The air directly behind the car at the rear bumper is not moving in laminar flow. It is moving in reverse, pushing on the rear of the car in an eddy. Think about it. Tie a streamer to your licence plate and drive around and have someone tell you if it's streaming straight back. I'd put money on it streaming straight up.

 

Some parachutes have holes in designed locations. I'm an airplane aerodynamicist, not a car or parachute designer, but my best guess is that the holes are primarily designed to help even out stresses in the parachute fiber.

It could well be the weight reduction that helps the Civics. Why do you assume it could not be? Have you looked at the power v. weight numbers and seen that the time difference is more than can be explained by weight? Keep in mind that most of the time in a drag race happens at relatively low speed, so aerodynamics don't matter all that much anyway.

Pickups do NOT get better mileage with the gate down or removed, they get worse mileage. But with so many other factors affecting mileage, it is very hard to measure any particular one unless the test is very well controlled. More importantly, CFD and wind tunnel results consistantly show pickups have less drag with the gate up than they do with it down or removed.

As for "misconstued" ideas, I guess I have to agree....

 

Yes, I know there is a recirculation zone at the back of the car. But the reason that zone is there is because there is a very low pressure there. The air under the car is at a higher pressure. That means the air under the car wants to move into the spot behind the car, which is why it recirculates.

The air in the bumper is at the same total pressure as the air under the car, but it is at an even higher static pressure because it is not moving. I think you would find that there is a positive pressure difference between the air inside the bumper and the air behind it.

Not too big of one, though, because there already is a big gaping hole where the license plate goes. And if there was all that much of a pressure difference the tow hook cover would pop off.

Honestly, other than removing weight, I don't know what removing the bumper cover would do. You would have to try it and see. But cutting a bunch of holes in it to let the air go through seems likely to me to be worse than either taking it off or leaving it on.

 

[QUOTE=Asura,May 4 2006, 11:14 PM] I made one too.

 

simple but makes complete sense.

good advice.

 

Hi All,

I have run many CFD cases on rear wings and underbody interactions of a vehicle. I've actually design many aero devices for the s2000 (just for fun). Here's a couple of suggestions from my analytical experience:

1. Generically, positioning a rear wing high relative to the trunk will provide more downforce than one that is low. Low ones, say 2-5 inches, practically does nothing. Also, don't position the wing so far back that it completely overhangs the trunk. The reason is that the low pressure wake region immediately behind the car will interact with the flow moving underneath the wing, causing it to separate off the surface earlier. This will reduce downforce.

2. If you look at a JGTC car, they have the lower portion of the bumper removed. This will reduce drag. Someone above stated that there is a pressure difference between trapped air and the wake region. This is infact true! Stagnant air has very high pressure compared to the wake region. The key is to minimize the surface area in which the pressure can act upon, therefore reducing drag. In aerodynamics, every little square inch counts.

3. To completely close off the body of the car with a panel, for the purpose of generating downforce, will account for more than 70% of total downforce created. As for the cooling issue, much thermal work will be needed. But for a quick fix, you can consider adding vent slots in key locations. The air that goes into the front openning of your car will fill all the little spaces that are in the body of the car. Eventually, this air will join the underbody flow or side flows. By placing vent slots, you can use the low pressure air flowing underneath/side of the car to help extract the hot air trapped in the body.

Hope someone will find this helpful.

 

Thanks for the info.

 

Sorry I don't. My car has only been on the alignment rack. She doesn't like high altitudes with the engine off

I'll have to do some testing to see if it actually helps with times.

Hey Derek, can I borrow your wind-tunnel?

 

Would it be possible to place the wing high enough so that it would not interact with the low pressure region behind the car even when positioned past the rear bumper?

German saloons seem to do well with this set-up -

dtm_1
dtm_2
dtm_3

Thanks!

 

Positioning the wing high away from the body will certainly reduce the interaction bwteen the wake and the wing. A wing that is close or at the same level as the top of the roof will definitely be more efficient than a wing half way up from the trunk. Take the EVO or STI for example. Their wings are fairly high off the trunk.

There are always exceptions, of course. One difference is the body style. Let's take the Merc DTM car. A longer roof gives the air more "time" to recover from separation, which occurs at the top corner of the front windshield. The s2k has much shorter/aggressive roof, which decreases the posibility of the air recovering or reattaching as it passes over. As the air separates going over the roof, it will continue to separate as it leaves the roof top and washes over the rear wind shield. This will result in more turbulent and slower air going toward the wing. (at high speeds, there's actually a wake/recirculation zone immediately after the rear windshield). This is the reason why the EVO and STI uses passive vortex generators and aero element to force the air downwards toward the rear wing.

Now, in regards to the DTM wings....If you look closely the rear wing assembly is a double decker configuration. The top deck is actually two axially offset elements, which will give more aerodynamic advantage over a single element design. The lower deck seems to be a flat plate, or a aero element (can't tell from pic). The key here is the lower element. It serves as a boundary between the wake and the air flowing underneath the upper deck, which is primary source of downforce. This lower deck also reduces drag.

If you look more closely, there's also a minor spoiler at the corner of the trunk. This spoiler, unlike nascar ones, is fairly parallel to the lower deck element. In the case where there is NO lower deck, a more aggressive spoiler will actually help keep the flow attached to the upper elements by deflecting the air upwards toward it or delaying the interaction as it approaches the wake. (in additional to creating downforce)

Remember, the DTM cars employ very high tech aerdynamic principles, which all work together to reduce drag caused by turbulence. The S2k has virtually non. One could expect there would be much of more drag, and therefore more drastic interaction between different regions of airflow.

(If you can imagine an auto airfoil in a windtunnel. The ground serves as a boundary for the air. Now, imagine that the boundary suddenly dissappears and is replaced by a vacuum. The air traveling will no doubt change direction toward the vacuum, regardless of how good the airfoil may be at creating laminar flow)

Hope I answered your questions. (or maybe too much blabbling...sorry...)