Diffuser Profile - Convex or Concave?

[shelly]

IIRC that feature was almost standard on every F1 car before the 2009 rule change.
It was related to a rule that imposed minimum radius for the transition from the flat floor to the diffuser. I think it is not adopted now because the ruling has changed.

I agree with gixxer: when cfd and wt give different result, it's not a matter of asking which is wrong - there's a big chance both results are wrong.

[hardingfv32]

Now if the boundary layer buildup exceeds what you calculate it to be (and this happens ALL the time) you kind of scratch your head, but depending on budgets and time you pursue what you must to get a car ready and performing at the best you can with the tools and money you have. Especially when you have a rolling floor, everyone thinks that fixes everything.

Someone I really look up to once told me that you never really understand the car until you designed and tested the wind tunnel as well.

If your CFD and your tunnel data don't converge, what do you trust? A mathematical system designed around giving the same results as a wind tunnel or a wind tunnel?

Nobody else copied it, it only worked in one tunnel. Might explain their car's performance if it were true. DId that feature appear on the new car after they pulled out?

In my vivid imagination F1 tunnels are so amazing they always give perfect values but reality may be a bit different. ;)

I think you paint too negative a picture, especially for those who have no actual tunnel experience. I don't think any of the issues you bring up are unknown to most major tunnel operators. Chances are good that many of the solutions are proprietary, so in fact they are not issues at all. We don't even have a good idea how much is spent doing tunnel development.

In the end I think you develop a numbers that represents the accuracy of the correlation between CFD, WT and actual track results. You view your results through the lens of these numbers.

Brian

[gixxer_drew]

Im throwing ideas out for consideration not absolute conclusions.

The way it was laid out was that one team had a part that nobody else had. Then thinking through calculated boundary layer over that short of a run and something didn't add up if the thing is to break up boundary. Doesn't mean it isnt something else, I presented a few different possibilities I would want to investigate, could be a sea of things.

Only one thing I suggested for investigation was a potential wind tunnel problem. If it was a rules thing, thats fine, but short of that... something is odd about that part. I am basing that on intuition and experience. Assign value to that at will.

The "why" is still the question, but it is definitely striking me as odd. Also I would disagree about these issues being "known" because I find problems in probably the majority of tunnels I use and have to just work around them. Presumably F1 tunnels are better though, I have no experience with them.

My real point in all of this is not to conclude that such a device will work well for you because it was on an F1 car.

[gixxer_drew]

Got a couple photos of the honda diffuser from 2007 season. I have no way to get access to the anything more recent than the photo which was posted before with the step.




I'm trying to find out more about the step and I'll ask if I can talk about it publicly.

[ringo]

Here we have 3 curves. No step on this one.

[astracrazy]

i know its an old topic, but it was an interesting one. So i'm wondering if we could bring it back to life with some more recent pictures etc.

rb9 diffuser

[trinidefender]

This is actually something I would like to see more of. Are there any guys out here still modelling diffusers?

[raymondu999]

I think we also have to consider the lateral aspect now, with so many top teams opting for diffusers that not only have an upward kink, but also sidewards

[KeithYoung]

This is actually something I would like to see more of. Are there any guys out here still modelling diffusers?

I am doing a series or articles related to this topic. Convex/Concave Diffusers will be covered later. At the moment I am running CFD Simulations on varying Ride Height.

Here is my Diffuser Angle article.

http://consultkeithyoung.com/content/cf ... user-angle

[SiLo]

This is actually something I would like to see more of. Are there any guys out here still modelling diffusers?

I am doing a series or articles related to this topic. Convex/Concave Diffusers will be covered later. At the moment I am running CFD Simulations on varying Ride Height.

Here is my Diffuser Angle article.

http://consultkeithyoung.com/content/cf ... user-angle

http://consultkeithyoung.com/sites/defa ... k=MGxVrlsA

Great article, keep them coming!

[marcush.]

you are changing difusser length with the angle adjustment - retaining the body length ..and this changes the area of the flat bottom .wouldn´t this have a significant influence on the test results?
why not shortening the difusser a little bit and fix the starting point of the difusser?
Also my view is a sharp edge ,kink at the transition will not make too much of a difference at small angles but as angles get steeper the flow will be separated easier ..So the transition should be rounded so as to avoid this effect ?(for comparability of testing results-I do not claim it is the way to go to get most out of a difusser design)

[KeithYoung]

Yeah marcush it does have an effect. However I did it in a way to keep the AR near constant.

Having a short diffuser with a low angle leaves a large area in the back of the car that's perpendicular to the airflow and it makes convergence difficult, driving iterations higher.

Yes, one thing I will do later is watch how the transition effects the flow. There is a radius there, just not a very large one. I'll likely need a larger radius for higher diffuser angles. We'll see later if the CFD backs up that theory.

[marcush.]

ah cool ..
I´m very interested in your findings and look forward to this thread evolving after its revival .

[BreezyRacer]

This is indeed an interesting thread to revive. I have learned though, that diffuser design in a vacuum (pun intended) where you stick a bluff body in the airflow and stick on a diffuser tells you nothing. Early on I took such designs and once attached to a real model, they died. Since the diffuser is the last part of the car virtually everything else determines much of diffuser design.

That is not to say that we cannot try to learn though. Still I'm a little vexed over the concave/convex nature of diffusers, but I've concluded that it comes down to size limitations rather than one being better than another. I guess that if I had the space and regulations allowed it, I would choose a pretty gradual taper with a slightly concave shape. Packaging drives these differences more than anything else.

And for the last couple of years, I've pondered the diffuser gurneys that today are in wide use and increasing sophistication. So does anyone have an opinion on this ..

Is a diffuser exit better released into a high flow surrounding or into a vacuum of sorts, as created with solid gurneys? Do today's slotted gurneys reduce drag, or increase diffuser exit and therefore downforce, or all of the above?

Just what we need .. more questions.

[Tommy Cookers]

Is a diffuser exit better released into a high flow surrounding or into a vacuum of sorts, as created with solid gurneys? Do today's slotted gurneys reduce drag, or .....

please remind me how there can be such a thing as a slotted gurney ??
without this being re-inventing the wheel (the single slotted flap) under a new name ?
sincerely

btw the so-called Gurney flap was patented around 1932 by some German fellow (so was not patentable by DG/Douglas)
and seems to do what was done by trimming around 100 years ago (when aircraft Re nos were similar to those of F1)
trimming being then the addition of one or other dia of cord to one trailing edge (then adjusted by trimming of cord length)