Vortex Generator for Diffuser
 

What exactly does this peice do?
Top Secret sells this.

It has to do with controlling air flow as it passes the sides and rear of the car.

The generators create a "Vortex" I would assume which is a wind funnel or swirl behind the car as it moves at high speeds.

r-34 skylines have this, and i belive the enzo also uses this. it acts just like a wing but underneath your car rather than using a wing, but use it with a wing and you have doube the effect i guess because its like you have two wings.

OK, here's the theory:

Lift (or downforce) works because of low pressure on the suction side of the wing (or in this case diffuser). The basic way this low pressure is created is by the wing pulling away from the airflow. Consider a diffuser -- it pulls away from the ground, expanding the airspace. The same amount of air in more space equals lower pressure.

However, the air will only follow the surface of the wing so far. When the energy in the boundary layer gets low enough, the air will separate from the wing. This drastically reduces lift and drastically increase drag. On an airplane, we say that the wing has stalled.

A vortex generator adds energy to the bounfary layer. But this energy does not come for free -- it costs in drag.

So IF the wing is stalling, and IF the vortex generator is placed correctly, then SOMETIMES it can keep the wing from stalling and decrease drag (or otherwise improve the airflow).

Now for the specific case you are asking about:

I have no idea if the diffuser works at all, I have no idea if it has separation problems, and I have no idea whether a vortex generator would help it.

wow, a simple plain language explaination of aerodynamics... awesome

So, Mike,

I don't know your expertise but you sound fairly knowledgeable. So let me run this by you.

In an effort to increase traction at speed, and pay for it with as little drag as possible, I am going to start experimenting with "ground effects" wings. I imagine them being maybe 16" long out each side of the car with maybe a 10" chord length.

Location would be immediately ahead of the rear wheels and about 1.5" to 2.5" from the ground. Don't forget we are talking about an S-2000 powered Land Speed Record car, here, not a stock bodied S-2000. The bottom of the car is about 2.5" from the ground, and currently, judging from the dust on the car, there is a lot of flow separation along the sides of the car about 4" to 5" up from the bottom edge.

So these wings might end up being located right where the boundary layer is dumping itself out to, and may not work. But who knows? All we can do it try it.

Your thoughts, please,

Jim Knapp

Well that answers that. Thanks guys!

FYI, I'm an aeronautical engineer. As in airplanes, not cars.

16" is fairly short, especially with a 10" chord length. However, I'm fairly sure that if it is outboard of the car by 16" you should be getting some clean air. Definitely will be in the front, and probably in the rear.

Remember, if you are getting any serious amount of lift off of these wings you will have to carry those loads back into the car. If you don't have some pretty serious structure involved, then either the wings won't be doing much or they will break off.

There is no such thing as lift (or negative lift aka. downforce) without drag. For a straight line speed record attempt, I would think you should be shooting for just a tad bit of downforce. Basically just enough to be sure you are not getting a dangerous amount of lift. Any more downforce than that is useless, unless you have insufficient friction for the tires.

I would think the dust is actually coming from the tires. It may be dust pulled up by low pressure under the car and spilled out the sides, I suppose. However, ideally you want the car as low as possible and you want as little air as possible to pass under the car. But of course you don't want the car to bottom out. As close as possible to bottoming out, though.

You could try some flow visualization techniques, like tufts or oil streaks. Tufts require access to high speed video of the surface with the tufts. Oil streaks will be very messy but might help, depending on what your question turns out to be.

the vortex generators are those spiral carbon fiber pieces that go on the sides...they disperse the air flow and create down force...

im also looking into the top secret diffusor, and im probably ordering mine today...its a nice mod for me since i want to stay wingless...

has anyone got any pics of this vortex generator?