Rear Diffusers?

[Relentless Rob]

[url]|http://thumbsnap.com/T2gta3NB[/url]

Rear diffusers? Any ideas what would be the best shape convex, concave or straight? The boot floor will be cut out completly and the battery re-located.

Also what material would you choose?

I already have an idea of what I'm going to do but I'm putting it to you to see if you agree with my (top secret) plan for the £999 car.

[Renrut]

From my tiny knowledge on fluid dynamics and from what I've read the key bit of making the diffuser work is a very smooth join between the flat bottom of the car and the angled section at the rear. If its not a clean, smooth and gradual change it simply doesn't work.

Think of the bottom of the car being like a spoon. Bear with me on this. When you put a spoon backwards into a running tap the water sticks to it and flows off in a direction that isn't gravity, thats the same effect you're trying to create but with the bottom of your car on air rather than a spoon in water. Coanda effect iirc. Bernoulli is just a red herring

[url]http%3A%2F%2Fen.wikipedia.org%2Fwiki%2FCoand%25C4%2583_effect&ei=B1xRTPvLBIuy4Aba_JTaAw&usg=AFQjCNEux4vpndCZ-dXOUKu9S7H1as_Dsw[/url]

Also I seem to recall 7degrees as an angle coming into it somewhere but I can't remember if that was optimal or was more to do with some race regulation limit.

10tenths might have something, have you searched on there?

FWIW unless this is the 300km/h car I wouldn't bother. Someone can probably work out the size vs speed relationship to give you a clue on how fast or how big it needs to be to help...


ETa just seen your picture, either the first one or the last one.

[winstar]

As an ex-aerodynamicist (turbo blades not external) I can try to answer this but as with most things aerodynamic it not very stright forward and very much depends on the application.

Firstly diffusers work by slowing the air flow down at the point it meets the air flowing over/around the car is smooth. Doing this speed the air up under the car and causes it to be at lower pressure. So this one is actually Bernoulli's the effect above applies to wings and ground effect. Diffusers are good as they are essentially free down force as they don't really have any drag. However to get them to work properly the car needs to be very low, the air entering the underside of the car as restricted as possible (ie splitter, skirts etc.), have a flat floor and the diffuser needs to extend as far as possible.

As for design then the accepted best angle before CFD was 7 degrees for race cars but it really depends on the speeds you want it be effective, as steeper angle will work better at low speed but the flow will seperate off it at high speed. This is where the curved, like your bottom right, come in as they will work better at a wide range of speeds if designed right, however if you don't get them rightthe flow will travel up the rear of the car and cause lift. This is the case with the one on the VX220 there are some rough models I did in this thread http://www.vx220.org.uk/forums/index.php?showtopic=65408

However after all that I'll say there is absoultly no point in putting one on your £999 car as it would be pointless in the current format as they only really work for high speed corners.

[Relentless Rob]

I'll leave a little while longer before I put the right answer up.

[bulletproofbob]

i took the route of flat boot floor and angled bumper... not for any other reason than boot was rotten and bumper needed paint so i cut it

[OilyFingers]

You need a flat floor, a low splitter to accelerate the air under the car and a diffuser to all work together. One without the other two doesn't work too well. As air is compressed it accelerates much like putting your finger over a hose and as the space under the car expands(the diffuser) it slows down sucking the car to the road. Because the transfer from under the front of the car to out the back is smooth it also has this neat trick of reducing gas turbulence and therefore smoother airflow and reduced low pressure behind the car. If the 7 degree angle is correct it must be measured from the leading edge of the front splitter in a straight line to the top of the diffuser.

You get high pressure in front of the car and low pressure behind it, kind of like having a hoover behind the car trying to suck it backwards. The holy grail of F1 is to design a package the reverses that effect but to do that you'd get lift and Mark Webber would need a new nappie. The ground effects as they are called use the high pressure at the front to give more downforce to the front wheels and accelerate it under the car while the back reduces low pressure.