Swirling vortices from vortex generators?

[PaleMelanesian]

Try this for an idea:
http://ecomodder.com/forum/showthrea...-6-a-6069.html

[aerohead]

Originally Posted by Hermie

According to a French study (Peugeot/Citroen), the large counter-rotating vortices I'm getting are really reducing rear lift while also slightly reducing drag.

Their tests show that vortex generators are MUCH more effective at reducing lift than drag, and the large counter-rotating vortices I'm seeing on my rear window means that the VG's are doing their job quite well:

"It appears that the largest drag and lift reduction is clearly associated to a strong increase of the size of the recirculation bubble over the rear slant."

Here's the paper:
SpringerLink Home - Main

This is exactly what I want- Reducing lift, with drag reduction as an added bonus.

By moving the separation point rearward,the base pressure is increased in the wake when it does occur,reducing both drag and lift as a consequence.
Circulation of any form is robbing your gas tank,as this kinetic energy cannot be converted to static pressure which leads to lower base pressure and higher form drag.
The VG is a palliative for inherently inefficient designs.
The Kamm-back offers no horizontal surface in the aft-body where lift can occur,as it moves all the 'lift' directly behind the vehicle,eliminating any vertical component.Also,the longer the tail,the smaller the wake,higher the pressure,and lower the drag.And no lift.
If you'd like to see a perfect low-drag,low-lift roofline,GOOGLE an image of the Ford Mk IV race car of the mid-1960s.It has a textbook Kamm body and track-proven stability.

[cfg83]

aerohead -

Is this it? :



(NSR slot car version)


CarloSW2

[AeroModder]

Actually, kammbacks do create lift from the low-pressure zone above them created by the downward angle. The air fills up the gap generated as it moves through the air, causing the molecules to spread out more. If you take a look at the Honda Insight's wind-tunnel smoke testing, you'll see the gap widen between the surface and the smoke trail as you come to the rear edge, indacative of lowering pressure. Though if you added a kammback to, say, a 3 box sedan (fastback), you'd most likely see a reduction of rear lift, but it won't be eliminated.

The rear on that race car pictured would be more of a fastback, and I'm sure that ducktail spoiler adds quite a bit of drag with it's downforce.

[aerohead]

Originally Posted by cfg83

aerohead -

Is this it? :



(NSR slot car version)


CarloSW2

Carlo,the number-9 car is it! I'm not sure what the upper one is maybe Alfa Romeo,Ferrari,perhaps a Chaparral.
With the #9 car,if you can imagine narrower tires and the body sides narrowing along with the roofline,that's Kamm's form.This car also chops the body at the 50% mark as Kamm advocated.It's one of only a hand full of cars to do it ( Ferrari F 40 and Bugatti EB 110 use it ).
PS the Ferrari F335 Berlinetta comes very close with it's rear spoiler.

[aerohead]

Originally Posted by AeroModder

Actually, kammbacks do create lift from the low-pressure zone above them created by the downward angle. The air fills up the gap generated as it moves through the air, causing the molecules to spread out more. If you take a look at the Honda Insight's wind-tunnel smoke testing, you'll see the gap widen between the surface and the smoke trail as you come to the rear edge, indacative of lowering pressure. Though if you added a kammback to, say, a 3 box sedan (fastback), you'd most likely see a reduction of rear lift, but it won't be eliminated.

The rear on that race car pictured would be more of a fastback, and I'm sure that ducktail spoiler adds quite a bit of drag with it's downforce.

AeroModder,I'm going to have to respectfully disagree with respect to your assessment of the Kamm-back.
It's true that the eruption of the roof and accelerated flow over the windshield causes lift,although that's something we've got to live with if we're going to have a greenhouse on vehicles.
With respect to the roofline,the smoke trails spreading apart is indicative of the streamline flow deformation caused by the passage of the car,rebounding to it's undisturbed orientation,with a deceleration in the flow and consequent pressure rise.
With the Insight,the curvature of the roofline,in my opinion,is too aggressive,and what you are witnessing is flow separation,as the streamline is unable to follow that contour.
I agree that there will be lift over the hatch on the Insight,as the pressure above the hatch is that of at the point of separation.With a 'proper' Kammback,separation is impossible( that's it's premise) over the hatch,and is moved to the 'back' of the car where vertical lift cannot occur.
If your 3-box car has good flow up to the attachment point of the Kamm-back,aside from the lift ocurring at the windshield,it is not possible for the Kamm-back to add lift,only reduce it or eliminate it,as the pressure is acting on a vertical wall.
The Ford Mk IV has fully attached flow all the way to the 'end' of the car,and since the rear is cantilevered,the base pressure will actually produce downforce due to the torque acting between top and bottom of car.
Please GOOGLE an image of the 1994 Bugatti EB 110.This is a $350,000,207-mph supercar with perfect Kamm-back.And feel free to compare it to the Aerodynamic Streamlining Template.

[AeroModder]

I see. So the "Kammbacks" used in mass-produced vehicles today fall short of the ideal angle. I was thinking more along the lines of what is out there than the idealized version. My mistake.

[cfg83]

Hello -

More Ford Mark IV pix :

Sports Car Silhouettes






Slot car pix are from here :

Espectacular Nsr Ford Mk.IV gt 40 Gulf | Noticias de scalextric de todoslot.es

CarloSW2

[Patrick]

Why does the Mk IV have the spoiler on the rear edge (it seems rather large on the sides)?

[cfg83]

Patrick -

Originally Posted by Patrick

Why does the Mk IV have the spoiler on the rear edge (it seems rather large on the sides)?

Do you mean along the side curving up? Maybe for downforce?

CarloSW2