Relationship Counseling: Keeping your boundary layer close

[Driver]

I think everyone here is familiar with the Gurney Flap, but perhaps fewer know the means by which it works.

Essentially, it helps to prevent flow separation from the underside of your wing, improving its efficiency (Laminar flow, being the buzz word).

One of the obvious drawbacks of using this device is the addition of drag. It is, after all, a vertical speed bump jutting up into the high velocity boundry layer. Sure, the downforce benefit outweighs the increase in drag, but it's still there.

 

37 years after it's invention, we still see it everywhere in motorsport. It's something that works and hasn't needed to be changed. Afterall, it's a brilliantly simple addition with great performance gains. But one would think that, over the years, some one has tried a better mouse trap. Especially with the money being spent in Formula One.

 

This is, in fact, the case in aircraft. Gurney actually signed patent rights over to McDonnel Douglass, and they saw use in aircraft for a number of years, in high-lift / low-stall applications. Unlike the racing world, though, the aircraft industry has not remained so stagnant. New devices exist these days to improve laminar flow over foils, and we're seeing them on aircraft... but not racecars.

I take that back, because there is a street car out there that employs such a device, but not to its wing.

The vortex generator is a means to draw potentially separated airflow back toward the surface of the rear glass. It pokes up into the boundary layer and creats spinning vortices that cling more to the surfaces that they pass over.

But wait, there's more. Examining the subject more closely, you'll discover that vortex generators existed on cars before they were called vortex generators.

The knee-jerk reaction to the canard is to call it a front wing, a means of providing downforce. They do this, true, but they do more. They also creat large vortices that travel the length of the car. They prevent flow separation at the nose of the car, where the boundary layer must make that sharp turn to travel down the side.

 

Where did the Votex Generator (VG) theorem (as it pertains to aerodynamics) get it's start? Why did I start this topic with the Gurney flap?

Wings

Fixed wing aircraft have moved on from the Gurney Flap. The VG's seen here are the most basic form:

Though more elaborate versions exist:

 

These devices have the same aim as the gurney. In the case of aircraft, it is to increase lift and reduce stall speed, as well as to keep the boundary layer close to the control surfaces.

As we know, a racing foil is an inverted aerofoil.

Has anyone seen application of VGs on racing cars, be it in testing or racing? I can't find any mention of it, and I'm wondering if they simply haven't felt the need to look beyond the gurney, or these have been tested and found to be lacking. They certainly aren't new, the concepts is likely just as old as the Gurney.

[mpowers]

I think everyone here is familiar with the Gurney Flap, but perhaps fewer know the means by which it works.

Essentially, it helps to prevent flow separation from the underside of your wing, improving its efficiency (Laminar flow, being the buzz word).

One of the obvious drawbacks of using this device is the addition of drag. It is, after all, a vertical speed bump jutting up into the high velocity boundry layer. Sure, the downforce benefit outweighs the increase in drag, but it's still there.

 

37 years after it's invention, we still see it everywhere in motorsport. It's something that works and hasn't needed to be changed. Afterall, it's a brilliantly simple addition with great performance gains. But one would think that, over the years, some one has tried a better mouse trap. Especially with the money being spent in Formula One.

 

This is, in fact, the case in aircraft. Gurney actually signed patent rights over to McDonnel Douglass, and they saw use in aircraft for a number of years, in high-lift / low-stall applications. Unlike the racing world, though, the aircraft industry has not remained so stagnant. New devices exist these days to improve laminar flow over foils, and we're seeing them on aircraft... but not racecars.

I take that back, because there is a street car out there that employs such a device, but not to its wing.

The vortex generator is a means to draw potentially separated airflow back toward the surface of the rear glass. It pokes up into the boundary layer and creats spinning vortices that cling more to the surfaces that they pass over.

But wait, there's more. Examining the subject more closely, you'll discover that vortex generators existed on cars before they were called vortex generators.

The knee-jerk reaction to the canard is to call it a front wing, a means of providing downforce. They do this, true, but they do more. They also creat large vortices that travel the length of the car. They prevent flow separation at the nose of the car, where the boundary layer must make that sharp turn to travel down the side.

 

Where did the Votex Generator (VG) theorem (as it pertains to aerodynamics) get it's start? Why did I start this topic with the Gurney flap?

Wings

Fixed wing aircraft have moved on from the Gurney Flap. The VG's seen here are the most basic form:

Though more elaborate versions exist:

 

These devices have the same aim as the gurney. In the case of aircraft, it is to increase lift and reduce stall speed, as well as to keep the boundary layer close to the control surfaces.

As we know, a racing foil is an inverted aerofoil.

Has anyone seen application of VGs on racing cars, be it in testing or racing? I can't find any mention of it, and I'm wondering if they simply haven't felt the need to look beyond the gurney, or these have been tested and found to be lacking. They certainly aren't new, the concepts is likely just as old as the Gurney.

 

 

Thanks for the great explanation, they do have a points penalty in the pt rules for a vortex generator, so some one some where has used it to an advantage.

 

 

 

mpowers

[vpnwiz]

A rear diffuser looks a lot like a vortex generator, upside down. But the vortex generator has a way cooler name.

[Driver]

A rear diffuser looks a lot like a vortex generator, upside down. But the vortex generator has a way cooler name.

Actually, the rear diffuser generates downforce in much the same way that the Gurney effects the airflow over the bottom of the wing.

The sharp drop-off at the rear of your car creates the same asymetrical horizontal vortices that the gurney flap does. The low pressure "bubble" draws the boundary layer up like a vacuum cleaner.

The strakes of a rear diffuser serve as... well, they serve as strakes. lol

[Rook]

So, if I understand correctly, all cars produce downforce by nature of design if they have a close-to-90 degree rear end profile? How does one maximize this?

[Driver]

No, actually the cars proximity has more to do with it. The area behind your car only aids in downforce with the use of a diffuser: You have to give the air a smooth means of expanding while still unde the car. If you just had a flat bottom that ended with an abrupt angle at the rear of the car, you would see no particular benifit.

 

To maximize the functionality of a difuser, you make it bigger. You're takign a stream of air with X volume/velocity and stretching it out to X volume. the more to stretch it and slow it down,t he lower the pressure under the car, and the more downforce it creates. If there were not rules, you could cut into your trunkspace.... but that would be excesive.

 

Keep in mind also that downforce = drag, finding your happy medium is up to you. You could have gobs of downforce helping you stick in the tightest of turns, yet have so much drag that you never breach 80mph.

[Mike Simard]

Driver, do you have any thoughts on rear wings that are not flat but somewhat mimic a roofline with a raised middle area?

APR makes some wings like that and I trust that they wouldn't do that strictly for appearance.

Would it not be better to keep as much of the wing as high as possible and therefore straight? Would there be a case when you would want a portion of the wing to be lower than the maximum height allowed by rules?

[Driver]

Driver, do you have any thoughts on rear wings that are not flat but somewhat mimic a roofline with a raised middle area?

APR makes some wings like that and I trust that they wouldn't do that strictly for appearance.

Would it not be better to keep as much of the wing as high as possible and therefore straight? Would there be a case when you would want a portion of the wing to be lower than the maximum height allowed by rules?

You know, I've been racking my brain over that exact question for months (because I'm trying to avoid making my own foil). It's just damn-near impossible to find a good straight foil at a reasonable price.

 

To answer your question, I've got an idea, but I'm not sure. Keep in mind, the realm of aerodynamics is, as Dan Gurney himself once said, "Monkey see - Monkey do". Meaning, one person will put in the time to develop something that works great on their car, and everyone else will copy it as best they can... regardless of weather it works with their cars shape or not.

 

So, since we don't have the development resources to do anything but watch the pro's, we must choose carefully which pro's we watch. Not to sound crass, but it's pretty safe to ignore anyone who doesn't have a wind tunnel, as they tend to be the ones who copy (like me! ). Fortunately, there are a good many teams out there who do have wind tunnels, and they employ aerodynamasists to use them properly.

Them:

vs

Me:

 

I have been able to find a couple of examples of this in top-tier motorsport, the C5R/C6R and the DBR9.

 

Possible reason the bulge exists:

-Chevy needed a wing for their "fast-back" C5R vette that did not fully clear the boundary layer (or they thought it didn't) because it wasn't allow to be higher than the roof. To get consistent downforce across the wing, they bent it up where they thought the boundary layer air would be hitting it. This would reduce drag (and downforce) to livable levels. The C5's roof line doesn't drop much, so if you are restricted to wings that aren't higher than your roof line, you can't get it up into clean air. Plus, the Vette has a "pointed" rear window, focusing airflow at the mid-point of the wing. You'll notice the the Vettes bulge is more pronounced than the Astons because the Astons hind profile isn't as pointed.

 

At LeMans, this is an issue because you're going so much faster down Mulsanne that even low Cd cars like the C5R and DBR9 can actually see separation at their back window.

 

So, wanting to do as the champions do, many people latched onto that design and now it's all over the place. Your roof line determines your wing shape. If you don't have a roof line similar to the DBR or the Vettes, then their wing may not be for you. Search the net for cars like yours and see what they have done.

 

In your case, Mike:

Your 928, from the side, has a similar issue with the roof line.... but unlike the vette, you're back glass is a big flat sheet. You're going to get cleaner and more uniform flow over the back of your car, so a flat-foil win is probably the best bet for you. Also, the roof line of your car drops more, meaning that a wing can get cleaner air by being mounted level with the roof line.

 

Me, I have a very short search as well.... because no one with a wind tunnel races notch-backs. lol

Those that did, however, seemed to do well.

[Mike Simard]

Driver, you need a subscription to Racecar Engineering. They often discuss aero testing and stuff that you're into.

For a notchback , might a stock type lip spoiler might help an elevated wing to work better? I know I've read of testing of just that with positive results on some late model Japenese car but have no specifics.

[Driver]

They do that in DTM (on of the best leagues to watch for Aero Nerds).

 

Lips are most effective on cars with good laminar flow over the rear, which isn't me. lol

You would probably make good use of that, and not pick up as much drag as a car with a square butt.

 

I tend to reference the PT rules when considering mods. The car may see TT use, probably not PT.... but, it'd be nice to comply if I decide to go that route, or sell the car.

 

I'd love a subscription to RE... but that $160 usually needs to go elsewhere.

[FlyingDog]

Has anyone seen application of VGs on racing cars, be it in testing or racing? I can't find any mention of it, and I'm wondering if they simply haven't felt the need to look beyond the gurney, or these have been tested and found to be lacking. They certainly aren't new, the concepts is likely just as old as the Gurney.

Some of the new F1 cars have various vortex generating components. The BMWs look like like and aero lab exploded on them...