vanes (fins) are just to tidy the streamlines up a bit and improve efficiency, especially in yaw.
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Old 27 Sep 2008, 05:22 (Ref:2298774)   #17
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actually, no disrespect but I am pretty sure this was discussed already earlier in the thread.

The whole point of a diffuser is to essentially feed the air through a bunch of funnels in reverse. This way the high pressure air under the car gets fed through the channels, in between the fins, and the fins spread apart and the pressure of the air decreases, allowing a much smoother transition to the rear of the car where all of the negative pressure is.

It cleans up the wake of the car and reduces drag.

I am just hoping someone can help me figure out at which angles to set those fins at.
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Old 27 Sep 2008, 09:28 (Ref:2298862)   #18
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Originally Posted by norbert24
actually, no disrespect but I am pretty sure this was discussed already earlier in the thread.

The whole point of a diffuser is to essentially feed the air through a bunch of funnels in reverse. This way the high pressure air under the car gets fed through the channels, in between the fins, and the fins spread apart and the pressure of the air decreases, allowing a much smoother transition to the rear of the car where all of the negative pressure is.

It cleans up the wake of the car and reduces drag.

I am just hoping someone can help me figure out at which angles to set those fins at.
The fins can be parallel - as the 7 degree diverging angle of the 'ceiling' of the diffuser is what creates the low pressure.

The fins themselves can be angled if you wish, but again 7 degrees should be the maximum angle. Any greater and the air flow is likely to become detached from the fins which will cause turbulence and loss of downforce.

The angled inner fins seen in pictures are usually to tidy up the flow round the gearbox, which commonly intrudes into diffuser space. Outer fins are usually parallel and without a wind tunnel to see what is going on with the airflow if you angle them I would stick with parallel to be sure of not introducing some undesirable turbulence.
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Old 27 Sep 2008, 09:50 (Ref:2298875)   #19
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Just to clarify, the low pressure area is not created IN the diffuser. The low pressure under the flat bottom of the car by creating a high velocity airflow between it and the track. (Bernoulli's Principle)

The diffuser increases the area of floor that 'sees' this low pressure by extending the low pressure area further rearward. As a result the area of the floor subjected to the lower pressure is greater so total downforce is increased.

The pressure gradient in the diffuser itself is low at the front (same as the under floor area of the car) and gradually increases to the higher ambient pressure at the rear.
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Old 27 Sep 2008, 16:10 (Ref:2299042)   #20
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Originally Posted by phoenix
The fins can be parallel - as the 7 degree diverging angle of the 'ceiling' of the diffuser is what creates the low pressure.

The fins themselves can be angled if you wish, but again 7 degrees should be the maximum angle. Any greater and the air flow is likely to become detached from the fins which will cause turbulence and loss of downforce.

The angled inner fins seen in pictures are usually to tidy up the flow round the gearbox, which commonly intrudes into diffuser space. Outer fins are usually parallel and without a wind tunnel to see what is going on with the airflow if you angle them I would stick with parallel to be sure of not introducing some undesirable turbulence.
Thanks! Now here is my question- if the fins are angled apart, but the outside ones are parallel, then the "channels" between the outer fins and the next ones in would have the opposite effect as the inner ones... ?

Do you see what I mean? If you set it up that way, there is an X degree difference between the inner fins, and then the parallel outer fins create a -X degree difference.



In this example, it looks like there are no parallel outer fins, but they are all angled. But I can't tell for sure.

Thanks for your help with all of this!
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Old 27 Sep 2008, 16:28 (Ref:2299051)   #21
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Originally Posted by norbert24
Thanks! Now here is my question- if the fins are angled apart, but the outside ones are parallel, then the "channels" between the outer fins and the next ones in would have the opposite effect as the inner ones... ?

Do you see what I mean? If you set it up that way, there is an X degree difference between the inner fins, and then the parallel outer fins create a -X degree difference.



In this example, it looks like there are no parallel outer fins, but they are all angled. But I can't tell for sure.

Thanks for your help with all of this!
From the picture, I can't tell for sure if they are angled either, but I would say they are parallel, as the outer fin on the right-hand side of the car is inside the inner edge of the rear right tyre.

If they angled, it looks like only a degree or two each would be possible, as there are so many.

Most diffusers I have seen have only a few vertical fins, if any. Some only have partial fins or ribs, if you like.

I guess the important factors to remember are that to work at all 1) the diffuser must be 'divergent nozzle' - i.e. the cross sectional area at the back/outlet must be greater than the cross sectional area at the inlet - where it meets the flat floor 2) the angle of divergence overall, and in any chamber formed by vertical fins, should be around 7 degrees be sure that the flow of air remains attached and non-turbulent, and 3) if there are multiple channels in the diffuser formed by vertical fins, none of the channels should be converging - i.e. have a smaller scross sectional area at the rear than at the front.

I think that, unless you have some part of the car protruding into your diffuser - like a gear box - you are unlikely to see much benefit from multiple vertical fins. But if you want to know for sure, you need to visit a wind tunnel!

Last edited by phoenix; 27 Sep 2008 at 16:32.
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Old 27 Sep 2008, 20:13 (Ref:2299162)   #22
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Originally Posted by phoenix
From the picture, I can't tell for sure if they are angled either, but I would say they are parallel, as the outer fin on the right-hand side of the car is inside the inner edge of the rear right tyre.

If they angled, it looks like only a degree or two each would be possible, as there are so many.

Most diffusers I have seen have only a few vertical fins, if any. Some only have partial fins or ribs, if you like.

I guess the important factors to remember are that to work at all 1) the diffuser must be 'divergent nozzle' - i.e. the cross sectional area at the back/outlet must be greater than the cross sectional area at the inlet - where it meets the flat floor 2) the angle of divergence overall, and in any chamber formed by vertical fins, should be around 7 degrees be sure that the flow of air remains attached and non-turbulent, and 3) if there are multiple channels in the diffuser formed by vertical fins, none of the channels should be converging - i.e. have a smaller scross sectional area at the rear than at the front.

I think that, unless you have some part of the car protruding into your diffuser - like a gear box - you are unlikely to see much benefit from multiple vertical fins. But if you want to know for sure, you need to visit a wind tunnel!
I completely agree with the wind tunnel deal.

So my question to you is this- if there are multiple fins all arranged in diverging positions, meaning the angles they sit at grow increasingly larger as they move away from the center, and then the outside fins are positioned parallel to the direction of the car, wouldnt those outside fins create converging channels?

Thanks!
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Old 27 Sep 2008, 22:10 (Ref:2299204)   #23
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No, because the angle of the top of the diffuser (the ceiling if you like) means that even mildly converging fins still results in a diverging CROSS SECTIONAL AREA.

It is the total area that is important, not merely a 2D view.

The fins are unlikely to be converging or diverging (from plan view) more than two or three degrees anyway, less near the sides where you've got to return to parallel.

Too many fins might cause problems with boundary layer accumulation - more area of the diffuser is 'stuck' to a surface rather than free flowing. How much effect that has on the car would be another question for the wind tunnel though.
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Old 28 Sep 2008, 07:33 (Ref:2299314)   #24
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Originally Posted by tristancliffe
No, because the angle of the top of the diffuser (the ceiling if you like) means that even mildly converging fins still results in a diverging CROSS SECTIONAL AREA..
ah HAH!!!

Thank you, sir!
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Old 28 Sep 2008, 09:43 (Ref:2299361)   #25
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Too many fins might cause problems with boundary layer accumulation - more area of the diffuser is 'stuck' to a surface rather than free flowing. How much effect that has on the car would be another question for the wind tunnel though.
And also wouldn't multiple find increase drag, because there is more surface area in the air flow?
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Old 28 Sep 2008, 22:22 (Ref:2299894)   #26
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And also wouldn't multiple find increase drag, because there is more surface area in the air flow?
Not really, the find direct air flow for high speed cornering, and to increase air velocity out the back.

Good Links
http://www.sae.org/servlets/productD...D=2007-01-0103

and

http://www.symscape.com/blog/secrets_of_diffusers
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Old 14 Oct 2008, 23:53 (Ref:2312622)   #27
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Edited 'cos I re-read norbert's post and realised I was missing his point.

Last edited by Malfunction Junction; 14 Oct 2008 at 23:58.
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Old 8 May 2017, 10:54 (Ref:3732189)   #28
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