tigra AERO PROJECT rear end question
 

I am estimating of modify my car like in the first photo, but I have some doubts.

In middle pic, you can see the upper vision of my car.
The rear window and the back end of car they have a particular shape and, I assume an air flow like red and blue, that I have marked.

http://img245.imageshack.us/img245/1039/provamh9.jpg

http://img177.imageshack.us/img177/6...perioreeq4.jpg

http://img405.imageshack.us/img405/4...ge11xg1tg7.jpg

http://img505.imageshack.us/img505/1379/vorticiqm3.jpg

Differents angles of two flow in the rear end Is it a possible source of drag?
Can I resolve this problem?

Similar rear end one has it the Bristol Fighter :

http://www.bristolcars.co.uk/pics/Br...roDynamics.jpg

but in this car, the two flow : from the flanks and from the rear windows has a minor angle difference .
On original tigra, the uper flanks has angled up by 6°, and roof-spoiler angle is 15°, but the real airflow from central car line minor of 15° because, the rear spoliler deviate upper the flow, consequently, I do not have a total by 21° (6°+15°), of angled flow (cause of cross vortex), but a minor value.
If I create new shape like the first photo, I can put the central flow at 15°, optimum for lowest drag, I go to increase the actuale cross vortex and I create more drag (I think).

have you, of the councils give to me?

Ah, the Jaray shape, I really like those designs from an aesthetic standpoint.

Honestly, I would not worry about the vortices, you're always gonna have two trailling vortices unless you have something close to a round boattail. Plus, you car is pretty well designed when you look at the c pillars and the slope of the back light.

One thing your car could probably benefit from is a lip spoiler that would otherwise be a decklid extension on a notchback as the B angle in your illustration is borderline. You should first check if you have an occurence of that arch vortex on you car or if the flow simply detach somewhere on the back light. One very easy way to ckeck would be to do some wool tuft tests at the bottom of the back light. You can even just apply wool tufts on one side, but do the test in to directions to account for sidewinds.

Hi tasdrouille.

my car has an Cd of 0.31, no bad, but also no good.
The opel has spold a lot hours in wind tunnel and the effects of this one they can are visible in some elements of car.
Rounded rear wheel arc (the profile), fill gapping in the front car, mirriors ect...
Although this one, the drag coefficient is "high".
So, I think that high drag contribution source is the rear end of car.

months ago I have applyed the wool tufts to look the rear flow in the rear windows.
Only detachments present is located between the rear windows and the decklid in a small bubble.

the bubble

http://img81.imageshack.us/img81/2981/bollarq0.jpg

I think you are on the right track with a center ridge. The diagram shows a vehicle having more or less the same cross section along the longitudinal axis.

As seen in an earlier posted about a pickup truck bed canopy, the optimal shape has a ridge.

Vortices have weight. A center ridge will tend to guide the "A" pillar vortices outboard where they can simply fall off the vehicle before they interact with the roof vortex. The other option would be a fence to seapate the "A" pillar and roof vortices, but a fence will aggravate and already compromised rear visibility sitation.

Can you extend your roof ridge out to the end of the "bubble?"

wow,it is an complex answer

I you would be pleasing if you made one outline me

H4MM3R -

Yeah, and it looks like an Opel-Insight in Fabrio's mockups!

CarloSW2

I like those chopped on porsche wheels.. :)

Ok, I finlly understand :)

A center ridge is an idea, and I wanted to discus with you only for undertand news.
If I build the bridge, do more 'small, that, from the cover the bubble.
Possible material is the transparent plexiglass, but the problem arises of how to anchor.

I like the original design. You will always have vortices. With your design they will be lessened.

Anchoring it depends on how detailed you want to get. Ideally, building a new lightweight fiberglass/polycarbonate hatch that mounts in the factory location but is "oversized" to fit the dimensions you laid out would be best.

Ofcourse, this method is not easy :p. I still think the best anchoring method will be to attach it to the hatch lid somehow. By doing so, you can make the mod reversible.

To decrease interference drag/vortices, be sure to apply large radii to all edges (if it's practical to do so...:)).

I really like the look, by the way. :D

- LostCause

It is hard....

I have exposed this project at the other member of Tigrafans (I am an user of this club).
Notthin has any experience and culture aboud the aerodinamic and ECO drive.
in the tigrafans, my project whas rejected :mad:...
But in this EM forum, I have goods reports...:)

My problem in the realization, is the curvature that it is necessary to create in the sides of ridge for an armonic anchorage.

I do not want that the new center profile (ridge) in order with the original curvature of rear windows, is cause more vortices.
Can you calm me?

Fabrio, I really like the look of your car in the picture you edited. It looks a lot like an Insight.
As far as any flow seperation on the back window, would you consider using vortex generators to keep the flow attached better ?
http://www.autospeed.com/cms/A_3061/article.html

You could attach them just below the roofline. This way they would reattach any flow seperation, but would also not add to the frontal area of the car and actually decrease your mileage.

Hi .Cd
I evaluate VG solution, but I have rejecter it beacose, the back lid isnt' horizontal, but upwards.
look this detail: in the the first, with VG, I think that the flow is like in red arrow

http://img168.imageshack.us/img168/9168/vgkk1.jpg

In this below, without the VG, but with the ridge cover, I think that the flow is like:

http://img260.imageshack.us/img260/410/coverhr7.jpg

Because some literatures say the best angle for rear window-beclid is approximately 15 degree, I think the good way it is the last solution.
Dubt rises, with lateral backlid flow that isnt' aligned with the central axis flow (look the fourth image) that, with the central ridge, is more unwaged than the original shape or VG elements.
I think, only best way to eliminate the bubble, it si to add central risge,but I do not want improve one thing and another worsen.
Perhaps, to apply VG in the lateral back lid? or add curves to guide down the outlet air flow?

If I was in your situation, I would remove the back glass and add a spacer between the glass and the trunk. This would allow you to modify the angle of it.
( I'll try and post an image as an example. )
Is the car a hatchback ?

( Like this : http://farm3.static.flickr.com/2019/...091d5502_o.jpg

wow....:)

You have big fantasy :)
Unfortunately, the glass is the rear cover and it is very dificult to add any material with required resistance.

Continues, I like people ingenious :)

Cannot see anything bad about the back glass, considering side winds and all the different conditions.

May raise the rear suspension 2 cm. at a time to see if any change.

The big challenge to is how to handle the air coming out from underneath the back.

The slope rules apply under there also.

Do not want to angle that belly pan up too fast.

Tigra backlight
 

Beautiful car! As the windshield is relatively flat,I suspect air is being directed over the roof in such a way,that it's velocity,as compared to that at the side of the car,is much higher,and leads to very powerful C-pillar vortices,common in fast-back style cars.And do the the upward rake of the rear quarter panel,I suspect the air is colliding together much as depicted in the graphic.On my CRX,I created a hatch wing that integrated the C-pillars,and provided increasing tumblehome,and increasing lateral radius as it extended towards the tail.Extending into the boattail,this added structure provided a mixing zone,where the roof and side airstreams could meet at nearer the same velocity,virtually eliminating the vortices.As seen from the rear,above the beltline of the car,the outline of the car is semicircular in cross-section.The slope of your Tigra's backlight seems a little "fast",at 15-degrees,and your "bubble"is evidence of minor flow separation,which is no big deal.I can't think of a fix that does not entail lengthening the car,which affects your tailights,and also raises the spectre of creating a blindspot in your rear field of vision,something I've had to live with, with the CRX.Lessening the backlight angle may compromise vision due to diffraction.Extending the rear spoiler straight back to a point where it's upper edge intersects the roofline at 12-degrees,instead of 15,while perhaps introducing a locked vortex above,may allow the flow above to exit at a slower velocity.By not "narrowing" the spoiler,the edges may help to "burst" the powerful C-pillar vortices,and allow easier mixing of the air.Without flow imaging,its a tough nut to crack.

Is a 15* slope on the rear window ideal, or is it the maximum? I've heard it both ways.

I am not sure to understand.
For you, the black glass is ok in standard car?

Move up the rear suspensione isnt' possible: I dont' whont any change, because, I love tha actual handling's car and all my modifications are to zero cost ot lowers :)

for the rests, yuo refer to the under floe car? rear extractor?
I do not have more action margin to improvement, but if I replace actual extractor with a new slim extractor, I have space for changing the rear slope.

The extractor slope, it is another interesting argument that I will face with you in future.

Interesting observations.
I would want to see some pictures of your cars and the hatch wing.
If I have correctly understand, your work it is similar or equal to my attempt.
About the ideal slope, reading the licterature, I know, that the ideal it is approximately at 15° not 12°.
For this I think that the rear dec lidt lengh do not require modification.
Mybe, the licterature studies isnt' based on my car, and the c-pilar vortices are differents.
In my car, the c-pilar flow is merged with the central car flow (excluded bubble area).
I think, that the tigra's C-pilar vortices are smaller, but the more external side flow from flanks (see image) runnig over the decklidt (remeber, this one is oriented up) creating vortices

http://img369.imageshack.us/img369/7...aleflowrc3.jpg

http://img505.imageshack.us/img505/1379/vorticiqm3.jpg