Swirling vortices from vortex generators?
 

I ordered a set of the delta-shaped "Vortekz" generators thinking that it would smooth airflow on my back end, but after some tuft testing, it showed something I wasn't expecting at all.

I don't have a camera for pictures, but airflow was attached down the middle of the window all the way to the base, where the airflow apparently strikes the trunk surface fairly hard and causes it to move towards the sides of the trunk. Behind that, airflow looks smooth rearward.

On the sides of the window, I noticed rotational movement of the tufts giving the impression of airplane-style swirls off of the tail end. From standing behind the car and looking at the passenger's side of the rear window, the air rotates in a counter-clockwise half-circle on the glass, and it looks like it'll make a full swirl off of the car.

So, has anyone else observed this, or know if that's a detriment to drag?

A pair of counter-rotating vortices are present at the rear of many vehicles. They were probably there before you added the vortex generators.

And yes, they're a "detriment to drag".

The way to avoid them is through shape optimization. Ideally you'd have a Kamm shape to your car (ie. extend & taper the greenhouse to the end of the vehicle) to get rid of them.

Actually, they weren't there before I added the vortex generators. Before I added them, tufts were striaght along the sides and "bubbled" in the middle of the glass to the middle of the trunklid where the flow detached.

I'm wondering if I may have put the VG's too far forward.

The VG's may have strengthened the vortices, but I'm betting they were more than likely there before.

Did they help your mpg?

ollie

Ollie: Not sure yet. I haven't had them on for long. I also got an alignment and new tie rod last week, so I won't be able to tell the difference in the MPG increase.

I also have a higher-flow muffler that I'll be installing this weekend.

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.

mitsubishi's work backs up your account of increased down force on the the trunk. i'm dying to try this on my blazer which has no trunk to be forced down upon. where did you get your vortekz?

Welcome to the site bgfshrcr. VGs have been covered many times over on this site. Time and time again they fail to produce any noticeable gain in mileage.

I hear you Doax. Thanks for the welcome. Has anyone ever tried them on an SUV with no trunk or is there another possible such fix like a rear spoiler?

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

want
 

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.

aerohead -

Is this it? :

http://www.sportscars.tv/cars13/lola%20t70%20mkIII.jpg

(NSR slot car version)
http://noticias.todoslot.es/wp-conte...d-mk-iv-03.jpg

CarloSW2

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.

No.9
 

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.

Kamm-back
 

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.

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.

Hello -

More Ford Mark IV pix :

Sports Car Silhouettes
http://www.sportscars.tv/cars13/ford...%20mk%20iv.jpg

http://noticias.todoslot.es/wp-conte...d-mk-iv-02.jpg

http://noticias.todoslot.es/wp-conte...d-mk-iv-01.jpg

Slot car pix are from here :

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

CarloSW2

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

Patrick -

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

CarloSW2

Yes. There's also a smaller lip running across the rear top edge of the car.

Patrick -

I am guessing that it is for "tripping" the airflow where it separates from the car. If that's the reason, other aero-people can explain it better than me.

CarloSW2

out there
 

I suspect that automakers would prefer we look at 'styling',not engineering.They may get to engineering one day.Don't know.
The most current example of a perfect Kamm-back is the 2008 Mercedes-Benz Boxfish concept.With only 32% aft-body they achieve Cd 0.19.
By comparison,The 2nd-gen VW 1-Liter car,with more aggressive ( steeper ) roof-line and diffuser,can't even get to Cd 0.19,and that's with 45% aft-body.
The air really likes the Kamm-back and occasionally the wind tunnels get a nice example for which to remind us.

spoiler
 

Patrick,the 1965 car had no rear spoiler at all.My guess would be that they modified the car to race at shorter tracks,where overall speed would be reduced.By adding the spoiler they could get the cornering downforce they needed at the lower velocity.
You will see examples of this with Porsche and other marques.GM's Oldsmobile AEROTECH ran a short-tail and a long-tail car,depending on the track.

Hello -

I overlayed the tear-drop and it looks pretty darn good :

http://ecomodder.com/forum/member-cf...rd-mark-iv.jpg

CarloSW2

Now it looks like a Rolex Series car.

overlay
 

Carlo,I found a number of 'matches' for the Template.Some were 'dead on',others close:
1922 Jaray 'pumpkin seed' wind tunnel model
1933 Walter E.Lay 'pumpkin seed' wind tunnel model
1935 Kamm 'pumpkin seed' wind tunnel model
1957 MG EX 181 streamliner with reflex camber tail
1957 Grumman TF-1/C-1A TRACER/E-1B 'Willy Fudd' radome
1961 Mickey Thompson' 'Pumpkin Seed' streamliner
1963 Alfa Romeo TZ
1964 Ford GT 40
1967 Ferrari 330 P4
1967 Chevy Chaparral 2F
1970 Porsche 917 Lange-Heck( Long-tail )
1978 Daimler-Benz C 111 III full-tail with reflex camber Cd 0.178
1981 Volkswagen Blunt Body Cd 0.15
1981 VW Drop Shape Cd 0.16
1981 VW Flow Body Cd 0.15
1987 GM/AeroVironment Sunraycer Cd 0.089-0.12
1988 Lotus Esprit Turbo
1991 Ferrari F-40
1994 Saleen Mustang S-351
1994 Bugatti EB 110
1995 Ferrari F355 Berlinetta
1999 GM PNGV Precept Cd 0.163
2000 Honda Insight Cd 0.25
2006 AeroCivic Cd 0.17
2008 Daimer-Benz Boxfish concept Cd 0.19
2009 Toyota Prius Cd 0.26
2009 VW 2nd-gen 1-liter concept Cd 0.195
2009 Bugatti Galibier
2010 Porsche Panamera

It looks like,that when 'real' performance matters,the design team will lean towards the 'ideal' aerodynamic parameters.Curious!