Failures!

[JulianEdgar]

https://www.youtube.com/watch?v=miNmX1baK1E

(Thanks to M_a_t_t here for the suggestion.)

[Stubby79]

Knowing what doesn't work is at least as important as knowing what does.

[AeroMcAeroFace]

https://www.youtube.com/watch?v=hgXni_LUgwc

I found this video, and indeed channel, to be very interesting. Not sure if it has ever been mentioned on ecomodder, he is an aerodynamicist who worked in F1, mostly race car focussed but still interesting

[Cd]

Great video.
As Stubby said, knowing what doesn't work is just as important as knowing what does !

You mention strakes on your diffuser didn't work.
They didn't produce any additional downforce.

But i added strakes to mine in the hope that it would staighten out airflow.
That, and I think they look cool ( which means even if ineffective, I plan to still use them unless they actually create drag.

Would strakes help guide air that is going turbulent ? ( It's related the questions I asked before about using fences on cars. )

[JulianEdgar]

Originally Posted by Cd

Great video.
As Stubby said, knowing what doesn't work is just as important as knowing what does !

You mention strakes on your diffuser didn't work.
They didn't produce any additional downforce.

But i added strakes to mine in the hope that it would staighten out airflow.
That, and I think they look cool ( which means even if ineffective, I plan to still use them unless they actually create drag.

Would strakes help guide air that is going turbulent ? ( It's related the questions I asked before about using fences on cars. )

Strakes / fences do not "stop the air going turbulent" (ie separating) in any situation of which I am aware. The approach I finally took, after much testing, is shown in this video:

https://www.youtube.com/watch?v=eepGdhi4daY

[JulianEdgar]

Originally Posted by AeroMcAeroFace

I found this video, and indeed channel, to be very interesting. Not sure if it has ever been mentioned on ecomodder, he is an aerodynamicist who worked in F1, mostly race car focussed but still interesting

He has a wide following but I find quite a lot of what he says is overly definite compared with the textbooks eg on how diffusers work.

It might just be that he is focused only on racing cars, where many aspects are quite different to road cars.

[AeroMcAeroFace]

It is mostly race car focussed, my reading of strakes is essentially, it separates the single diffuser into multiple diffusers. If there is a lower point, say a rear differential, you can start the outer diffuser further forward and the one after the differential or exhaust further back. In your insight, and most eco cars, the flow doesn't need to be separated into different channels because the floor is reasonably flat at the back and diffusers are usually much shallower so would not interact with components anyway.

[Vman455]

A lot of what the guy in the video says, though, is contradicted in this article by F1 aerodynamicist Willem Toet. Toet makes no mention of using fences to separate flow in a diffuser, and he goes so far as to say vortex generators "of any design really" may improve downforce.

(Also, note the last CFD image--especially where the vortex appears in relation to an L-shaped vane!).

[AeroMcAeroFace]

From what I remember of the video and reading that article I see no contradictions, maybe things aren't mentioned but that doesn't mean it is a contradiction.

The video says nothing about separating the flow. My post said "my reading of strakes is essentially, it separates the single diffuser into multiple diffusers" which you could argue is not mentioned, and you could argue is not technically correct.

The part you are quoting says "If you add an additional fence or strake (a vortex generator of any design really) in each side of the floor it is possible to roll up two more vortices" The video definitely agrees with that.

The vortices help to "seal" the gap between the diffuser and the road. The last image shows a vortex at the edge of the L and one inside the diffuser, the video just mentions the vortex at the edge of the L.

Do you have timestamps or quotes that show the contradiction?

[Vman455]

The video specifically states that the strakes and their tip vortices separate the flow coming off the rear wheels from the rest of the diffuser, at 7:44. He also says several times (and takes as his initial premise) that the vortices rolling off the strake edges stay at those edges, but the flow-viz patterns in the article clearly show that is not the case; the vortices from longitudinal fences are quite large and attached to the diffuser surface itself until its angle is extreme enough that one bursts, and the chart shows that before that burst angle, these attached vortices reduce lift. The video also asserts that the vortices rolling off an L-shaped vane occur at the tip of the horizontal portion, but images in the article show the opposite--the vortices roll up the opposite side from the L. Finally, the article ends with the explanation that fences in the middle of diffusers are there specifically to generate vortices to improve generation of downforce, whereas the guy in the video says that the fences are there to separate wheel flow from the inward part of the diffuser and that this improves downforce by not messing up the flow, essentially (last two minutes of the video).

Two different explanations, but I don't know which one is correct. Perhaps neither, but one seems more credible to me.