rear diffuser

[trebuchet03]

Originally Posted by Compaq888

That was the most effective shape possible on a $20 budget...Yes I could of done a lot more and could of had something better but that would require a lot of money. You got to draw the line of how much spending to get better FE you're going to do...

No worries I'm totally in the same boat (plus making sure nothing is permanent)

[Compaq888]

Originally Posted by trebuchet03

No worries I'm totally in the same boat (plus making sure nothing is permanent)

Yea..that's what I did...all my aero mods were designed where I could take them off and nobody would notice a thing...and that's exactly what I did. I took them all off and sold the car.

[aerohead]

Hi Fabrio, in Hucho's book,there is a section on bellypans.Within this section is a piece on diffusers,which maximize the performance of the bellypan.I do not have the book in front of me,however,if my memory serves me,the article claims that a diffuser should have an angle of divergence of no more than 2 to 2.5 degrees,with respect to the horizontal plane or you'll be looking at separated flow.The work presumes that you have a complete bellypan ahead of the diffuser.The angles must be established with "load" in the car,so as to mimic the cars attitude,as it will demonstrate while driving with occupants and cargo.Sandbags,sacks of concrete mix,etc.,can be used to ballast the car's interior,to simulate the "load" while you set up the diffuser's angle.

[fabrio.]

thankyou aerohead, the image come from Hucho's book, and I have supposed the same consideration, but I do not understand because full belly pannelled cars (loremo, ecc...) they have an angle divergence mote than 5 degrees.
Its possible, conseguence from hi grounf clearance of this cars?

[Piwoslaw]

I found this in Piechna's book. It's based on Kohri and Numata's paper[1].



As can be seen, a diffuser hurts aerodynamic drag in vehicles with a square rear (vans, station wagons). It's something about the diffuser causing eddies that cancel with countereddies from the sloped rear end. In the square rear there are no countereddies, so there is no cancellation.

[1] Kohri I., Numata N., Effects of rear end configuration of vehicles on aerodynamic drag, JSAE Review, December 1985.

[aerohead]

Originally Posted by fabrio.

I am confused from two image below, can you help me to understand wich is the reality?

From the diagram,if the diffuser begins ahead of the rear suspension,go with 2-1/2-degrees maximum.
If it begins behind the rear axle go with 4-degrees maximum.
The 7-degree recommendation may be a compromise to road clearance and not really about aerodynamics.

[puddleglum]

Originally Posted by Piwoslaw

I found this in Piechna's book. It's based on Kohri and Numata's paper[1].



As can be seen, a diffuser hurts aerodynamic drag in vehicles with a square rear (vans, station wagons). It's something about the diffuser causing eddies that cancel with countereddies from the sloped rear end. In the square rear there are no countereddies, so there is no cancellation.

[1] Kohri I., Numata N., Effects of rear end configuration of vehicles on aerodynamic drag, JSAE Review, December 1985.

I was looking at this thread again to determine which angle to use for the rear pan/diffuser and this picture really confused me. Is the graph at the bottom showing drag or lift?
Piwoslaw is saying drag, but the numbers on the left correspond to the lift graph in the other diagram. Could one of you who really understand this please explain. If this is showing drag, is a rear pan going to do more harm than good on a bluff back car? If this is showing lift, is it saying that lift is increased or decreased on a bluff back car?

[Piwoslaw]

Puddleglum, I would also like someone to comment on this, since I like to have more than one source of info.

I checked once more to make sure, and that chart is for drag (Cx), not lift (Cz). As mentioned, for sedan and sloped-back (notchback and fastback) cars, the sloped C pillars cause a pair of eddies (I posted a scan of them somewhere, but can't find it now), while the diffuser produces counterrotating eddies that cancel. The angle of the diffuser determines how strong the countereddies are, and at about 3°-4° is the minimum, after which the diffuser's countereddies are strong enough to survive cancellation and bring drag down again. In a squareback car there is no sloped C pillar to produce eddies, so the diffuser's eddies have nothing to cancel with, and their drag increases more of less proportionally to the angle.

I'll update this later today when I scan another drawing.

I the mean time, here's a related thread:
Underbody panel testing

EDIT: Here are the promised drawings:

On the left is a sloped-back or notch-back vehicle (notice cancellation of eddies and countereddies), on the right a square-back vehicle (no countereddies to cancel).


Wakes (and eddies) from different vehicle shapes.


Drag (Cx) and lift (Cz) change with diffuser angle. This is from a chapter on race and performance cars, so I don't know how close it is for non-race vehicles. Based on:
Bearman P.W., De Beer D., Hamidy E., Harvey J.K., The effect of a moving Floor on Wind-Tunnel Simulation of Road Vehicles, SAE 880245.


Drag (Cx) and lift (Cz) of different downforce devices:
1 - Spoiler,
2 - Rear diffuser,
3 - Diffuser plus front air dam.

[puddleglum]

I see what you are saying, but your first diagram seems to show that a rear pan is useless on a bluff back car. The middle graph on this last post seems to still show a slight improvement. I hope that one of the experts will give some clarification on this. It doesn't make sense to me that there would not be any benefit to smoothing the underbody, even on a hatchback

[Piwoslaw]

Originally Posted by puddleglum

I see what you are saying, but your first diagram seems to show that a rear pan is useless on a bluff back car. The middle graph on this last post seems to still show a slight improvement.

I think those two graphs are based on different papers, so maybe the researchers used different vehicles? Anyway, I'd like to know what Hucho has to say about it, and I'd be honored if Phil dropped a comment.

One thing makes me wonder: my car is a square-backed station wagon, but it now has a Kamm-style roof extension, so can it still be regarded as "bluff"? In such a case, would a rear diffuser help more than if I didn't have the Kammy?