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Vortex Generator for Performance Cars

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Sep26-11, 06:04 AM
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Hi Physics Guys,

I'm not a physic guy myself but some google searching lead me to this site so I signed up so I could ask a question.

I'm into amateur motor sport and I'm currently driving a 1999 Subaru WRX. Anyway I'm always looking out for anything that will increase it's performance.

However, I've been around cars long enough to know that there are plenty of gimmicks out there for those who are easily parted from their money.

Recently soeone locally has started advertising these vortex generators:

I googled Vortex Generators and did a bit of reading but it was all about their use on planes.

My question is this: Is a vortex generator a worthwhile upgrade for a car? Or is it just a gimmick or something you would only consider putting on a pro-level racing machine? Thanks

Also if it does work on a car. How?
I don't know a lot about aerodynamics. I have a basic idea of how a spoiler works and that's about it.

Thanks in advance.
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Sep26-11, 08:58 AM
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Did a web search and found this article:

Quote Quote by kiwiscott View Post
I googled vortex generators and did a bit of reading but it was all about their use on planes.
Most powered planes fly at faster speeds than cars, but gliders typically glide at 70 mph or less. However gliders are extremely low drag devices compared to cars, with thin boundary layers, and they use "turbulators" instead of vortex generators. The idea is to trip laminar flow in the boundary into turbulent flow at a controlled point in a cambered surface since turbulent flow will follow cambered surfaces a bit better. The goal is to trade off the initial increase in drag at the trip point for a thinner boundary layer aft of the trip point to reduce the effective profile drag. In the case of gliders, just roughing up surfaces with 600 grit sandpaper may be enough, or sometimes thin "turbulator" tape is used. The finned vortex generators generally aren't used on gliders, only on faster moving and heavier aircraft. The article above mentions that the vortex "bumps" need to be relatively small, about the same size at the boundary layer on a car, which was 3 cm (a bit over 1 inch) for the car they were testing.
Sep27-11, 02:28 AM
P: 1,008
Just looking at that, I doubt it will do anything significant. Vortex generators, even the kind used on large aircraft, tend to be much smaller and more closely spaced than that to be effective, and I'm not sure they would even fulfill their purpose on most cars (which is to delay flow separation). Where is that supposed to be mounted, and what specific claims are being made about it by the manufacturer?

Sep27-11, 04:38 AM
P: 4
Vortex Generator for Performance Cars

Thanks for the response guys.

rcgldr: Thank you for the links. I read both of them and found the first one very useful and the second one quite interesting.

cjl: The generator is intended to be attached at the back of the roof above the rear windscreen. The same as can be seen on late model mitsi evos:

These are the manufacturers claims about the aftermarket one for the WRX:

"Our carbon look vortex generator helps direct air flow over the rear of your car to improve down forces and keep your REX more stable at high speeds."

Are there any introductory books or websites you guys would recommend about aerodynamics for someone like myself who does not know a lot about physics but is keen to learn?

Thanks again.

P.S: After a bit more searching I found this article from Mitsubishi that some of you might find interesting.
Sep27-11, 09:17 AM
P: 1,008

At that location, they might actually make a difference. If designed properly (which is by no means a guarantee), they would improve the flow characteristics down the rear windshield, assuming there is some flow separation there. That article supports this theory, although the change in Cd given in the article is pretty minimal (you'd probably never notice, unless you frequently do precisely-measured top speed runs).
Sep28-11, 06:02 AM
P: 4
Thanks cjl. After reading the article from Mitsubishi last night I figured the knock off ones would not be worth it as they wouldn't have been tested and developed the same way as Mitsubishi developed theirs for the Evo VIII.

Thanks to the both of you for your help and advice. I'm quite keen to learn more about aerodynamics know after reading that article from the Mitsibushi engineers.
Sep28-11, 08:09 AM
P: 22,234
Actually, don't they have the logic backwards? The placement on a car is similar to the placement on a wing, so they would do the same thing: keeping the flow attached will delay/reduce flow separation, decreasing drag and INCREASING lift. A street car, of course, doesn't have aerodynamic downforce - it is shaped like an upright wing. For actual downforce, you need ground effects or an actual upside-down
wing (not a spoiler).

Do nascar cars have them...?
Sep28-11, 10:07 AM
P: 4
It is only the manufacturer of the cheap knock-off that is claiming an increase in downforce. Mitsubishi are only claiming a decrease in drag, as I understood it.

I know that the spoilers on most street cars don't produce true downforce but many of the high quality after market racing wings do actually follow the correct principles. The limited edition 22B WRX road car came with the same adjustable wing as the WRC cars so I imagine that also constituted an upside down wing design.

NASCAR is not big in my part of the world but from a quick google image search it does not appear that they use VGs.
Sep28-11, 11:37 AM
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NASCAR provides body templates for each racing team to conform to as they build and mount their bodies. You won't find extraneous stuff such as vortex generators or spoilers being bolted onto NASCAR cars because then the body would no longer match the body templates and would be disqualified from competition.

Not to say such added gear wouldn't work, at least to some extent - it's just against the rules to make such modifications.
Ranger Mike
Sep28-11, 11:44 AM
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a while back we did research this. We found empirical data supporting the little vortex globulations KIWISCOT has,,good show!!

Personally with your door slammer i think there are more areas that will give you way more competitive edge before using hard to come to dollars for this. Don't get me wrong..great racing idea and these do work but lowering the car and channeling the air on top and underneath will help the aero big may want to read my delusional post in the Mech. Eng. forum..Race car Suspension class
Sep28-11, 11:52 AM
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Quote Quote by russ_watters View Post
Actually, don't they have the logic backwards? The placement on a car is similar to the placement on a wing, so they would do the same thing: keeping the flow attached will delay/reduce flow separation, decreasing drag and increasing lift.
That would depend on the car. If it's a hatchback, then there should be some increase in lift. If that downwards flow is directed towards a horizontal surface, like the trunk of a sedan or coupe, then the net vertical force may cancel out. If the trunk has a spoiler or wing, and there's a net upwash of flow, then there should be a bit of downforce.

Quote Quote by russ_watters View Post
decreasing drag
Drag is increased at the location of the vortex generators. Drag may be decreased overall if the resultant turbulent flow reduces the effective profile drag more than the vortex generators increase the drag, by causing the boundary layer (now turbulent) flow to remain attached and thinner than it would otherwise be without the vortex generators.

Quote Quote by russ_watters View Post
A street car, of course, doesn't have aerodynamic downforce - it is shaped like an upright wing. For actual downforce, you need ground effects or an actual upside-down wing (not a spoiler).
Spoilers can produce downforce.

Quote Quote by russ_watters View Post
Do nascar cars have them...?
From what I understand, the wings used on the "cars of tomorrow" are gone, making those the "cars of yesterday". Nascar is back to using spoilers at the rear of the car. Over time, there have been various methods for Nascar race cars to produce downforce.

Air dams below front bumper - these prevented the air from getting directly under the car. The forwards acceleration of air aft of the air dam resulted in reduced pressure at the front half of the cars, while a spoiler produced downforce at the rear of the cars. The allowed height of the spoiler was different depending on the body style, but over time, the body styles ended up using almost identical templates. The air dams were replaced with spiltters that were already in use on other classes of race cars.

Splitters - a forwards protuding plate at the bottom of front bumper - These almost scrape the ground. In addition to preventing air flow from getting under the car, the pressure differential on the forward protuding surface provides additional downforce on the front end of the car.

Spoilers - (in reference to cars, not aircraft) - an upwards angled protuding plate at the rear of a car. With sufficient height and angle, these will produce downforce.

Diffusers - The lower surface of the tail end of the car is angled upwards towards the rear, providing a low pressure area that ends up drawing air from under the car, which reduces pressure under a car.

Ground effects on under body - These may include tunnel type shapes. Indy Racing League cars use this. Formula 1 bans it and requires "skid boards" used to make sure car height isn't too low (the cars wear down the skid board during a race if too low).

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