rear diffuser

[fabrio.]

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

[Compaq888]

Originally Posted by fabrio.

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]

Originally Posted by Compaq888

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]

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

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]

Just going to chime in here to the original post with this....



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.


Those angles were not arbitrarily chosen


------

Originally Posted by compaq

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

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

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

[fabrio.]

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]

Originally Posted by fabrio.

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.]

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]

Originally Posted by fabrio.

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]

Originally Posted by trebuchet03

Just going to chime in here to the original post with this....



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.


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

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..