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I have searched quite a bit trying to get some ideas for diffuser angle profiles. It seems a lot of the CFD study information I found hasn't included rear wings when defining optimal angles which as I understand "drive" the diffuser and allow for greater angles without flow separation.

I have to create a new diffuser for my car and thought it the ideal time to improve upon what is there. What I am now wondering about is exactly what profile to use for the new design.

Any help / ideas appreciated.

Are the wishbones the only restrictions to the space you have available? What about the chassis and the differential? I've also found it difficult to find what's optimal, so I've just gone with what looks right. How well does your existing diffuser work? Is air remaining attached at 17 degrees? It seems quite an aggressive angle, especially if you're running any sort of rake. Do you have flow over the top of the diffuser such that a gurney on the top could help pull the air out underneath and reduce separation?

Other than the gearbox, basically, yes no other restrictions. I was just concentrating on the profile of the diffuser sections either side of the gearbox for now, the diagram ignores the gearbox.

How well does my diffuser work? Well not very well now as it's in about 30 different pieces after I stuck it in the barrier backwards! My car is an 88' F3 car so I doubt it was every really optimised, the lack of curvature looked wrong as most diffusers you see these days blend with a curve to the floor line, not just a straight angle like the original.

There should be some flow over the top of the diffuser. I also have later side pods which look to direct more flow over the top of the diffuser. I will be using these at some point in the future, but there is work involved in fitting so not high up my priority list at the moment. Where are you suggesting the gurney would sit though? I was thinking of adding one to the lower wing element as shown on the diagram.

There's very little reliable information in the public domain about diffuser and rear wing interaction. I've never found anything useful. If you don't have access to a wind tunnel or CFD, then the best bet is a test day and an adjustable diffuser. Solidly mount the leading edge of a plywood floor in line with the floor of the car and use an adjustable rear mount (threaded bar) and the plywood will flex to a nice arc. Try a few rear wing heights and adjust the diffuser up and down until you're getting good attached flow. Use wool to tuft-test it and mount a camera on a bracket hanging out the back. You can run different angles each side to halve the test time. If you want some empirical data, then use a data logger and some suspension pots to get some data at a constant speed. A soft-cut rev limiter would be useful here!

Most importantly, take photos and post back here so we can see the results!

I might also be useful to use some sort of flow viz paint if available? Or at least your own concoction of it?

I've always believed tufts can be effective, but also quite invasive in quite a critical area? But I suppose it all depends on how, and where they are placed.

Flow-viz might be a good plan, but you'll need to clean it off between runs if you're altering the angle of the diffuser and it won't show separation when the nose dives under braking, effectively increasing the angle of the diffuser. This is a 25 year old F3 car, not a modern F1 car with today's advanced pitch control. Most importantly, you'll spend more time cleaning than driving, which is not a good use of track time. Take video and data with 5 minutes of adjustment written in a notebook and spend the next few evenings analysing.

andylaurence wrote:Flow-viz might be a good plan, but you'll need to clean it off between runs if you're altering the angle of the diffuser and it won't show separation when the nose dives under braking, effectively increasing the angle of the diffuser. This is a 25 year old F3 car, not a modern F1 car with today's advanced pitch control. Most importantly, you'll spend more time cleaning than driving, which is not a good use of track time. Take video and data with 5 minutes of adjustment written in a notebook and spend the next few evenings analysing.

Good points, thank you for making better use of your brain than I

That kink line from the floor to the diffusor might not be as bad as it appears to be. It generates a area of local low pressure and therefore helps in generating downforce. These are things which should be tried out with CFD.

My priority at the moment is to get the car fixed so I will just "eye ball" the angle to what looks about right. Once I have more time I will look into CFD and then perhaps make up a different version. I'm interested in a concave version although then I have less space under the car.

As requested here are a couple of videos from under the car:

The first is with a gurney flap on the lower wing element:

https://vimeo.com/46549906

The second is without:

https://vimeo.com/46563924

It seems there was less disturbance without so it was left off for the rest of the day. I had other things to work on so this is as far as the testing went on the diffuser for now.

The following image I took from this discussion: viewtopic.php?f=6&t=10943&start=90



You can imagine the same thing happening under my car. Is the vortex a bad thing? The 10 and 15 degree convex profiles in that post produced similar downforce with the same drag. I think one of the considerations from looking at the video is how best to deal with the bow wave of air coming off the wheels as you can see it spilling in from the right of the shot. I will try and get a camera mounted for the centre tunnel at some point as it would be very interesting to see the difference. The centre tunnel has less of a angle as it has to fit around the gearbox and you would presume there would be less of an effect from the rear wheels.

nacnac wrote:You can imagine the same thing happening under my car. Is the vortex a bad thing?

If that's flow separation then it's an area of the diffuser that's generating no downforce, just stalling.

-not an aero guy.

That certainly looks cleaner without the gurney. I'm no aero guy either, but I'd expect it to be cleaner if it was working properly. How did your lap times compare? I assume those are back-to-back tests.

New track and fairly new car to me so lap time data was pretty meaningless as I was improving all day. I've got some other ideas about cleaning up the flow some more which I will try at some point. I also have Simon McBeath's book on order which may also help some.

In the screenshots you can see the vortex is blue, that is moving at near zero. Therefore its pressure will be up around static pressure, that is it will be generating no downforce.

Therefore, to no one's great surprise, angles of 10 or 15 degrees are not useful in diffusers because they cause separation. The rule of thumb used to be 7 - 8 degrees. Adding a wing is unlikely to increase that useable angle significantly, but it does extend the negative pressure/high velocity area, so you get more downforce.

The convex diffuser shown in the pictures is at its steepest right were the flow has the least energy left. IMHO it's no surprise that the flow seperates. The "bell shaped" diffusers used in F1 nowadays might be a better idea. I.e. get high expansion at the front edge of the diffuser and the flatten out the diffuser "roof" towards the end. That should keep the flow attached, yet maximize the expansion. Tunring vanes might also help. Or you could blow the diffuser to keep the boundary layer attached. That's going to be a lot of development work, though.