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14 Apr 2007, 13:31 (Ref:1891081) | #1 | ||
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Rear Diffuser angles
I've recently had a diffuser made for my car but the angles appear far too high, i'm told that 7-10 degrees is the right compromise between drag and downforce, does anyone know about these things?
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14 Apr 2007, 18:08 (Ref:1891247) | #2 | ||
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yes, its easy to fall into the bigger is better trap with diffuser angles and widths, believe me, I did!....... based on experience I would go for no bigger than 7 degrees, although it might look shallow and not very racey at 7 degrees, rest assured it will be fine and generate loads of downforce.......forget what you see on the back of F3 and F1 cars.......
the trick with a diffuser is to get the air to pass through it faster than the actual air speed around the car, so a good diffuser acually has a bigger entry width than the exit width, from the birds eye view it will taper towards the rear slightly.........I ran a very small diffuser on my F4 single seater and it glued the rear of the car to the floor, the exit was no wider than about 2 feet. with about a foot of flat floor pan either side of the diffuser, this is what the diffuser sucks on and pulls the rear down.......the entry was about 6 inches wider than the exit in the birds eye view, this is what acellerates the air faster than the car speed. for your beemer, I'd take a look at the sort of diffusers that the supersports cars run, a Juno would be an ideal starting point, from memory they have about 2.5-3 feet working width, have loads of strakes, and have plenty of panelling either side, deffo a good starting point. |
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14 Apr 2007, 19:47 (Ref:1891301) | #3 | |
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No disrespect chief but that's exactly the opposite of what a diffuser is for. it's purpose is to slow air back down to near-freestream speeds before exiting the underside of the car. If there is a big speed differential you effectively have a jet mixing with the car wake and the energy loss is huge, resulting in a much reduced mass flow in the channel.formed by the underbody and the ground.
The downforce comes from two aspects. Reducing the resistance of the underbody increases mass flow under the car. Given that the size of the channel is effectively fixed and we're well away from Mach number effects, the more air flows through the faster it has to go and, by Bernoulli's principle, the static pressure has to drop. Lower pressure under the car gives you the downforce. Secondly, the upsweep of the diffuser means you're changing the momentum of the flow and giving it a vertical component. Action & reaction means air goes up = car goes down. More downforce. With respect to the original question, 7-10 degrees is about the limit, depending on how smooth you can keep the floor all the way along the car, where your cooling air exits (hopefully not underneath), whether you have deepend/sharpened/extended sills, what rear wing angle you're running, how close it is to the back of the car and whether you run proper endplates or strakes in the diffuser. If you optimise all of these then 15-20 degrees may be possible. If you have a close-mounted rear wing (like on a single seater) then 30degrees may be possible (like F1). Critical things to sort are no upstream disturbances, having a smooth transition in the floor surface between flat and inclined (no panel seams at the apex of the diffuser) and shielding it from the rear wheel wakes with deep endplates which come as far forward along the inside of the rear wheelarch as possible. Then it should give you a healthy of downforce (both at the rear AND at the front) but for minimal drag. |
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15 Apr 2007, 04:02 (Ref:1891530) | #4 | |
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Yeah their fiddly things those diffusers. But locost I didn't have a tunnel to sort the Supercarts out and it seems I wasn't to far away. Nice to know..........trikes
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15 Apr 2007, 09:40 (Ref:1891643) | #5 | ||
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Yeah that's what I thought, we have too much angle, the unnderfloor is not flat, we do have an adjustable twin element rear wing up high on the back running between 15-40 degrees and the diffuser is quite long, extends under the diff and we plan to panel in as much as we can around it and around the wheels but by no means is it going to be optimised as you say so need to look at changing the angles
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15 Apr 2007, 10:48 (Ref:1891698) | #6 | ||
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But a rear wing such as you have can change dramatically what is required of the diffuser, rather than one simply stuck under the boot of a hatchback, for example. This is because the area under the wing is low pressure too and quite different from the rear of an 'ordinary' car. The aero effects interact and that is why you will see F1/CART cars and LMPs of course with much steeper angles on their diffusers. To summarise, a diffuser stuck under a car with no other aero devices will work withouth creating turbulence at 7 to 7.5 degrees angle. More angle MIGHT be possible, but only time in a windtunnel will allow that judgement to be made, so 7 degrees is 'safe' for everyone. If you have a rear wing, then all that is still true, but the likelyhood is, if the package was designed together and a wind tunnel and/or accurate modelling used to come up with the design, then the greater angle you have may work without seperation and going to a lower angle may reduce your downforce from the underbody, and may even upset the downforce generated by the rear wing. |
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15 Apr 2007, 13:50 (Ref:1891920) | #7 | ||
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But there's nothing at the rear of the car at the moment so anything surely has to be an improvement, the wing has not been designed, its an off the shelf job and we haven't had any wind tunnel time, that's too much for what we need, basically we need something simple that'll reduce drag and improve downforce and that is what i'm really enquiring about, 7degrees seems to be the way to go
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16 Apr 2007, 19:18 (Ref:1893305) | #8 | ||
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glad we all agree on 7 degrees then!......I think we have differing opinions on how it all works, but the parcticalities of the design advice above still stand.
as your racing a 3 series beemer, the chances of using the rear wing to pull the air through the diffuser is zilch.......thats for LeMans, F3 and F1 cars........ I just dont understand that a diffuser is supposed to slow the air?......if so then why do people use the depression under the rear wing on LeMans F1 and F3 cars to "pull" the air through the diffuser.......a pull would indicate that faster is better also looking at all diffusers from the birds eye view, why are they wider at the front/entry and narrower at the rear/exit.......an F1 front wing is a classic example......not wanting to argue, just curious thats all I always remember Adrian Kidd pointing to his diffuser on his F4 car and saying the faster that air go's through that, the more down-force you generate.......from an aero perspective - that makes sense to me |
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16 Apr 2007, 19:27 (Ref:1893316) | #9 | |||
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Bernoulli's Principle states that in an ideal fluid (low speed air is a good approximation), with no work being performed on the fluid, an increase in velocity occurs simultaneously with decrease in pressure or a change in inertia. Source: http://en.wikipedia.org/wiki/Bernoulli's_principle |
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16 Apr 2007, 19:36 (Ref:1893327) | #10 | ||
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Oh well, so much for my input, you guys have got it covered!
I wanted to have a good long discussion about diffusers etc...never mind! |
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16 Apr 2007, 20:44 (Ref:1893383) | #11 | |
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We've talked about the physical mechanisms behind the operation of diffusers before:
http://tentenths.com/forum/showthrea...light=locost47 there's stuff in there about increasing massflow and Bernoulli and interactions with rear wings. Just for the record, rear wings *can* assist diffuser (and front splitter!) performance on saloon cars - i've done it myself on at least half a dozen different cars, including the current Team Dynamics Civic which has its rear iwng right up high on the rear hatch but still helps out. It doesn't have to be a wing, either. The NASCAR-style flat plate spoilers work too. Pretty much anything that can lower the base pressure or raise the height of the effective rear stagnation point will help increase flow under a car. |
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17 Apr 2007, 17:15 (Ref:1893992) | #12 | |||
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27 Apr 2007, 11:53 (Ref:1901610) | #13 | ||
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LARGE Pics Sorry
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27 Apr 2007, 11:58 (Ref:1901613) | #14 | ||
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The diffuser angle is dependent on the upper body too! So its better to spend a bit of money and take the car to MIRA and find out the aero effect of the diffuser on your specific car.
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26 Sep 2008, 18:32 (Ref:2298584) | #15 | |
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Ressurection!
Sorry to dig up such an old post... but this seems to be the only place on the net where people seem to know what they are talking about!!
I sold my soul and decided to trade my 1989 325i for a 1998 honda civic hx. I am doing more commuting these days and I just needed better mpg. In my quest to get better gas mileage, I have decided to make a rear diffuser for the honda. Its a little besides the point, but if you have to know, its because the underside of the car is fairly consistent until after the rear control arms. There is an expanse under the trunk that allows the rear bumper to act as a parachute. So at the least I am going to seal that off, but also, since I am a nerd, I am going to add in diffuser fins to try and decrease the overall drag of the car. So... yes. This is for a street car, and not a race car. Furthermore its for a very slow and underpowered street car. And yes, I understand that the benefits of a rear diffuser are probably not very substantial at free way speeds. But I am going to do it anyways. Can anyone clarify which angles you are talking about here? Are these angles of the "bottom" of the fins relative to the ground? Or are these angles of the fins relative to each other? Thanks! Last edited by norbert24; 26 Sep 2008 at 18:35. |
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26 Sep 2008, 18:56 (Ref:2298599) | #16 | ||
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The 7° 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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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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27 Sep 2008, 09:28 (Ref:2298862) | #18 | ||
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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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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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27 Sep 2008, 16:10 (Ref:2299042) | #20 | ||
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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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27 Sep 2008, 16:28 (Ref:2299051) | #21 | |
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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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27 Sep 2008, 20:13 (Ref:2299162) | #22 | ||
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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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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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28 Sep 2008, 07:33 (Ref:2299314) | #24 | ||
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Thank you, sir! |
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28 Sep 2008, 09:43 (Ref:2299361) | #25 | ||
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