Top speed vs Coefficient of drag

[fatoni]

Originally Posted by old greg

There have been a few actually. Streamlining was very fashionable in the 30's so there were are a couple of prewar cars with very low coefficients of drag. The Tatra 77 (Cd=0.212) comes to mind, and more recently there was the GM EV1 (Cd=0.19). And if it ever makes it into production, there's the Aptera (Cd=0.15).

as much as i hate that newage kinda future stuff the aptera is pretty cool

[blur]

Originally Posted by old greg

Your understanding of undertray venturi tunnels is a little off. Firstly you need air to pass under the car, the more the better in fact (see the high noses on F1 cars, or the audi R15 for proof). So a splitter scraping the ground is counter productive (unless you are required to run a flat floor), you need the splitter to not choke the airflow going to the venturis. Following that, you need to have the undertray slope down towards the ground a bit to reduce the cross sectional area of the tunnel (but not so much as to choke the flow, that would create huge drag and lift). This forces the airflow to speed up relative to the airflow passing over the vehicle. Bernoulli's principle and dynamic pressure mean that you've just created a pressure differential. In a true ground effects car (ala F1 circa 1980) this is where the majority of the downforce is generated, and it's the reason why the FIA mandated flat floors for the whole length of the wheelbase. Contrary to popular belief, the diffuser doesn't create a negative pressure zone, it's simply there to slow the air back down and recover some of the energy (drag) that went into speeding it up in the first place. As a side effect of that process though, some of that high speed flow of air is pulled upward due to it's attachment to the surface of the diffuser, as a result of this the diffuser is pulled down (equal and opposite reactions) creating downforce. An interesting side effect of this is that the majority of the downforce generated in the diffuser is generated in the forward most portion. An airfoil mounted just behind and above the trailing edge of the diffuser (see F1 Beam Wing) will create a negative zone low and directly behind the vehicle which, while it creates drag, actually helps pull even more air through the venturis (by creating a higher front to rear pressure gradient) creating even more downforce. This is often reffered to as using a wing to "drive" the diffuser.

The reason why the sides should be sealed is to prevent air from escaping the nozzle (front) section and not speeding up like it should, and to prevent slow moving high pressure air from being pulled into the Venturi section. Sealing up the diffuser though isn't particularly critical. Air bleeding in from the sides of the car simply means that the diffuser needs (and can can have) a greater angle of attack.

Great explanation. Are there any examples of street cars that have taken advantage of ground effects?

Here's a diagram for anyone that was confused of the applied principle in the Lotus 79



edit: As I was reading up on wiki about ground effects, I came across this: "as of 2007, Formula One cars still generate a proportion of their overall downforce by this effect: vortices generated at the front of the car are used to seal the gap between the sidepods and the track and a small diffuser is permitted behind the rear wheel centerline to slow down the high speed underbody airflow to free flow conditions." The vortices they speak of, how do they work and how are they used?

[old greg]

Yes and no. Many exotics have functional undertrays, but they mostly create downforce in the diffuser section as I described earlier. The ride heights of street cars prevents the creation of low pressure regions under the car, high pressure air would simply rush right in.

[blur]

Originally Posted by old greg

Yes and no. Many exotics have functional undertrays, but they mostly create downforce in the diffuser section as I described earlier. The ride heights of street cars prevents the creation of low pressure regions under the car, high pressure air would simply rush right in.

About the diffuser, is bigger always better? The way I see it the more air you can displace the greater the downforce...

[fatoni]

Originally Posted by blur

About the diffuser, is bigger always better? The way I see it the more air you can displace the greater the downforce...

no. just think about it for a little

[old greg]

Originally Posted by blur

The vortices they speak of, how do they work and how are they used?

Ok, so most people have probably seen wingtip vortices. These are the big swirly things that you sometimes see in pictures of airplanes.

They are caused when air tries to flow from the high pressure side of a wing to the low pressure side.

The problem with those is that they create a lot of induced drag, which is shy you'll see little winglets on the ends of airplane wings, or great big end plates on the ends of racecar wings. If you can stop the flow of air around the ends of a wing you can reduce drag and increase lift/downforce. Anyway, you can do the same sort of thing on a smaller scale using a sharp edge. The reason you'd want to (because you're creating drag by doing so) is that vortices will stay attached to a surface in situations were laminar air flows won't. Fighter jets use chines ahead of their wings to create vortices over their wings at high angles of attack in order to turn tighter without causing flow separation (stalling).

The Evo IX did sort of the same thing with the vortex generators along the top of its rear window. The little vortices helped prevent the airflow separating from the rear window and did the same job as the decklid spoiler I mentioned earlier.

If you generate a vortex just behind the front wheel it will attach itself to the outside edge of the underbody. If it's powerful enough it will stay intact all the way up to the rear tire. The thing is, you can't have one flow of air intersect another flow of air, air molecules are solid things and bump into each other. So if you've got this swirling mass of air along the bottom edge of your car you can't very well have air flowing from the side of your car to the underside of your car. If you have a low pressure region underneath your car, that high pressure air next to your car will want to be underneath instead. It can't flow through the vortex, but it will interfere with it and start to break it up / detach it, so a wimpy little vortex won't do much good. This doesn't create a perfect seal with the road surface (air can flow under it, so you need to be low), and it's not as effective as sliding skirts, but it's a whole lot better than nothing.

[old greg]

Originally Posted by blur

About the diffuser, is bigger always better?

The surface of the diffuser can only pull up on a flow of air to a certain extent. If you try to pull up too hard or quickly the air flow detaches and the diffuser stalls (lots of drag, no downforce). If you had a perfect awesome airtight seal with the ground, you'd only want ~7 degrees of rise in the diffuser. Air leaking in from the side will allow/require you to have more of an angle. The extra pressure will reduce downforce, but will help your airflow stay attached to the diffuser at a greater angle (more downforce). And since more and more air will have leaked into the diffuser along its length, you can/should increase the angle of the diffuser along its length (have it curve up). How long/tall/wide it should be really depends on how much air is coming through the venturi section. The diffuser's main purpose is to reduce the drag induced in the nozzle by slowing down the air after it passes through the venturi. If you don't slow it down enough, you lose efficiency (more drag), if you slow it down too much you lose efficiency (more drag). There is a Goldilocks zone where the the diffuser is sized just right, and the drag caused by the undertray is at a minimum.

[Longhorn248]

Originally Posted by fatoni

i think that would be really expensive to do and i dont think looking at the shapes of cars is as great an indicator as you think. weve already brought up the fact that my brick of an ls430 is more aerodynamic than just about everysports car including the frs

It's not as expensive as you would think. Various designs can be tested with CFD in a computer and then small scale mock ups can be sent to a wind tunnel for validation. Pick a design that works then build a full scale mock up and test again.

I was thinking along the same lines as recumbent bicycles that break speed records. The shells around the rider have very low cd, it would be cool to see a manufacturer come out with something similar, even if it was just a research/marketing vehicle.

[old greg]

Originally Posted by Longhorn248

Various designs can be tested with CFD in a computer and then small scale mock ups can be sent to a wind tunnel for validation. Pick a design that works then build a full scale mock up and test again.

They do that anyway. It's just that other factors are usually given precedence. Styling, interior space, safety requirements (Thanks a lot, European pedestrians), wind noise, etc.

They even go so far as to have rapid prototyping departments that can pop out a redesigned trim piece in an hour or two. Then they test it on the full scale, pre-production prototype in their on-site wind tunnels. Then they redesign it and have another prototype made, etc.

[Sully]

Man did this thread get academic.

[Longhorn248]

Originally Posted by old greg

They do that anyway. It's just that other factors are usually given precedence. Styling, interior space, safety requirements (Thanks a lot, European pedestrians), wind noise, etc.

They even go so far as to have rapid prototyping departments that can pop out a redesigned trim piece in an hour or two. Then they test it on the full scale, pre-production prototype in their on-site wind tunnels. Then they redesign it and have another prototype made, etc.

Which is why it would be cool to see a manufacturer forego the usual requirements such as styling, space, safety, etc just to have something unique and "super efficient" to show off at a car show.

It would be awesome to have rapid prototyping capabilities at home for designing aftermarket parts. There's more and more relatively cheap machines that can print out small .STL parts coming to market. One day I'll have to add one to my stable of nerdy tools/toys.

[carbonBLUE]

Im pretty sure they have done some aerodynamic testing for this car, lexus does it for most of thier cars, toyota does it for their economic lineup. Knowing the CoD means its been in the wind tunnel a few times at least. Im wondering how well the aerodynamics were tuned to relieve lift and how smooth the underbody is and if there is a strong suction under the creating more downforce and less drag

[serialk11r]

Originally Posted by Longhorn248

Which is why it would be cool to see a manufacturer forego the usual requirements such as styling, space, safety, etc just to have something unique and "super efficient" to show off at a car show.

It would be awesome to have rapid prototyping capabilities at home for designing aftermarket parts. There's more and more relatively cheap machines that can print out small .STL parts coming to market. One day I'll have to add one to my stable of nerdy tools/toys.

VW "1L" concept car. Gears are hollowed out to save weight, no mirrors.

[Allch Chcar]

Originally Posted by Longhorn248

Which is why it would be cool to see a manufacturer forego the usual requirements such as styling, space, safety, etc just to have something unique and "super efficient" to show off at a car show.

The boxfish concept by BMW was one recent design displayed. There are plenty of examples, but they are often too nerdy/impractical to be mainstream news.

Originally Posted by carbonBLUE

Im pretty sure they have done some aerodynamic testing for this car, lexus does it for most of thier cars, toyota does it for their economic lineup. Knowing the CoD means its been in the wind tunnel a few times at least. Im wondering how well the aerodynamics were tuned to relieve lift and how smooth the underbody is and if there is a strong suction under the creating more downforce and less drag

Exactly and I also wonder about the other results. CoD is only one factor in aerodynamics.

And BTW, the CoDs reported aren't 100% comparable to street versions. They do not always measure with mirrors, antenae, spoilers, etc. The fact that they are even advertising the CoD is a gift. They could write a small book just on the aerodynamics and related measurements of this car.

old greg, thanks for sharing!