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Old 06-26-2022, 07:55 AM   #11 (permalink)
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The underbelly angle must be kept very shallow to retain attachment (2 or 3 degrees if I recall correctly), otherwise turbulence results.

The diffuser fences/strakes allows a roughly doubling of inclination angle. The 10 degree angle I drew was considered undo-able.

Going off memory of sharing a coffee with a professional aerodynamicist about five years ago that had work experience with several automakers.

The fences work because air attaches to the vertical surface much the same way as the near horizontal surfaces, I suspect as parasitic drag hence people's words of caution about assuming a rear diffuser will always lower drag.

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Old 06-26-2022, 08:54 AM   #12 (permalink)
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Quote:
Originally Posted by kach22i View Post
The underbelly angle must be kept very shallow to retain attachment (2 or 3 degrees if I recall correctly), otherwise turbulence results.

The diffuser fences/strakes allows a roughly doubling of inclination angle. The 10 degree angle I drew was considered undo-able.

Going off memory of sharing a coffee with a professional aerodynamicist about five years ago that had work experience with several automakers.

The fences work because air attaches to the vertical surface much the same way as the near horizontal surfaces, I suspect as parasitic drag hence people's words of caution about assuming a rear diffuser will always lower drag.
These are of course generalisations and not necessarily applicable to every car type/groud clearance.

"im going to keep it fairly flat and a mild angle like the eqxx also. pretty much just extending the existing underbody shape out another foot or so"

That is rarely a bad thing.

Generally, high diffuser angles (greater than 18 degrees) cause seperation and vortices and increase drag. Low diffuser angles (less than 12 degrees) allow attached flow and can reduce drag.

What that exact optimum (for low drag) angle is on your specific car? Who knows, likely somewhere between 0-12 degrees.

If you are talking about just extending out the diffuser a long way, past the end of the car then there is a paper on this. In their tests, on their car, they saw around 9 degrees was the optimum angle. Click image for larger version

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ID:	32471 Paper name: Drag and lift reduction of a 3D bluff body using flaps

Last edited by AeroMcAeroFace; 06-26-2022 at 08:56 AM.. Reason: Addition of necessarily
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Old 06-26-2022, 11:05 AM   #13 (permalink)
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www.researchgate.net/publication/226210472_Drag_and_lift_reduction_of_a_3D_bluff_bo dy_using_flaps

Quote:

During the data acquisition, both laser source and camera placed outside the test-section perform the measurements through transparent walls or sufficiently small holes to avoid perturbations of the flow (Fig. 5). The flow is seeded 12 L upstream from the model with olive oil spherical droplets whose average diameter is about 300 lm. The particles have very satisfactory reflection properties. Two Nd:Yag laser sources (120 mJ during a 10 ns pulse) provide double-pulsed light sheets which are approximately 3-mm thick.Images are recorded using a 1,280 91,024 pixels CCD camera. The typical physical dimensions of the PIV images are 400 mm 9500 mm. We use a 16 916 pixels interrogation window with a 25% overlap leading to 4.8 mm spatial resolution. 250 instantaneous velocity fields were recorded to obtain a converged time-averaged velocity field.
Interesting data aquisition via olive oil.
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Old 06-26-2022, 01:47 PM   #14 (permalink)
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The way the rear of my car is shaped, it looks like my diffuser will have to get more narrow the farther out it goes. In theory, shouldn’t that also benefit since a more narrow path increases flow and thus should spit out even faster flowing air into the wake area? I’m just going off what I know from sluice boxes when gold panning in water lol
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Old 06-26-2022, 03:06 PM   #15 (permalink)
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Any success with the sluice box? Or do you keep that sort of knowledge held close?

Depending on tuft testing, the long skirts should have some taper in plan. It should transition from the rectangular body section to an increasing radius. (pace Morelli) A skeg at 45&deg wouldn't add side area in crosswind conditions. I imagine something like the Cadillac Biarritz turned upside down, at an angle.

https://bestcarmagz.net/sites/defaul...ar_3-4_Web.jpg
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Old 06-26-2022, 06:56 PM   #16 (permalink)
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The way the rear of my car is shaped, it looks like my diffuser will have to get more narrow the farther out it goes.
That's not optimal. Ideally, the diffuser would be a diverging nozzle, i.e. bigger at the back.
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Old 06-26-2022, 09:43 PM   #17 (permalink)
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Originally Posted by Blacktree View Post
That's not optimal. Ideally, the diffuser would be a diverging nozzle, i.e. bigger at the back.
Well the way the back bumper is shaped on the corners and how I’m “ supposed to integrate the flares coming out of the rear wheel skirts), I don’t see how that’s possible
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Old 06-26-2022, 09:45 PM   #18 (permalink)
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See. Tapers in
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Old 06-26-2022, 11:16 PM   #19 (permalink)
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Convergent on the outside, divergent on the inside.
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Old 06-26-2022, 11:33 PM   #20 (permalink)
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is it even worth building the diffuser from the rear wheel spats though? theres going to be a lot of turbulance under body behind the rear wheels anyways with detached airflow

whats the point of trying to reattach the airflow for another inch, just for it to become separated after the diffuser? obviously between the wheels is the most important part

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