Stepped shape for flow attachment.

[UltArc]

Mach-Speed 12013 Ford Mustang Coupe Aluminum Rear Window Louver - 2005-2013 - Black : Amazon.com : Automotive

Guys. I almost started. A new thread, but this one is pretty good compared to others, so I bring this up. The attached page, can I receive feedback? Please, any feedback at all. I am very interested and on a strict schedule, so I need to know if people think there will be a benefit. I believe so, but would like more feedback.

Thank you to all who help!

[ChazInMT]

No benefit you'll see in MPG gains I feel, remember we need to change the shape of the car in the back in order to realize gains, this adds 3 inches to the rear window on a car that has been wind tunnel optimized by Ford I'm sure. When I say I think it has been "optimized" I mean the shape of the rear cabin pillars, the rear glass, and trunk have been designed so it does not create large MPG sucking vortices. When you upset this balance, you run the risk of creating vortices due by having created unintended pressure differences.

So yeah, maybe you have better "Attached Flow" (which I feel is crap since attached flow is NOT the goal, it is merely a condition that needs to exist for optimum efficiency to be achieved. It's like saying if you get the spark on your engine to work, it'll run perfect, never mind the fuel & air thing.) but, if you create an undesirable vortex situation by having better attached flow, your modification will be a break even at best, or will have a detrimental effect on the MPG at worst. In the case of the louvers for the Mustang, my gut feeling is it looks to be a tossup. But I'm pretty certain it would not be a significant gain.

[Sven7]

Do a tuft test on the rear window (easy half hour, one man job). If it's attached, forget about the louvers.

[jack29]

Hello, don't know if I should start a new thread or not, seeing as this is exactly the subject that's been bugging me for a bit : louvers.
And specifically louvers on a '79 Scirocco that I've owned for 12 years now (now I use it once a month).
I find it a very interesting case study in aerodynamics for the simple fact that it's sister model ,the vw Golf, in spite of the fact that it has a steeper and taller windscreen it has a lower Cd 0.38 vs the Scirocco's 0.42
It's all down to that rear hatch angle , on the golf the air detaches at the roof, and the sciroccos hatch is basically a huge vortex generator

So the question is: would louvers that bring the hatch angle from 25-26(not sure about this) ,as it is now, to 15-20 make a difference in Cd?

I've attached 2 pics of example louvers that don't follow the hatch angle.

[jack29]

Also this an excerpt from a aero study that I think I found on this very forum,it talks about how the designers of the Golf went about the hatch angle.

"The fact that two completely different types of flow occur depending upon the inclination of the rear end, which lead to different drag, was observed for the first time during the development of VW Golf I. Janseen and Hucho varied the angle of inclination of the rear end <p (see Fig 5.14) in increments. The drag coefficient is shown versus the angle of
inclination <p in Fig 5.14.



Fig 5.14 Influence of rear end slope angle <p on drag coefficient, separation line and wake, measured on VW Golf I (Rabbit)

The development work in the wind tunnel began with a high angle of inclination at the rear end, <p = 45o. At this angle, flow separated at the end of the roof. The drag coefficient was
0.40. As the angle of inclination was diminished, the drag suddenly increased by 10 percent at <p = 30o. The line of separation jumped down to the lower edge of the inclined rear end. Two strong inward-rotating longitudinal vortices were observed which induced very low pressure on the slanted part of the back. As the angle was reduced further, the drag dropped again. At <p = 15o, a very flat angle, a drag minimum resulted. At still smaller angles the same flow forms were reached as with a square back: CD = 0.40. In the area 28o< <p <32o
a bistable condition was observed. Depending upon the curvature of the rear edge of the roof, separation occurred at the top or bottom of the inclined rear end."

[aerohead]

Originally Posted by jack29

Also this an excerpt from a aero study that I think I found on this very forum,it talks about how the designers of the Golf went about the hatch angle.

"The fact that two completely different types of flow occur depending upon the inclination of the rear end, which lead to different drag, was observed for the first time during the development of VW Golf I. Janseen and Hucho varied the angle of inclination of the rear end <p (see Fig 5.14) in increments. The drag coefficient is shown versus the angle of
inclination <p in Fig 5.14.



Fig 5.14 Influence of rear end slope angle <p on drag coefficient, separation line and wake, measured on VW Golf I (Rabbit)

The development work in the wind tunnel began with a high angle of inclination at the rear end, <p = 45o. At this angle, flow separated at the end of the roof. The drag coefficient was
0.40. As the angle of inclination was diminished, the drag suddenly increased by 10 percent at <p = 30o. The line of separation jumped down to the lower edge of the inclined rear end. Two strong inward-rotating longitudinal vortices were observed which induced very low pressure on the slanted part of the back. As the angle was reduced further, the drag dropped again. At <p = 15o, a very flat angle, a drag minimum resulted. At still smaller angles the same flow forms were reached as with a square back: CD = 0.40. In the area 28o< <p <32o
a bistable condition was observed. Depending upon the curvature of the rear edge of the roof, separation occurred at the top or bottom of the inclined rear end."

I took Hucho's data and drew the lowest drag roof slope,then superimposed the 'Aerodynamic Streamlining Template' on top of that and found a fairly good fit.
If you were doing the backward-facing step louvers you might push its contour up to the 'Template' and have confidence that it would come in near the drag mimimum.

[AndrzejM]

Originally Posted by jack29

And specifically louvers on a '79 Scirocco that I've owned for 12 years now (now I use it once a month).

Sciroccos are great cars, I've owned four of those MK1 from '78 and the second one from the same year, then MK2 GTi from 81, and GT from '82. Then I had a Corrado G60 '89 which I converted to diesel

Good luck with your car. Pity you barely can see them on the streets.

[kach22i]

Originally Posted by beer

Look at these race mirrors, implaying the same princible.

http://www.racemirrors.com/engineering.html

Several different topics in this single thread.

The original idea expressed is very exciting in that the step via vortexes increases the aerodynamics.

This is how all rear spoilers (Gurney Flaps and wickerbills) work, right?

[aerohead]

Originally Posted by kach22i

Several different topics in this single thread.

The original idea expressed is very exciting in that the step via vortexes increases the aerodynamics.

This is how all rear spoilers (Gurney Flaps and wickerbills) work, right?

As long as a spoiler is configured such that a portion of the flow will reattach to it,it is in effect capturing a vortex out ahead and the outer flow will skip over it relatively unmolested.
You do pay a penalty for all the air circulation,but lift and drag can be mitigated to some degree with a light,and economically low cost structure.And you can have good rear vision.
The light weight is an advantage with short track racing.If you want to set really fast land speed records,then the proper tail is the answer.

[Xist]

There kind of seems to be a debate on here whether solar panels are worth the additional weight on a car. I have seen some members do it and others say that it is a waste. This guy added them to his Prius and insists they help: Green Car Congress: Solar-Power-Augmented Prius Takes the Grid Out of Plug-in

On a house, you would want them angled to be at 90° with the sun, but on a car, I would think that you would want them to be flat, and one of the comments that I have read is that there is very little usable space on a car.

Well, if you need a flat area, wouldn't louvres provide that?

If I bought solar panels, I would put them on my house, and then park my car in the garage.