Rear Diffuser angles

[topwelshman]

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?

[knighty]

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.

[Locost47]

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.

[trikesrule]

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

[topwelshman]

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

[phoenix]

Originally Posted by topwelshman

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

Take care. With a rear wing such as you have the steeper angle on the diffuser may well be correctly designed; I recently (like two days ago) discovered I know someone who is an aero man at Red Bull Technology/Racing working under Adrian Newey and in talking about the downforce on our car (see another thread) he pointed out that at the rear of our car we have a void, more or less the full width of the car and extending back about a metre behind the (transverse) engine & box. This void is in fact acting as a diffuser and will allow the floor area of the car to give downforce. A correctly designed diffuser 'tidies up' this area and matches the pressure under the car more accurately to the pressure behind the car. The angle in the diffuser is to prevent seperation and the resulting turbulence and drag.

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.

[topwelshman]

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

[knighty]

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

[AU N EGL]

Originally Posted by knighty

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

Same way sails on a ship work, Bernoulli's principle and diffenence between high and low pressure air zones.

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

[AntonioVacos]

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