Vortex Generators

[turbof1]

I realise this is an F1 forum but I am primarily talking about road cars.

I would imagine there is a large amount of flow separation at the back of most road cars as the air goes over the roof and doesn't stay attached as it suddenly drops into the rear window. As seen here:

http://farm4.static.flickr.com/3096/249 ... a86e5c.jpg

So why don't more cars have generators such as these?

http://www.recumbents.com/car_aerodynamics/diffuser.jpg

Ah, you are speaking in the sence of using vortices to clean up the boundary layer, on road cars.

I think other factors are involved here. Primary the speed: aerodynamics don't play a huge role in city traffic. It does a bit on a highway. It's a cost and benefit analysis: most manufacturers don't bother because it costs too much for the benefits it brings.

[olefud]

I realise this is an F1 forum but I am primarily talking about road cars.

I would imagine there is a large amount of flow separation at the back of most road cars as the air goes over the roof and doesn't stay attached as it suddenly drops into the rear window. As seen here:

http://farm4.static.flickr.com/3096/249 ... a86e5c.jpg

So why don't more cars have generators such as these?

http://www.recumbents.com/car_aerodynamics/diffuser.jpg

It’s easier to use the Kamm spoiler air dam on the trunk. This builds high pressure air over the trunk so the slipstream flows relatively attached and turbulence free. Then the spoiler detaches the slip stream from the vehicle for minimum turbulence and drag.

http://en.wikipedia.org/wiki/Kammback

[ACJJ619]

I realise this is an F1 forum but I am primarily talking about road cars.

I would imagine there is a large amount of flow separation at the back of most road cars as the air goes over the roof and doesn't stay attached as it suddenly drops into the rear window. As seen here:

http://farm4.static.flickr.com/3096/249 ... a86e5c.jpg

So why don't more cars have generators such as these?

http://www.recumbents.com/car_aerodynamics/diffuser.jpg

Ah, you are speaking in the sence of using vortices to clean up the boundary layer, on road cars.

I think other factors are involved here. Primary the speed: aerodynamics don't play a huge role in city traffic. It does a bit on a highway. It's a cost and benefit analysis: most manufacturers don't bother because it costs too much for the benefits it brings.

Ah okay, thanks for that. Having said that - companies will sell you stick-on vortex generators very cheaply. It can't cost much for a manufacturer to just stick some on the roof can it?

I realise this is an F1 forum but I am primarily talking about road cars.

I would imagine there is a large amount of flow separation at the back of most road cars as the air goes over the roof and doesn't stay attached as it suddenly drops into the rear window. As seen here:

http://farm4.static.flickr.com/3096/249 ... a86e5c.jpg

So why don't more cars have generators such as these?

http://www.recumbents.com/car_aerodynamics/diffuser.jpg

It’s easier to use the Kamm spoiler air dam on the trunk. This builds high pressure air over the trunk so the slipstream flows relatively attached and turbulence free. Then the spoiler detaches the slip stream from the vehicle for minimum turbulence and drag.

http://en.wikipedia.org/wiki/Kammback

Interesting. Surely it still leaves a large area of low pressure behind the car though? I've found a diagram and added some pink shading in the section I'm talking about:

[turbof1]

Ah okay, thanks for that. Having said that - companies will sell you stick-on vortex generators very cheaply. It can't cost much for a manufacturer to just stick some on the roof can it?

Yes but who would buy it? There isn't really a market for this, and you still need to factor in startup costs like seting up proper manufactury.

[ACJJ619]

Ah okay, thanks for that. Having said that - companies will sell you stick-on vortex generators very cheaply. It can't cost much for a manufacturer to just stick some on the roof can it?

Yes but who would buy it? There isn't really a market for this, and you still need to factor in startup costs like seting up proper manufactury.

You wouldn't advertise the vortex generators themselves so to speak, but the MPG figure on the brochure would be higher surely?

[thepowerofnone]

More food for thought for you on this topic ACJJ619, most of these aerodynamic features add weight, and since the drag improvements are extremely small at the sort of speeds common for road cars, does the aerodynamic gain outweigh the additional weight?

The truth is, whether you're talking about downforce or drag, road cars are pretty terrible aerodynamically. Even cutting edge supercars only generate hundreds of kilos of downforce at outrageous speeds of 180mph, which is poor by most standards. You probably know of the new super efficient Volkswagen (here if you don't http://www.volkswagen.co.uk/about-us/futures/xl1) which obtains by far the best mpg ratio of any commercial combustion vehicle, and even that isn't a particularly aerodynamic shape. The practicalities of cars and their safety regulations are a major restriction to drag performance, and additionally there are plenty of areas like engine efficiency where much larger gains are realistic.

Why not include devices like those vortex generators as standard? Most likely answer to me is because some accountant decided that they were sufficiently ugly as to dissuade more buyers put off by the aesthetics than would be attracted by the 0.1 mpg improvement (or whatever it is) they offer.

[ACJJ619]

More food for thought for you on this topic ACJJ619, most of these aerodynamic features add weight, and since the drag improvements are extremely small at the sort of speeds common for road cars, does the aerodynamic gain outweigh the additional weight?

The truth is, whether you're talking about downforce or drag, road cars are pretty terrible aerodynamically. Even cutting edge supercars only generate hundreds of kilos of downforce at outrageous speeds of 180mph, which is poor by most standards. You probably know of the new super efficient Volkswagen (here if you don't http://www.volkswagen.co.uk/about-us/futures/xl1) which obtains by far the best mpg ratio of any commercial combustion vehicle, and even that isn't a particularly aerodynamic shape. The practicalities of cars and their safety regulations are a major restriction to drag performance, and additionally there are plenty of areas like engine efficiency where much larger gains are realistic.

Why not include devices like those vortex generators as standard? Most likely answer to me is because some accountant decided that they were sufficiently ugly as to dissuade more buyers put off by the aesthetics than would be attracted by the 0.1 mpg improvement (or whatever it is) they offer.

That clears things up. Thanks!

[NoDivergence]

More food for thought for you on this topic ACJJ619, most of these aerodynamic features add weight, and since the drag improvements are extremely small at the sort of speeds common for road cars, does the aerodynamic gain outweigh the additional weight?

The truth is, whether you're talking about downforce or drag, road cars are pretty terrible aerodynamically. Even cutting edge supercars only generate hundreds of kilos of downforce at outrageous speeds of 180mph, which is poor by most standards. You probably know of the new super efficient Volkswagen (here if you don't http://www.volkswagen.co.uk/about-us/futures/xl1) which obtains by far the best mpg ratio of any commercial combustion vehicle, and even that isn't a particularly aerodynamic shape. The practicalities of cars and their safety regulations are a major restriction to drag performance, and additionally there are plenty of areas like engine efficiency where much larger gains are realistic.

Why not include devices like those vortex generators as standard? Most likely answer to me is because some accountant decided that they were sufficiently ugly as to dissuade more buyers put off by the aesthetics than would be attracted by the 0.1 mpg improvement (or whatever it is) they offer.

Not fully accurate in its entirety, although the tone is for the most part correct. Accountants dominate everything. For the most part, the Evo is quite special in that is has a very large slope for the roof to the trunk. This high pressure gradient directly leads to separation, and given that these Rally style cars typically have a spoiler or wing on the trunk, something needs to be done if they want that wing to be functional. The vortex generators were an elegant way to do that. For the most part, the vast majority of cars don't have such a huge boxy roof to trunk shape anymore. They are far more gently curved for the sake of aerodynamics. So VG's really aren't necessary. These cars also now use a trunk spoiler for the larger wake behind the car. But the Evo is quite special with regards to between the roof and rear window area

[thepowerofnone]

Not fully accurate in its entirety, although the tone is for the most part correct. Accountants dominate everything. For the most part, the Evo is quite special in that is has a very large slope for the roof to the trunk. This high pressure gradient directly leads to separation, and given that these Rally style cars typically have a spoiler or wing on the trunk, something needs to be done if they want that wing to be functional. The vortex generators were an elegant way to do that. For the most part, the vast majority of cars don't have such a huge boxy roof to trunk shape anymore. They are far more gently curved for the sake of aerodynamics. So VG's really aren't necessary. These cars also now use a trunk spoiler for the larger wake behind the car. But the Evo is quite special with regards to between the roof and rear window area

That's very fair, although my response was more about vehicles in general and drag reduction devices in general than the specific case of the Evo running VGs - certainly its fair to say virtually all vehicles leave significant drag reductions aside for the sake of one restriction or another. e.g front bonnet height for EU pedestrian impact regulations is a huge cause of vehicle drag, and certainly many vehicles sacrifice aerodynamics for aesthetics. All valid comments by you however in that specific case.