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fabrio. 04-13-2008 10:07 AM

rear diffuser
 
Hi boys

I am going to make the rear diffuser on my car, but serching in all my documentations, I do not understand what is the ideal rear diffuser angle in order to drag reduction.
I suppose, that this angle, he is conseguence of same car measure like the lengh of total body and the lengh of rear diffuser, but also the ground clearance.

Do you have some indications to tell to me?

thanks

tasdrouille 04-13-2008 03:14 PM

Don't go with more than 10 degrees and you'll be fine.

fabrio. 04-13-2008 03:38 PM

thankyou tasdrouolle, tonigth I have installed the rear diffuser.
Small spaces do no permit me to change the actual inclination that is aboud 2°, but afterday, I will go to measure him.
Do you think that this small 2° inclination is bond or bad ?

tasdrouille 04-13-2008 05:51 PM

2 degrees is ok. A couple degrees more would be better, but that would mean cutting your bumper so it's a no go. You just don't want your diffuser to be too steep else the flow will not stay attached to it.

By looking at the brackets you made to support the diffuser I'd say it's more than 2 degrees for sure, but you measure and tell us.

fabrio. 04-13-2008 06:14 PM

yes, afterday, I measure him correctly, and tell you.
For increase the actual angle, I must change the actual rear diffuser with other material more slim of actual.
The limit from spare wheel compartment!
With slim element, I can retrive few centimeters for gain angle.
I think that you have look my last work about the rear diffuser.

Ok, for now, I thank you ;)

Compaq888 04-15-2008 02:40 PM

I used common sense..make sure the air flows and doesn't get stuck anywhere...

diffuser taken off
http://i32.photobucket.com/albums/d2...g?t=1208284416

diffuser on car
http://i32.photobucket.com/albums/d2...g?t=1208284607

http://i32.photobucket.com/albums/d2...g?t=1208284708

http://i32.photobucket.com/albums/d2...g?t=1208284775

tjts1 04-15-2008 03:46 PM

Quote:

Originally Posted by Compaq888 (Post 19718)

It looks very smooth. What kind of material is that?

Compaq888 04-15-2008 05:56 PM

honestly I have no idea...I just went to the home depot roofing section and they had a lot of these sheets...

tjts1 04-16-2008 02:21 AM

Quote:

Originally Posted by Compaq888 (Post 19753)
honestly I have no idea...I just went to the home depot roofing section and they had a lot of these sheets...

Cool thanks, I'll check it out.

fabrio. 04-16-2008 12:12 PM

good clear work Compass, thankyou for your prensence :)

I think (but), that in your rear diffuser, the sepaation point of flow, isnt' fixed because, the rear end of pannel is too up curved
This one, cause not predictable turbolence (i think)

fabrio. 04-16-2008 12:38 PM

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

http://img72.imageshack.us/img72/145...eldesrisa9.jpg

http://img72.imageshack.us/img72/4629/venturism8.jpg

Compaq888 04-16-2008 01:30 PM

Quote:

Originally Posted by fabrio. (Post 19939)
good clear work Compass, thankyou for your prensence :)

I think (but), that in your rear diffuser, the sepaation point of flow, isnt' fixed because, the rear end of pannel is too up curved
This one, cause not predictable turbolence (i think)

It's Compaq...
it's suppose to curve...imagine blowing into a straw...

_____________
____________/ <-------straw A


_____________
_____________| <---- straw B

Now which one are you going to have an easier time blowing...A or B?
The answer is A...because B your air get concentrated and you have less space to work with...
A on the other hand flows more freely and exits more smoothly cause less drag...

RH77 04-16-2008 02:01 PM

Quote:

Originally Posted by Compaq888 (Post 19955)
It's Compaq...
it's suppose to curve...imagine blowing into a straw...

_____________
____________/ <-------straw A


_____________
_____________| <---- straw B

Now which one are you going to have an easier time blowing...A or B?
The answer is A...because B your air get concentrated and you have less space to work with...
A on the other hand flows more freely and exits more smoothly cause less drag...

Right, but I think what Fabrio means is that it abruptly curves upward, which can create turbulence. "Straw A" does not equate to your design:

Straw "C":
______________
____________J

It looks like it would need to be flush with the rear bumper as well -- the air goes upward and gets trapped.

RH77

Compaq888 04-16-2008 02:33 PM

Quote:

Originally Posted by RH77 (Post 19961)
Right, but I think what Fabrio means is that it abruptly curves upward, which can create turbulence. "Straw A" does not equate to your design:

Straw "C":
______________
____________J

It looks like it would need to be flush with the rear bumper as well -- the air goes upward and gets trapped.

RH77

It does abrupty turn upward but it's curved such a way that air exits after the bumper and not inside the bumper...

I went through 2 previous designs until I got it right on this one...


_____________
____________J

I made a little improvement on your design which clearly shows that air never hits the inside of the bumper...

All the aero stuff on my car and increased tire pressure only increased my hwy mpg +10...giving me about 39mpg hwy...but for the city I got 23-24mpg and I drove most city so I had to get rid of the car...

The car I currently have gets 35-36mpg city and 48.x on the freeway...those numbers are hypermiling of course...If you aren't satisfied get a more efficient car...

trebuchet03 04-16-2008 03:16 PM

1 Attachment(s)
Just going to chime in here to the original post with this....

http://ecomodder.com/forum/attachmen...1&d=1208372369

That's a Noble M12 GTO

We measured the diffuser angle to be a tiny tiny bit less than 10 degrees with a digital angle finder (horizontal was set as the shop floor and measured with the car on the ground).

Note the little winglets pointing down to keep air from the tires/side from interfering with the faster flow through the diffuser....


For last year's HPV (The one in foreground), diffuser angle = 10 degrees with something like an 8 degree inlet in the front. These planes don't intersect - there's a flat region from the wheel well area to a little bit behind the wheels.
http://primitiveengineering.com/ucfh...es/splash1.jpg

Those angles were not arbitrarily chosen ;)


------
Quote:

Originally Posted by compaq
I used common sense..make sure the air flows and doesn't get stuck anywhere...

Quote:

Originally Posted by RH77
Right, but I think what Fabrio means is that it abruptly curves upward, which can create turbulence. "Straw A" does not equate to your design:

Straw "C":
______________
____________J

Quote:

Originally Posted by Compaq
It does abrupty turn upward but it's curved such a way that air exits after the bumper and not inside the bumper...

RH77, I think Compaq is fixing a problem of a parachute type effect as opposed to what Fabrio is looking for (something to do stop "parachuting" and work as an effective diffuser). But, Compaq, as a diffuser - that shape is not optimal. The curvature eventually reaches an angle that is way too steep for flow to stay attached without separating "cleanly." Optimal? No. Effective? Possibly. There's no room for all or nothing :D

fabrio. 04-16-2008 04:52 PM

sorry for your nik Compaq, and sorry for my explanation, my English isnt' good, but trebuchet has answered correctly.
I do not refer at the flow direct to bumper, but at the excessive curvature at the end of your diffuser.
The energy of flow, is to small for to follow the profile of diffuser...
Do you know the reserce by mitzubishi about the VG?
Mitzubishi has applyed the VG at the rear of roof for to direct the air flow down direction because, the rear window of lancer sedan is too angled and the flow distact from window.

trebuchet03 04-16-2008 05:03 PM

Quote:

Originally Posted by fabrio. (Post 19990)
sorry for your nik Compaq, and sorry for my explanation, my English isnt' good, but trebuchet has answered correctly.
I do not refer at the flow direct to bumper, but at the excessive curvature at the end of your diffuser.
The energy of flow, is to small for to follow the profile of diffuser...
Do you know the reserce by mitzubishi about the VG?
Mitzubishi has applyed the VG at the rear of roof for to direct the air flow down direction because, the rear window of lancer sedan is too angled and the flow distact from window.

Here's the research paper published by Mitsubishi

http://www.mitsubishi-motors.com/cor...004/16E_03.pdf

fabrio. 04-16-2008 05:09 PM

trebuchet, thankyou for your exhamples.

In my case, I reserch only drag reduction, all my work its focuses they.
Cases A and B of diagram above posted by me, says that the best angle for drag reduction is from 2 and 4 degrees, I know that the reserce of diagram its realative at model with flat underbody, with or widhout wheels I do not know, the height from ground , I do not know.
I know, that some aerodynamic cars like the Loremo, Bionic by mercedes, Opel G90 and more others cars, they have diffusers angle whit more 7° degree.
I think, that optimal angle of diffuser depend also by inclination roof, rearwindow and declidt...corrected ?
It is possible, that the optimal diffusor angle, depend also by point of intersecation of upper flow with the under car flow? may be, that the intersecation point of two flow, it must at the centre of car in according with the front stagnation point?

trebuchet03 04-16-2008 09:19 PM

Quote:

Originally Posted by fabrio. (Post 19994)
trebuchet, thankyou for your exhamples.

In my case, I reserch only drag reduction, all my work its focuses they.
Cases A and B of diagram above posted by me, says that the best angle for drag reduction is from 2 and 4 degrees, I know that the reserce of diagram its realative at model with flat underbody, with or widhout wheels I do not know, the height from ground , I do not know.
I know, that some aerodynamic cars like the Loremo, Bionic by mercedes, Opel G90 and more others cars, they have diffusers angle whit more 7° degree.
I think, that optimal angle of diffuser depend also by inclination roof, rearwindow and declidt...corrected ?
It is possible, that the optimal diffusor angle, depend also by point of intersecation of upper flow with the under car flow? may be, that the intersecation point of two flow, it must at the centre of car in according with the front stagnation point?

For our cars, I don't think flow interaction from the top and bottom are significant - because our cars are bluff bodies. For very streamlined shapes, yes - that will be a bigger concern. But, in my opinion, most of the losses for that section of our cars are due to wake rather than intersecting flow regions... That's just a guess - testing/analysis would be really useful here.

It's actually a good idea to look at other cars with diffusers to see what they have. Being that they're cars, they'll have a similar (at least somewhat comparable) ride height and a comparable height to length ratio.

Compaq888 04-17-2008 12:37 AM

Quote:

Originally Posted by trebuchet03 (Post 19975)
Just going to chime in here to the original post with this....

http://ecomodder.com/forum/attachmen...1&d=1208372369

That's a Noble M12 GTO

We measured the diffuser angle to be a tiny tiny bit less than 10 degrees with a digital angle finder (horizontal was set as the shop floor and measured with the car on the ground).

Note the little winglets pointing down to keep air from the tires/side from interfering with the faster flow through the diffuser....


For last year's HPV (The one in foreground), diffuser angle = 10 degrees with something like an 8 degree inlet in the front. These planes don't intersect - there's a flat region from the wheel well area to a little bit behind the wheels.
http://primitiveengineering.com/ucfh...es/splash1.jpg

Those angles were not arbitrarily chosen ;)


------






RH77, I think Compaq is fixing a problem of a parachute type effect as opposed to what Fabrio is looking for (something to do stop "parachuting" and work as an effective diffuser). But, Compaq, as a diffuser - that shape is not optimal. The curvature eventually reaches an angle that is way too steep for flow to stay attached without separating "cleanly." Optimal? No. Effective? Possibly. There's no room for all or nothing :D


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...My design was simple and effective...I wanted to do the whole underbody of the car and finish off with a crazy diffuser but that would of wasted a lot of money..

trebuchet03 04-17-2008 12:40 AM

Quote:

Originally Posted by Compaq888 (Post 20089)
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 :thumbup: I'm totally in the same boat (plus making sure nothing is permanent) :D

Compaq888 04-17-2008 03:06 AM

Quote:

Originally Posted by trebuchet03 (Post 20090)
No worries :thumbup: I'm totally in the same boat (plus making sure nothing is permanent) :D


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 04-17-2008 11:25 AM

Diffuser angle
 
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. 04-19-2008 04:03 PM

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 03-26-2010 03:10 PM

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

http://ecomodder.com/forum/attachmen...4&d=1269629073

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 03-26-2010 05:49 PM

angle determined by origin of diffuser
 
Quote:

Originally Posted by fabrio. (Post 19944)
I am confused from two image below, can you help me to understand wich is the reality?

http://img72.imageshack.us/img72/145...eldesrisa9.jpg

http://img72.imageshack.us/img72/4629/venturism8.jpg

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 05-14-2010 10:11 PM

Quote:

Originally Posted by Piwoslaw (Post 167791)
I found this in Piechna's book. It's based on Kohri and Numata's paper[1].

http://ecomodder.com/forum/attachmen...4&d=1269629073

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 05-15-2010 01:54 AM

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:
http://ecomodder.com/forum/member-pi...loped-back.jpg
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).

http://ecomodder.com/forum/member-pi...cle-shapes.jpg
Wakes (and eddies) from different vehicle shapes.

http://ecomodder.com/forum/member-pi...r-diffusor.jpg
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.

http://ecomodder.com/forum/member-pi...ce-devices.jpg
Drag (Cx) and lift (Cz) of different downforce devices:
1 - Spoiler,
2 - Rear diffuser,
3 - Diffuser plus front air dam.

puddleglum 05-16-2010 01:20 AM

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 05-16-2010 02:16 AM

Quote:

Originally Posted by puddleglum (Post 174772)
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?

puddleglum 05-17-2010 12:36 AM

It would seem that your roof extension would make your car less bluff (if that is a proper term). I read through your build thread today, nice work. Oh, and my wife likes your yard ;) I take it you are thinking about a belly pan this year as well? Our cars are actually quite similar in configuration and profile. mine is taller, wider and heavier though, and uses twice as much fuel. I have the same spare tire issue as you do if I do a rear pan. If you beat me to it I'll be looking to see what you did. It's quite impressive that you got a 10% improvement in FE even though your Cd numbers didn't change that much. This is off topic, but I still find it very cool that I can sit here and chat with someone half way around the world.


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